agronomy

Communication
Greater Nitrogen Availability, Nitrous Oxide
Emissions, and Vegetable Yields with Fall-Applied
Chicken Relative to Horse Manure
Gabriel Maltais-Landry 1,2, * , Zoran Nesic 2 , Nicholas Grant 2 , Brianna Thompson 2 and
Sean M. Smukler 2
1 Soil and Water Sciences Department, University of Florida, Gainesville, FL 32611, USA
2 Faculty of Land and Food Systems, University of British Columbia, Vancouver, BC V6T 1Z4, Canada
* Correspondence: maltaislandryg@ufl.edu; Tel.: +1-352-294-3159




Received: 21 June 2019; Accepted: 9 August 2019; Published: 12 August 2019


Abstract: Optimal manure management can maximize agronomic benefits and minimize
environmental impacts. Field experiments were conducted in the Pacific Northwest (Vancouver,
Canada) to determine how chicken and horse manures that were fall-applied to meet nitrogen crop
demand affect soil ammonium (NH4 + ) and nitrate (NO3 − ), apparent net mineralization (ANM) and
nitrification (ANN), crop biomass and nutrient concentration, and fluxes of nitrous oxide (N2 O),
carbon dioxide (CO2 ), and methane (CH4 ). Relative to horse manure, chicken manure increased soil
NH4 + by 60-fold, ANM by 2-fold, and ANN by 4-fold. Emissions of N2 O (+600%) and CO2 (+45%)
were greater and growing season CO2 emissions (−40%) were lower after application of chicken
than horse manure. Productivity of cover crop (+30%), legume cover crop (−25%), and squash cash
crop (+20%) were affected by chicken relative to horse manure. Overall, fall-applied chicken manure
increased yields, N availability, and environmental impacts relative to horse manure.

Keywords: Nitrogen mineralization; Nitrification; Greenhouse gas; Squash; Rye; Clover; Cover crop

1. Introduction
Proper management of manures in agricultural systems recycles nutrients, maintains crop
productivity, and increases soil organic matter (SOM), which improves water holding capacity and
reduces soil erosion [1]. Manure applications can also lead to excessive nitrogen (N) inputs that
increase N leaching and/or runoff [2], greenhouse gas (GHG) emissions [3], and potentially soil
salinity [4]. Thus, manures must be managed optimally to maximize agronomic benefits and minimize
environmental impacts.
The fraction of manure N that is plant-available N (PAN) is key to manure management because
most N in solid manures consists of organic forms that must be converted to ammonium (NH4 + )
or nitrate (NO3 − ) prior to crop uptake [5]. This can be estimated with the carbon to nitrogen (C:N)
ratio [6], although other variables (e.g., the chemical species for carbon (C) and N in manures) can be
better predictors of the magnitude and timing of N mineralization [5]. However, as N release from
organic sources is often decoupled from plant N demand [7], total N inputs typically exceed crop N
requirement to maintain yields, which increases the potential for N losses [2]. Applying manures to
match crop N demand can also lead to P overfertilization [8] and soil P loss [1] because manures are
enriched in P relative to crop N needs [9].
Applying N-rich manures to soils typically increases nitrous oxide (N2 O) emissions, with
nitrification and denitrification as the main N2 O sources in coarse- and fine-textured soils,
respectively [10]. The N2 O emission factor (EF) is typically lower in manures vs. fertilizers and in solid

Agronomy 2019, 9, 444; doi:10.3390/agronomy9080444 www.mdpi.com/journal/agronomy

Agronomy 2019, 9, 444 2 of 15

vs. liquid manures [11]. Manure mineralization increases organic N release [12] and carbon dioxide
(CO2 ) emissions [13], although only a fraction of organic C inputs from manure is mineralized in the
short-term, resulting in a net accumulation of SOM [4]. Solid manures have a negligible impact on soil
methane (CH4 ) fluxes [3], likely because most agricultural soils are well aerated.
Spring manure application can maximize nutrient uptake by summer cash crops, but it requires
adequate winter storage infrastructure and early applications to ensure sufficient mineralization and
pathogen reduction. In climates with a mild winter (e.g., the Pacific Northwest), fall manure application
could help reduce storage requirements, address the challenges of application timing, reduce N2 O
emissions via lower soil temperatures, and enable farmers earlier planting opportunities in the spring
by allowing for the regulatory 120-day period between manure application and harvest of crops whose
edible portion is in direct contact with soils. However, it may increase nutrient losses via leaching
when precipitation in fall and winter is high [14]. Matching manure N inputs with the expected uptake
from a winter cover crop may reduce environmental impacts (especially with non-legume cover crops)
and improve N availability for the summer cash crop [14–16]. High C:N ratio (>25:1) manures may
limit N losses via N immobilization that would reduce soil residual N, but spring mineralization of
this immobilized N must be synchronized with subsequent crop uptake for this to be an effective
soil N management practice [7,17,18]. Low soil N resulting from immobilization may also promote
N-fixation from legume cover crops [19], although the latter may increase N leaching compared to
non-legumes [15].
The objective of this study was to quantify the effects of a fall application of chicken (N-rich)
and horse (C-rich) manures on crop productivity, soil N availability, and GHG emissions in organic
vegetable production of the lower Fraser Valley (British Columbia, Canada). This area has the highest
density of livestock in Canada, 80% of chicken production in the province of British Columbia, and
a high density of horse stables. Thus, it is critical to identify optimal manure management in this
area with substantial nutrient surpluses due to animal production, limited cropland area for manure
application, and aquifers that are vulnerable to nitrate contamination.
We hypothesized that N-rich chicken manure would increase cereal dominance in cover crops,
N availability and N cycling rates, and GHG emissions. In contrast, horse manure was expected to
reduce fall N losses and GHG emissions, increase legume dominance in cover crops, and immobilize N.
Cash crop yields were expected to be similar with both manures via greater spring N mineralization
with horse manure.

2. Materials and Methods

2.1. Experimental Setup

A field experiment was established at the University of British Columbia (UBC) farm (Vancouver,
Canada), a mature certified organic farm where sandy soils (Typic Haplorthod) are high in SOM
(>9%). The climate is typical of the Pacific Northwest (Figure 1). Mean daily temperature was 11.3 ◦ C
during the experiment (2 October 2015 to 5 October 2016) with a range from −2 ◦ C (2 January 2016) to
21.3 ◦ C (19 August 2016) and five consecutive freezing days from 30 December 2015 until 3 Jan. 2016.
Total rainfall was 1255 mm: 997 mm during the cover-cropping season (roughly seven months from
2 October 2015 until 26 April 2016) and 258 mm for the growing season (roughly five months from
27 April 2016 until 7 October 2016).
 Agronomy 2019, 9, 444 3 of 15
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25 50
Precipitation

Mean Daily Temperature (C)

Daily Precipitation (mm)

20 Temperature 40

15 30

10 20

5 10

0 0

-5 -10
1 Oct 2015 31 Jan 2016 1 Jun 2016 1 Oct 2016
Figure 1. Mean daily temperature and daily precipitation during the study period.
Figure 1. Mean daily temperature and daily precipitation during the study period.
The experiment was designed to provide the total N demand (200 kg N/ha) of spaghetti squash
The experiment
(Cucurbita wastwo
pepo L.) with designed to provide
amendment the totalbased
applications N demand (200 kgPAN
on estimated N/ha) of spaghetti
values: 100 kg squash
PAN/ha
(Cucurbita pepo L.) with two amendment applications based on estimated PAN
supplied by fall-applied chicken or horse manures and 100 kg PAN/ha with spring-applied municipal values: 100 kg
PAN/ha supplied by fall-applied chicken or horse manures and 100 kg PAN/ha with spring-applied
compost. Due to space constraints, there were no control check plots, which reflects common practices
municipal compost.organic
among small-scale Due tovegetable
space constraints, there
farmers in the areawere
that no control
always add check
organicplots, which reflects
amendments. Spring
common
municipal practices
compostamong small-scale
applications organic
are also vegetableof
representative farmers
farmerin the areaalthough
practices that always add organic
the interpretation
amendments. Spring
of the effects due municipal
to each type of compost
manure isapplications
complicatedare alsoadditional
by this representative
input.of farmer practices
although the interpretation of the effects due to each type of manure is complicated by this
Manure application rates were based on the University of Minnesota Extension [20] and Colorado
additional input.
State Extension [21], with PAN estimated as the sum of NO3 -N, NH4 -N and the fraction of organic N
Manure application rates were based on the University of Minnesota Extension [20] and
that mineralizes during the growing season. For horse manure, values of 0.7% N (wet weight) and 20%
Colorado State Extension [21], with PAN estimated as the sum of NO3-N, NH4-N and the fraction of
mineralization of organic N were used, and 2.8% N (wet weight) and 40% mineralization of organic N
organic N that mineralizes during the growing season. For horse manure, values of 0.7% N (wet
for poultry manure. These “book values” for spring-applied manure may not apply directly to a fall
weight) and 20% mineralization of organic N were used, and 2.8% N (wet weight) and 40%
application at the UBC farm, as cooler conditions may decrease the mineralization rate and ultimately
mineralization of organic N for poultry manure. These “book values” for spring-applied manure
the amount of N released. However, they are representative of the information and data that was easily
may not apply directly to a fall application at the UBC farm, as cooler conditions may decrease the
available to small-scale organic vegetable farmers in the area, and our goal was to be as representative
mineralization rate and ultimately the amount of N released. However, they are representative of
as possible of their practices. Since we conducted this experiment, the BC government has established
the information and data that was easily available to small-scale organic vegetable farmers in the
its own online nutrient management calculator that will be better suited for BC agriculture.
area, and our goal 2was to be as representative as possible of their practices. Since we conducted this
Eight 22.5 m plots (5 m × 4.5 m) were set up with four 1.12 m rows in four randomized blocks.
experiment, the BC government has established its own online nutrient management calculator that
Individual plots were amended on 2 October 2015 with chicken (12.5 Mg/ha based on wet weight)
will be better suited for BC agriculture.
or horse (98.5 Mg/ha) manures, which were distributed uniformly with a rake (Table 1). Manure
Eight 22.5 m2 plots (5 m × 4.5 m) were set up with four 1.12 m rows in four randomized blocks.
subsamples were analyzed by an external laboratory (A&L Canada—London, ON, Canada): total C
Individual plots were amended on 2 October 2015 with chicken (12.5 Mg/ha based on wet weight)+or
and N by combustion, P by nitric-hydrochloric acid digestion and quantification via ICP, and NH4 by
horse (98.5 Mg/ha) manures, which were distributed uniformly with a rake (Table 1). Manure
distilled water extraction and colorimetry. The nutrient concentrations measured in the laboratory
subsamples were analyzed by an external laboratory (A&L Canada—London, ON, Canada): total C
differed from the “book values” used to make manure applications, thus the actual PAN application
and N by combustion, P by nitric-hydrochloric acid digestion and quantification via ICP, and NH4+
rates differed from our original calculations. As a result, PAN inputs were higher for chicken manure
by distilled water extraction + and colorimetry. The nutrient concentrations measured in the
(106 kg PAN/ha; 23% as NH4 ) and lower for horse manure (80 kg PAN/ha; 4% as NH4 + ; Table 2)
laboratory differed from the “book values” used to make manure applications, thus the actual PAN
than our original 100 kg PAN/ha target. While this difference in application rate is a limitation, PAN
application rates differed from our original calculations. As a result, PAN inputs were higher for
targets are always approximate given the variability in nutrient content of individual manures, the
chicken manure (106 kg PAN/ha; 23% as NH4+) and lower for horse manure (80 kg PAN/ha; 4% as
mineralization factor used for each manure, and edaphic and climatic conditions at a given site in
NH4+; Table 2) than our original 100 kg PAN/ha target. While this difference in application rate is a
a given year. Despite these limitations, studies such as this one are critical to inform local manure
limitation, PAN targets are always approximate given the variability in nutrient content of
management that must be done in terms of nutrient rather than mass inputs to optimize nutrient
individual manures, the mineralization factor used for each manure, and edaphic and climatic
management, despite greater variability in computing application rates.
conditions at a given site in a given year. Despite these limitations, studies such as this one are
critical to inform local manure management that must be done in terms of nutrient rather than mass
inputs to optimize nutrient management, despite greater variability in computing application rates.
Agronomy 2019, 9, 444 4 of 15

Table 1. Nutrient content (expressed as dry weight) of amendments used in the current study, where
PAN = estimated plant-available nitrogen.

Amendment Moisture Carbon Nitrogen NH4 + -N PAN a Phosphorus C:N N:P

________________ g/kg ______________ mg/kg __________ g/kg __________

Chicken manure 640 404 50 5400 23 19 8 2.6

Horse manure 690 358 13 93 2.6 3.0 28 4.2
Municipal
250 339 25 1800 6.3 5.9 14 3.8
compost
a For manures, this is computed as NH4 + -N + (Organic N x mineralization rate), where mineralization rate equals
0.4 (chicken) or 0.2 (horse). For compost, PAN was estimated by the external laboratory that performed the
measurements on compost.

Table 2. Mass and nutrient inputs (expressed as dry weight) for the two treatments used in the current
study, where PAN = estimated plant-available nitrogen.

Treatment Amendment Mass Carbon Nitrogen PAN Phosphorus

_____ Mg/ha _____ _______________ kg/ha _______________

Chicken Manure 4.5 1.8 227 106 86

Compost a 24.2 8.2 603 151 143
Total 28.7 10.0 831 257 229
Horse Manure 30.5 10.9 390 80 92
Compost a 24.2 8.2 603 151 143
Total 54.7 19.1 993 232 235
a For municipal compost, input rates apply to the banded area.

Fall PAN supply was comparable to the 96 kg N ha−1 recovery in the aboveground biomass (AGB)
of a cereal rye (Secale cereale L.) and crimson clover (Trifolium incarnatum L.) cover crop reported for the
area [22], although total N inputs for both manures exceeded this. Cover crop seeds were broadcast
on 5 October 2015 (clover: 43 kg/ha; rye: 195 kg/ha), incorporated with manures using a rotary tiller
(depth: 15 cm) on 6 October 2015, and mowed and incorporated with a moldboard plough (depth:
20 cm) at flowering (29 April 2016). There was no additional fertilization, irrigation, or pest/weed
management conducted during the cover crop season, although differences in nutrient inputs between
the two manures could affect cover crop growth and further complicate the analysis of the effects of
each type of manure on soil properties.
Municipal compost (Net Zero Waste Inc.—Abbotsford, BC, Canada) was added to provide an
estimated 100 kg PAN/ha, using nutrient analyses obtained from the supplier. The specific compost
batch used was analyzed by an external laboratory (Soil Control Lab—Watsonville, CA, USA) for total
C and N by combustion, P by Aqua Regia digestion and ICP, and NH4 + and NO3 − with distilled
water extraction and colorimetry. This laboratory estimated PAN by summing NH4 + -N, NO3 − -N, and
organic N mineralization (10–40% depending on biologically available carbon). This PAN estimate
was used because it was specific to this compost and because extension documents [20,21] did not
have data on municipal compost. Measured N concentrations (23,300 mg organic N kg−1 ; 1770 mg
N-NH4 + kg−1 ; <1 mg N-NO3 − kg−1 ) differed from those provided by the supplier, thus PAN inputs
were estimated at 150 kg PAN/ha, i.e., 50% greater than expected. Higher than expected PAN inputs
are also partially due to the relatively high ammonium content of this specific compost relative to other
composts. Combined P inputs from manures and compost were high (Table 2).
On 25 May 2016, municipal compost was applied as a 1.12 m wide band centered on the row
where squash would later be planted, and incorporated with a rotary tiller (15-cm depth) on 27 May
2016. Squash was transplanted on 28 May 2016 with 2.2 m between-row spacing, within the range
recommended for squash in the area [23]. A drip irrigation system was used to irrigate as needed (two
to three times a week, depending on weather) and was carefully managed to avoid excess watering
that could lead to N losses. Squash was harvested on 27 August 2016, and plots were tilled (rotary
tiller, 15-cm depth) on 15 September and 29 September 2016.
Agronomy 2019, 9, 444 5 of 15

2.2. Soil Sampling and Analyses

Soils were sampled for initial conditions on 25 September 2015, after manure application, and
every 2–3 weeks (April to June 2016) or 6–8 weeks (November 2015 to April 2016; June to September
2016). Four cores (0–20 cm) were collected per plot using the whole plot area (excluding a 1 m edge
per plot), focusing on the area banded with compost after 24 May 2016. Soil cores were mixed into a
composite sample for each plot, stored in a refrigerator (at 4 ◦ C) until N analyses, and then air-dried and
sieved (2 mm) for P analyses. While focusing on the banded area will increase nutrient concentrations
relative to a homogeneous sample taken from the whole plot, it better represents the conditions
experienced by squash roots that are directly related to squash N uptake and yield. Manure-compost
interactions would also take place in the banded area, and combining soils from the banded area with
those that did not receive compost would likely reduce our capacity to identify these interactions.
Within 48 h of collection, NH4 + and NO3 − concentrations were measured by extracting 5 g of
field-moist soils with 25 mL of 2 M potassium chloride (KCl) for 30 min on a reciprocal shaker. Samples
were centrifuged (5000 rpm for 5 min), filtered (Fisherbrand Q2 filters), and analyzed for NH4 + [24]
and NO3 − [25] by colorimetry.
Apparent net mineralization (ANM) and net nitrification (ANN) were measured by incubating
8 g of soil in specimen cups at in situ soil temperatures for 21 d. Samples had their moisture content
adjusted (60% of water holding capacity) to standardize incubation conditions among sampling points,
sample cups were loosely capped to allow for oxygen exchange, and samples were incubated in a
plastic container buried at a shallow depth near the squash field. Soil N was measured with 2 M
KCl, and ANM and ANN rates were computed based on Robertson et al. [26]. For ANM, the sum of
NH4 + and NO3 − at the beginning of the incubation was subtracted from the sum of NH4 + and NO3 −
at the end of the incubation, and this difference was divided by the number of incubation days. For
ANN, only NO3 − was used. Rates of ANM and ANN were reported in different figures on the date of
sampling, not the date of termination of the incubation. Cumulative rates were computed using linear
interpolation between measurements [27].
Resin P was measured on four sampling dates (initially, after manures and compost inputs, and at
the end of the experiment) using 2.5 g of air-dried soil extracted with 25 mL of distilled water and
one anion-exchange resin (charged with 0.5 M NaHCO3 ) for 16 h on a reciprocal shaker. Resins were
eluted with 25 mL of 0.5 M hydrochloric acid (HCl) for 1 h, and P concentrations were determined by
colorimetry [28]. Resin-extractable P was used because it measures soil P that is freely exchangeable [28]
and represents a smaller background P pool than other methods used in the area (e.g., Mehlich 3).
All nutrient analyses are expressed as concentrations per unit of dry weight, unless specified
otherwise. Moisture content was measured on samples dried at 105 ◦ C for 48 h.

2.3. Plant Sampling and Analyses

Cover crop biomass was sampled just before incorporation (26 April 2016) with a 0.36 m2 quadrat per
plot, separated as rye, clover, or “others” (all other plants), and dried at 65 ◦ C until constant weight. Dried
biomass was ground with a Wiley mill, followed by ball-milling. Ball-milled samples were analyzed
for total P, using a sulfuric acid and hydrogen peroxide digest [29]. Cover crop samples were analyzed
for total C and N using Fourier-transform infrared (FTIR) spectroscopy on a TENSOR 37 instrument
(Bruker Instruments—Ettlingen, Germany) with a subset of samples analyzed by combustion on a Vario
EL Cube Elemental Analyzer (Elementar—Langenselbold, Germany) for calibration. The calibration
models selected for FTIR analyses (Opus v. 7.2) had high validation R2 (0.99 for N, 0.98 for C) and low
validation RMSE (0.03 for N, 0.10 for C) (Supplementary Materials, Figure S1).
Marketable squash biomass was harvested on 27 August 2016, using all fruits located in a 3 m
length of row located in the center of every plot (i.e., excluding a 1 m edge at each end of each plot).
Fresh weight was recorded for individual fruits, and no additional analysis was performed.
Agronomy 2019, 9, 444 6 of 15

2.4. Greenhouse Gas Sampling and Analyses

Individual plots were sampled for N2 O, CO2 and CH4 fluxes weekly (October to December 2015 and
April to October 2016) or biweekly (January to April 2016), using a PVC collar (inner diameter: 20 cm)
inserted to 5–10 cm depth. This resulted in a headspace varying between 2 and 3 L among plots, although
headspace volume did not vary through time among measurements of a given plot. Collars were inserted
at the center of each plot (cover-cropping season) or in a planted row (growing season), amended with the
same manure and compost application rates as the rest of the plot or row on which they were installed,
and weeded weekly to ensure that fluxes measured were due solely to soil processes.
Gases were measured by cavity ring-down spectroscopy using a Picarro G2508 analyzer (Picarro
Inc.—Santa Clara, CA, USA) and non-steady-state chambers. Headspace gas accumulation was
measured during a 5-min enclosure, which is optimal in this system [30]. Air was recirculated at a rate
of 2 L/min using a vacuum pump during measurements, and the system was flushed with outside air
(10 L/min for 2 min) between measurements.
GHG fluxes were computed with MATLAB v. R2014a [31] and the following formula.

F = (ρ * S * V)/(A) (1)

where F is the flux (µmol/m2 /s), ρ is the molar density (mole/m3 ) of dry air (i.e., corrected for the average
mixing ratio of water measured during measurements), S is the slope of headspace GHG accumulation
computed by linear regression (µmol/mol/s), V is the enclosure volume (m3 ), and A is the collar area (m2 ).
Cumulative fluxes were computed using linear interpolation between measurements [27]. Individual
flux measurements were not included in computations due to occasional issues (e.g., incomplete
soil-collar seal after thawing), and two complete sampling dates (17 and 24 June 2016) due to a
malfunctioning vacuum pump that led to underestimated fluxes.

2.5. Statistical Analyses

Repeated measures ANOVAs were used with manure type as the between subject factor and time as
the within-subject factor, followed by a post hoc comparison of means using the Tukey HSD test when
more than two treatments were compared. Results were considered statistically significant at p < 0.05
and marginally significant at p < 0.1. Individual sampling dates were used as the time factor for soil
analyses, and cumulative fluxes over a given time period (cover-cropping season, growing season or
whole-study) for GHG to reduce the number of comparisons for GHG (2 time points vs. 45 individual
sampling dates). Whenever the manure x time interaction term was significant, individual one-way
ANOVAs were computed to determine the effect of manure on individual sampling dates. Conditions of
normality of residuals and homogeneity of variances were verified, and appropriate data transformations
were applied when necessary. All statistical analyses were conducted in SPSS v. 24 [32].

3. Results

3.1. Soil Nutrient Cycling

Soil NH4 + peaked immediately after the application of chicken manure (63 mg N/kg), decreased
gradually until 22 February 2016, and remained below 3 mg N/kg for the remainder of the experiment
(Figure 2A). Soil NH4 + was always below 5 mg N/kg with horse manure, peaking immediately after
application. Soil NH4 + concentrations were significantly smaller with horse than with chicken manure
on 8 October and 18 November 2015.
Soil NO3 − did not increase immediately after manure application, dropped from 20 mg N/kg to
less than 5 mg N/kg on 18 November 2015, and remained low until 5 May 2016 (Figure 2B). Soil NO3 −
increased to 20 mg N/kg after the incorporation of cover crops and municipal compost, and varied
between 6 and 12 (chicken) or 4 and 6 (horse) mg N/kg for the remainder of the experiment. Overall,
soil NO3 − was greater with chicken than with horse manure (time x manure interaction not significant).
Agronomy
Agronomy2019, 9, 9,
2019, 444x FOR PEER REVIEW 7 of 1515
7 of

Chicken
A M anu re: F = 109.5 *** B M an u re: F = 6.7 *
● Horse
T im e: F = 87.0 *** T im e: F = 21.8 ***
60 In teractio n : F = 74.1 *** In teractio n: F = 1.0 n s 60

Soil NO3−−N (mg/kg)

Soil NH4 −N (mg/kg)
40 40
+

20 ●●
● 20

● ● ●
● ●
● ● ● ● ● ● ●
● ● ● ● ● ●
0 ● ● ● 0
c c M anu res
4 M anu re: F = 45.4 ** M an u re: F = 48.1 *** 4
C T im e: F = 4.7 *
D T im e: F = 17.6 ***
In teractio n : F = 4.0 * In teractio n: F = 11.1 ***
3 3

C o m p o st C o m p o st
ANM (mg N/kg/d)

ANN (mg N/kg/d)

M an u res

2 2

1 1
●
●
● ● ●
● ● ● ●
● ● ●
● ● ● ● ● ● ●
● ● ●
0 ● ● 0
c c c c cc c c c c c c cc c

2015−09−01 2016−01−01 2016−05−01 2015−09−01 2016−01−01 2016−05−01 2016−10−01

Figure 2. Soil ammonium (A; NH4 nitrate (B; NO3 + -N),

apparent net mineralization (C; ANM), − -N),

and apparent net nitrification (D; ANN) during the study (mean ± standard error). Arrows indicate the
Figure 2. Soil ammonium (A; NH4+-N), nitrate (B; NO3--N), apparent net mineralization (C; ANM),
date of manure (2 Oct. 2015) and municipal compost (25 May 2016) application. Results from ANOVA
and apparent net nitrification (D; ANN) during the study (mean ± standard error). Arrows indicate
factors refer to the full model with repeated measures, where *** = p < 0.001, ** = p < 0.01, * = p < 0.05,
the date of manure (2 Oct. 2015) and municipal compost (25 May 2016) application. Results from
ns = p > 0.05. Letters along the x-axis indicate that chicken (“c”) was significantly greater (p < 0.05)
ANOVA factors refer to the full model with repeated measures, where *** = p < 0.001, ** = p < 0.01, * =
than the other manure type in one-way ANOVAs computed on individual dates when the manure x
p < 0.05, ns = p > 0.05. Letters along the x-axis indicate that chicken (“c”) was significantly greater (p <
time interaction was significant.
0.05) than the other manure type in one-way ANOVAs computed on individual dates when the
manure
Soil ANM x time interaction
varied between was significant.
0.3 and 0.8 mg N/kg/d with chicken manure, peaking on 27 June
2016, whereas it varied between 0 and 0.6 mg N/kg/d with horse manure, peaking on 8 August 2016
Soil NO3− did not increase immediately after manure application, dropped from 20 mg N/kg to
(Figure 2C). Soil ANM was significantly greater with chicken than horse manure on 18 November 2015
less than 5 mg N/kg on 18 November 2015, and remained low until 5 May 2016 (Figure 2B). Soil NO3−
and from 22 February to 27 June 2016. Cumulative ANM (data not shown) at the end of the experiment
increased to 20 mg N/kg after the incorporation of cover crops and municipal compost, and varied
was twice as high with chicken relative to horse manure (193 vs. 92 mg N/kg; F = 63.3, p < 0.001).
between 6 and 12 (chicken) or 4 and 6 (horse) mg N/kg for the remainder of the experiment. Overall,
Soil ANN peaked at 2.8 mg N/kg/d immediately after chicken manure incorporation, then decreased
soil NO3− was greater with chicken than with horse manure (time x manure interaction not
to values between 0.4 and 0.7 mg N/kg/d from 6 January 2016 (Figure 2D). Soil ANN varied between
significant).
0 and 0.6 mg N/kg/d in plots amended with horse manure, peaking on 8 August 2016. Soil ANN was
Soil ANM varied between 0.3 and 0.8 mg N/kg/d with chicken manure, peaking on 27 June
significantly smaller with horse relative to chicken manure on 8 October and 18 November 2015, and
2016, whereas it varied between 0 and 0.6 mg N/kg/d with horse manure, peaking on 8 August 2016
from 22 February to 27 June 2016. Cumulative ANN (data not shown) at the end of the experiment was
(Figure 2C). Soil ANM was significantly greater with chicken than horse manure on 18 November
three-fold greater with chicken relative to horse manure (291 vs. 99 mg N/kg; F = 89.1, p < 0.001).
2015 and from 22 February to 27 June 2016. Cumulative ANM (data not shown) at the end of the
Soil available P increased after manure application and was greater with chicken than horse
experiment was twice as high with chicken relative to horse manure (193 vs. 92 mg N/kg; F = 63.3, p <
manure for the duration of the experiment, although that difference was not statistically different
0.001).
(Supplementary Material, Figure S2).
Soil ANN peaked at 2.8 mg N/kg/d immediately after chicken manure incorporation, then
decreased to values between 0.4 and 0.7 mg N/kg/d from 6 January 2016 (Figure 2D). Soil ANN
3.2. Crop Response
varied between 0 and 0.6 mg N/kg/d in plots amended with horse manure, peaking on 8 August
Cover crop biomass and the relative abundance of rye were greater and clover biomass and
2016. Soil ANN was significantly smaller with horse relative to chicken manure on 8 October and 18
relative abundance were smaller with chicken compared to horse manure, but only the latter was
November 2015, and from 22 February to 27 June 2016. Cumulative ANN (data not shown) at the
statistically significant (Table 3). Clover %N was greater and C:N was smaller than rye, with no effect
end of the experiment was three-fold greater with chicken relative to horse manure (291 vs. 99 mg
of manure type. Cover crop P concentrations were greater and C:P was smaller with chicken relative
N/kg; F = 89.1, p < 0.001).
to horse manure, with no effect of cover crop species; both differences were statistically significant.
Soil available P increased after manure application and was greater with chicken than horse
The amount of C, N, and P recovered in cover crop aboveground biomass (AGB) was greater with
manure for the duration of the experiment, although that difference was not statistically different
chicken relative to horse manure (not significant for N). Recovery in cover crop AGB was equivalent
(Supplementary Material, Figure S2).
to 85% (chicken) or 95% (horse) of manure inputs for PAN, 20% (chicken) or 13% (horse) of manure
inputs for P, and 130% (chicken) and 17% (horse) of manure inputs for C.
Agronomy 2019, 9, 444 8 of 15

Table 3. Cover crop biomass, concentration, and recovery in aboveground biomass of carbon (C), nitrogen (N), and phosphorus (P), and nutrient ratios (mean ±
standard error). Different letters denote significant differences (p < 0.05) among cover crops in chicken manure only (a, b, c), horse manure only (f, g, h), or in both
manures when the interaction term is not significant (x, y, z). Hash signs (#) denote a significant difference between chicken and horse manures for a given cover crop,
whereas plus signs (+) denote a significant difference between chicken and horse manure that is identical among all cover crops (i.e., the interaction term is not
significant).

Manure Crops Biomass Carbon Nitrogen Phosphorus C:N C:P

Mg ha−1 % total Mg C ha−1 g kg−1 kg N ha−1 g kg−1 kg P ha−1
Chicken Clover 1.6 ± 0.29 b 28 ± 3.1 b 0.7 ± 0.13 b 23 ± 1.0 x 38 ± 6.5 x 3.2 ± 0.20 + 5 ± 0.8 b 19 ± 0.6 y 136 ± 16.7
Rye 3.9 ± 0.39 a # 67 ± 2.9 a 1.7 ± 0.17 a # 14 ± 1.5 y 52 ± 4.0 x # 3.0 ± 0.20 + 11 ± 0.8 a # 33 ± 3.6 x 150 ± 19.0
Weeds 0.3 ± 0.06 c 5 ± 0.5 c
Total 5.8 ± 0.67 # 2.4 ± 0.27 # 90 ± 9.3 17 ± 1.4 #
Horse Clover 2.2 ± 0.51 f 46 ± 6.5 f 0.9 ± 0.22 f 22 ± 1.0 x 47 ± 9.5 x 2.9 ± 0.31 6 ± 1.0 f 19 ± 0.8 y 154 ± 32.7 +
Rye 2.2 ± 0.15 f 51 ± 6.6 f 1.0 ± 0.07 f 13 ± 0.5 y 29 ± 2.9 x 2.7 ± 0.15 6 ± 0.4 f 34 ± 1.4 x 164 ± 16.7 +
Weeds 0.1 ± 0.02 g 2 ± 0.4 g
Total 4.5 ± 0.55 1.9 ± 0.23 76 ± 8.9 12 ± 0.8
F, p F, p F, p F, p F, p F, p F, p F, p F, p
Manure 4.7, 0.047 0.0, 0.994 2.6, 0.142 1.1, 0.317 1.6, 0.243 7.0, 0.026 4.0, 0.076 0.4, 0.561 6.7, 0.030
Species 70.0, <0.001 89.9, < 0.001 18.0, 0.002 125.8, <0.001 0.2, 0.684 3.7, 0.085 10.9, 0.009 77.5, <0.001 3.8, 0.084
Interaction 10.2, 0.002 8.3, 0.003 12.2, 0.007 0.0, 1.000 8.6, 0.017 0.0, 0.890 8.2, 0.019 0.0, 0.897 0.1, 0.801
Total 15.0, 0.031 n.s. 15.8, 0.028 n.s. 3.2, 0.173 n.s. 10.4, 0.048 n.s. n.s.
Agronomy 2019, 9, 444 9 of 15

Squash yields were 20% greater with chicken relative to horse manure (58 vs. 45 Mg/ha), a
marginally significant difference (F = 7.4, p = 0.073). Fruit size (1.16 vs. 1.05 kg/fruit; F = 5.8, p = 0.053)
was also greater with chicken relative to horse manure, but not fruit number (51 000 vs. 45 000 fruits/ha;
F = 4.7, p = 0.118).

3.3. Greenhouse GAS emissions

Nitrous oxide emissions peaked after manure application and were seemingly greater with
chicken relative to horse manure until 14 January 2016, after which there were no differences between
manures (Figure 3A). Cumulative N2 O fluxes were greater with chicken relative to horse manure for
the cover-cropping season (289 vs. 28 mg N2 O/m2 ) and the full experiment (335 vs. 68 mg N2 O/m2 ),
whereas cumulative fluxes for the growing season were similar between manures (46 vs. 39 mg
N2 O/m2 ; Figure 3B). The fraction of N inputs emitted as N2 O was five times greater with chicken
Agronomy 2019, 9, x FOR PEER REVIEW 9 of 15
relative to horse manure (0.26% vs. 0.05%).

0.6
A Chicken ● Horse B M anu re: F = 4.4 (*) Chicken

Cum. N2O emissions (g/m2)

T im e: F = 3.3 n s Horse
N2O emissions (mg/m2/d)

6
In teractio n : F = 4.0 (*) 0.5

0.4
4

0.3
●
2
●
0.2
●
●
●
●
●
●
● ● ●●● ● ●
● ● ●● ●●●●●●●
●
0.1
0 ●● ●●●●
●●
●●
●
● ● ●● ●
●
●

0
25
C D M anu re: F = 3.5 n s 2

Cum. C02 emissions (kg/m2)

C o m p o st
T im e: F = 173.5 ***
CO2 emissions (g/m2/d)

20 M an u res In teractio n : F = 30.7 ** 1.6

●
15 ●
1.2
● ●
● ●●
● ● ●
10 ● 0.8
● ●
● ● ●
● ●●●
●
5 ●● 0.4
● ●
● ●
● ●●●● ●● ● ● ●
● ●●● ● ● ● ●
0 0

0.1 E F Cum. CH4 emissions (mg/m2)

0
CH4 emissions (mg/m2/d)

● ●
0.0 ● ●● ●●●
●
●●●● ● ● ● ●
● ● ●
● ●●
● M anu re: F = 0.2 n s −10
−0.1
●●●
●●
● T im e: F = 180.8 ***
● ●● ●
●●
−0.2
●
● In teractio n : F = 1.5 n s
●
●
−20
●●
−0.3 ●
● ●
−30
−0.4

−0.5 −40

2015−09−01 2016−01−01 2016−05−01 2015−09−01 2016−01−01 2016−05−01 2016−10−01

Figure 3. Temporal and cumulative fluxes (mean ± standard error) of nitrous oxide (A,B; N2 O), carbon
dioxide
Figure(C,D; CO2 ), and
3. Temporal methane
and (E,F; CH
cumulative 4 ). Arrows
fluxes (mean ±indicate
standard theerror)
date ofofmanure
nitrous (2 Oct 2015)
oxide (A,B; and
N2O),
municipal compost (25 May 2016) application. Results from ANOVA factors refer
carbon dioxide (C,D; CO2), and methane (E,F; CH4). Arrows indicate the date of manure (2 Oct to the full model
2015)
with
andrepeated measures,
municipal compostwhere
(25 ***
May = p2016)
< 0.001, ** = p < 0.01,
application. (*) = from
Results p < 0.1, ns = p >factors
ANOVA 0.1. Chicken manure
refer to the full
had higher
model N2 O
with emissions
repeated than horse
measures, manure
where *** = during the**
p < 0.001, cover-cropping
= p < 0.01, (*) =season and
p < 0.1, nsthe
= p whole study,
> 0.1. Chicken
and it had higher and lower CO 2 emissions than horse manure during the cover-cropping
manure had higher N2O emissions than horse manure during the cover-cropping season and season andthe
thewhole
whole study,
study, respectively.
and it had higher and lower CO2 emissions than horse manure during the
cover-cropping season and the whole study, respectively.
Peaks of CO2 emissions were measured immediately after applications of manure and municipal
compost and after ploughing, with sharp declines in emissions following these events (Figure 3C).
Peaks of CO2 emissions were measured immediately after applications of manure and
Cover-cropping season cumulative fluxes were greater with chicken relative to horse manure (608
municipal compost and after ploughing, with sharp declines in emissions following these events
vs. 420 g CO2 /m2 ), whereas cumulative fluxes during the growing season (1018 vs. 1425 g CO2 /m2 )
(Figure 3C). Cover-cropping season cumulative fluxes were greater with chicken relative to horse
and the full experiment (1625 vs. 1844 g CO2 /m2 ) were smaller with chicken relative to horse manure
manure (608 vs. 420 g CO2/m ), whereas cumulative fluxes during the growing season (1018 vs. 1425
2
(Figure 3D). The proportion of C inputs (manure + municipal compost) emitted as CO2 was greater
g CO2/m2) and the full experiment (1625 vs. 1844 g CO2/m2) were smaller with chicken relative to
with chicken than horse manure (44% vs. 26%).
horse manure (Figure 3D). The proportion of C inputs (manure + municipal compost) emitted as CO2
was greater with chicken than horse manure (44% vs. 26%).
Methane fluxes were comparable after the application of chicken or horse manure, and
consistent CH4 consumption was observed starting in April (Figure 3E). Cumulative CH4
consumption for the whole experiment was similar with horse (29.4 mg CH4/m2) and chicken (31.5
mg CH4/m2) manure (Figure 3F), and CH4 fluxes accounted for an insignificant fraction of C inputs
Agronomy 2019, 9, 444 10 of 15

Methane fluxes were comparable after the application of chicken or horse manure, and consistent
CH4 consumption was observed starting in April (Figure 3E). Cumulative CH4 consumption for the
whole experiment was similar with horse (29.4 mg CH4 /m2 ) and chicken (31.5 mg CH4 /m2 ) manure
(Figure 3F), and CH4 fluxes accounted for an insignificant fraction of C inputs with both manures
(<0.003%).

4. Discussion

4.1. Effects of Manures During the Cover-Cropping Season

Soil NH4 + was higher after chicken than horse manure application, which was expected given
the 60-fold greater NH4 + concentration measured and 8-fold greater NH4 + inputs with the former
(24 kg NH4 + -N ha−1 vs. 2.8 kg NH4 + -N ha−1 ). Larger PAN inputs with chicken relative to horse
manure may have also contributed to this difference. Cumulative ANN was four-fold greater and N2 O
emissions were six-fold greater with chicken relative to horse manure. These results are consistent with
high soil NH4 + [33] and N2 O emission [13] reported after chicken manure application. In addition,
the coupling between greater ANN rates and N2 O emissions with chicken manure suggests that
nitrification may dominate N2 O emissions in this study, consistent with results from Rochette et al. [10]
after manures were applied to coarse-textured soils. However, the current study did not collect direct
evidence (e.g., stable isotopes) that would indicate which of nitrification or denitrification was the
main N2 O source.
Soil NO3 − was expected to increase after manure application, but NO3 − remained low in the
plough layer (0–20 cm). It is unlikely that cover crop N uptake prevented NO3 − accumulation because
fall growth was minimal, although fall N uptake was not measured. Leaching below the rooting
depth is more likely in these coarse-textured soils experiencing high rainfall in the fall (Figure 1), but
direct measurements of N leaching (e.g., lysimeters) are lacking in the current study. If confirmed,
these hypothetical N losses could deplete N in the rooting zone, reduce growing season N availability,
and affect water quality [14], although earlier cover crop seeding and establishment could potentially
mitigate these losses.
The cumulative amount of N mineralized was twice as large with chicken than horse manure,
which had lower percentages of estimated PAN and manure PAN found as NH4 + , a higher C:N, and
lower estimated PAN inputs than chicken manure. Given the strong correlation between manure
mineralization and CO2 emissions [12], the 45% greater CO2 emissions measured with chicken relative
to horse manure, despite six-fold smaller C inputs, could be used as a proxy to estimate the doubling
in cumulative ANM measured with the former. The smaller ANM rates measured with horse relative
to chicken manure are similar to the lower N mineralization rates reported by Chae and Tabatabai [34]
and Gagnon and Simard [35]. Despite a high C:N ratio (>25:1), soils amended with horse manure
did not seem to immobilize N in the current study however, which could be due to slower microbial
activity via lower temperatures (although temperatures were relatively high) or most likely C and N
speciation that was unfavorable to decomposition [5].
Clover biomass was more abundant with horse than chicken manure, and lower soil inorganic
N might have stimulated a greater proportion of N derived from N-fixation, as measured by
Büchi et al. [19]. Cover crop N concentrations were comparable to values reported for late April in the
area [22], resulting in a similar cover crop N recovery (76–90 kg N/ha) between manures. However,
cover crop N recovery was smaller than total N inputs and the 110 kg N/ha threshold proposed by
Tonitto et al. [16] for cover crop N inputs to result in comparable yields relative to synthetic fertilizers.
Although ANM increased after cover crop incorporation (especially with horse manure), soil N was
relatively unaffected, suggesting that ANM rates measured in specimen cups incubated in situ were
not directly transferable to field conditions, similar to observations from Sistani et al. [33].
Carbon inputs from cover crop AGB accounted for less than 20% of C inputs from horse manure
as opposed to C inputs of similar magnitude compared to chicken manure. Cover crop C inputs should
Agronomy 2019, 9, 444 11 of 15

contribute less to SOM accumulation than manures [4] because of faster mineralization. However, the
contribution of cover crop AGB to SOM may still be important in farming systems with smaller SOM
concentrations than those found in the current study.
Despite a greater P uptake with chicken manure via greater N availability, cover crop P recovery
was low for both manures (<20% of manure P inputs), suggesting a limited impact in these high P
soils, similar to Kuo et al. [36] and Maltais-Landry et al. [8].

4.2. Effects of Manures During the Growing Season

Rates of ANM were lower with horse relative to chicken manure after municipal compost
application, consistent with lower N mineralization with horse manure [34,35]. This limited N
availability to squash early in the growing season suggests that N was not released at the optimal time,
despite our sampling focusing only on the area banded with compost that should have experienced
increased N availability vs. unfertilized, unplanted rows. The lack of fall N immobilization also
suggests a limited potential to reduce fall N losses. Thus, this horse manure did not seem to be as
effective as a soil N management tool compared to other amendments with a high C:N (e.g., used
cooking oil [18]). However, fall application of horse manure might still be more advantageous than
spring application, and this should be investigated in future studies.
Squash yields were 20% greater with chicken relative to horse manure, consistent with the greater
fertilization value of the former [6,34]. This could also be due to the greater (+11%) estimated total
PAN inputs with chicken relative to horse manure, although estimated PAN inputs were excessive
relative to local recommendations of 200 kg PAN/ha with both manures. High PAN inputs and SOM
concentration in these soils may have masked potential losses with chicken manure however, which
could have led to N deficiency under more N-limiting conditions. Unamended control plots would
have allowed to test this hypothesis, but we did not include control plots due to space constraints and
to be consistent with practices that represent those of local farmers as closely as possible (i.e., cover
crops and organic amendments used in all fields). Ultimately, excessive PAN and total N inputs likely
resulted in N surpluses relative to crop demand, although the extent of those surpluses were not
quantified in this study.
Growing season CO2 emissions were 40% greater with horse relative to chicken manure but total
CO2 emissions were similar between manures despite larger C inputs with the former, suggesting that
C was more recalcitrant and decomposition rate was slower with horse manure. Total N2 O emissions
were greater with chicken relative to horse manure (growing season N2 O emissions unaffected), but
this could be affected by larger NH4 + content and PAN inputs with the former, indicating a higher
environmental footprint for NH4 + -enriched manures applied in the fall. There was a small peak in
N2 O emissions after municipal compost application, consistent with low EF for composts [11]. Manure
type did not affect CH4 emissions, similar to what Chadwick et al. [3] and Sistani et al. [13] reported.

4.3. Trade-Offs of Chicken and Horse Manure Use

Greater N availability and PAN inputs with chicken relative to horse manure (total PAN inputs
11% larger with chicken manure) benefitted crop productivity and soil fertility but resulted in greater
environmental impacts (Figure 4). For example, chicken manure had greater cumulative N2 O fluxes
(3-fold) and fraction of N inputs emitted as N2 O (5-fold) relative to horse manure, consistent with
the greater EF reported by Charles et al. [11] for amendments that mineralize quickly. Greater N2 O
emissions with chicken relative to horse manure were largely driven by a 2.5 month peak in the
fall, lasting longer than the 40 days or less reported previously for N2 O emissions peaks in warmer
conditions [10,37]. The environmental impacts of chicken relative to horse manure could be even
more important, as large NH3 losses with chicken manure [37] could exacerbate N gaseous losses and
impacts on air quality. Overall, this indicates the need to better estimate PAN inputs from chicken
manure, through cheaper characterization methods (e.g., FTIR as used for cover crop in Figure S1) and
Agronomy 2019, 9, 444 12 of 15

locally-adapted
Agronomy 2019, 9, x nutrient
FOR PEERcalculators
REVIEW (e.g., the new BC nutrient calculator), as this would have reduced
12 of 15
chicken manure inputs and associated risks of environmental N losses.

Soil PAN
Chicken
before planting
● Horse
Residual 1
ANM
soil PAN
0.75
●

0.25 ●
●
N2O
Efflux 0 Yields
●
●

●
●

Soil Cover
resin P crop
build−up biomass

●
●

CO2 C inputs
Efflux from manure

Figure 4. Radar plot displaying average values for each treatment scaled to the maximum values
observed for each variable. All differences were statistically significant (p < 0.05; except for yields that
Figure
were 4. Radar at
significant plot
p <displaying average
0.1 and resin valuesthat
P build-up for was
eachnot
treatment scaledalthough
significant), to the maximum values
the difference in
Cobserved for each
inputs was variable.Shaded
not testable. All differences were
variables statistically
represent significant
negative (p < 0.05; impacts
environmental except for yields
(i.e., that
smaller
were significant
values indicate a at p < desirable
more 0.1 and resin
outcome). C = carbon;
P build-up that was = nitrogen;
Nnot significant), = estimated
PANalthough the difference in C
plant-available
N; P = phosphorus; N2 O = nitrous oxide; CO2 = carbon dioxide; ANM = apparent net mineralization.
inputs was not testable. Shaded variables represent negative environmental impacts (i.e., smaller
values indicate a more desirable outcome). C = carbon; N = nitrogen; PAN = estimated
Horse manureN;had
plant-available smaller crop
P = phosphorus; N2Oproductivity relative
= nitrous oxide; CO2 = to chicken
carbon manure,
dioxide; ANM =likely
apparentbecause
net of
lowermineralization.
PAN inputs and poorly synchronized N mineralization. However, yields per unit of N2 O
emitted were more favorable for horse (0.79 ± 0.182 Mg yield/mg N2 O emitted) than chicken manure
(0.24 Horse
± 0.059manure released
Mg yield/mg N2 O40% less of which
emitted), its C inputs
may alsoas be
COpartially
2 relative to chicken
driven by lower manure, consistent
PAN input rates
with
for thesmaller
former.CO 2 emissions
Horse manurewhen may manure
provide C andbenefits
more N is more recalcitrant
in systems with[5,12].
less SOM Smaller yields
or after with
several
horse ofrelative
years to chicken
application, manuretowould
as compared require
the current studyhigher application
that focused rates
on the and/or
effects of a additions
single horse of
synthetic fertilizers to increase yields however, where higher horse manure
manure addition on a high SOM soil. Greater legume cover crop abundance with fall-applied horse inputs to meet PAN
requirements
relative would
to chicken ultimately
manure couldincrease environmental
also increase N-fixationimpacts
benefits(e.g., GHG
if the N emissions,
fixed by cover N surpluses,
crops is
and soil P build-up). These results suggest a greater potential for horse manure
transferred efficiently to subsequent cash crops [7,15,17]. However, smaller squash yields with horse to help build SOM
rather than
relative increasemanure
to chicken crop productivity,
suggest thatwhich may be
N-fixation especially
benefits wereuseful in degraded
insufficient soils.
to compensate for the
Phosphorus
smaller estimated inputs (>200made
PAN inputs kg P/ha/yr)
with horsewere excessive
manure in thewith bothstudy.
current manures relative to typical
vegetable
Horsecrop P removal
manure released of40%
less less
thanof20
itskg P/ha/yras[9].
C inputs COSoil resin Ptoincreased
2 relative only with
chicken manure, chicken
consistent
manure
with however,
smaller consistentwhen
CO2 emissions with manure
the increase
C andin Nplant-available P reported
is more recalcitrant [5,12].with chicken
Smaller butwith
yields not
manureto
horse relative inchicken
the incubation
manurestudywouldofrequire
Gagnon and Simard
higher [35]. rates
application Long-term
and/orPadditions
overfertilization will
of synthetic
result in higher
fertilizers soil Pyields
to increase ultimately,
however,similar
whereto higher
other farming systems
horse manure targeting
inputs to meet cropPANN demand with
requirements
manures
would [8].
ultimately increase environmental impacts (e.g., GHG emissions, N surpluses, and soil P
build-up). These results suggest a greater potential for horse manure to help build SOM rather than
5. Conclusions
increase crop productivity, which may be especially useful in degraded soils.
Phosphorus
Overall, a single inputsfall
(>200 kg P/ha/yr)
application were manure
of horse excessiveprovided
with both manures
benefits relative to
compared to chicken
typical
vegetable
manure incrop termsP removal of less
of C inputs andthan 20 kg cover
legume P/ha/yrcrops
[9]. Soil resin
while P increased
having a smallonly with
effect onchicken
residualmanure
soil N
and GHG. The lack of unamended control plots and larger PAN inputs with chicken relative to
horse manure in this study prevents us from quantifying the net effect of each type of manure
accurately, which is further complicated by the use of cover crops and spring municipal compost
inputs in both treatments. Nevertheless, our results highlight that the use of horse manure must be
Agronomy 2019, 9, 444 13 of 15

however, consistent with the increase in plant-available P reported with chicken but not horse manure
in the incubation study of Gagnon and Simard [35]. Long-term P overfertilization will result in higher
soil P ultimately, similar to other farming systems targeting crop N demand with manures [8].

5. Conclusions
Overall, a single fall application of horse manure provided benefits compared to chicken manure
in terms of C inputs and legume cover crops while having a small effect on residual soil N and GHG.
The lack of unamended control plots and larger PAN inputs with chicken relative to horse manure
in this study prevents us from quantifying the net effect of each type of manure accurately, which is
further complicated by the use of cover crops and spring municipal compost inputs in both treatments.
Nevertheless, our results highlight that the use of horse manure must be optimized to maximize fall N
scavenging and limit excessive P overfertilization, and that higher fertilizer N inputs will likely be
required to increase yields, despite excessive total N inputs. For N-rich chicken manure, application
immediately before the growing season may be more appropriate given its high estimated PAN and
NH4 + concentrations, which could reduce N leaching risks by better timing manure PAN supply with
crop demand. However, given the significant P overfertilization and excessive total N inputs observed
with both manures, application rates will need to decrease to minimize P loss risks, requiring N inputs
from fertilizers and/or significant N-fixation in cover crops.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4395/9/8/444/s1.

Figure S1: Calibration between cover crop N (A) and C (B) measured by combustion with an Elemental Analyzer
(EA) or by Fourier-Transform Infrared Spectroscopy (FTIR); Figure S2: Soil phosphorus during the study (mean ±
standard error).
Author Contributions: Conceptualization, G.M.-L. and S.M.S.; methodology, G.M.-L., Z.N., N.G., and S.M.S.;
validation, G.M.-L., Z.N., N.G., and S.M.S.; formal analysis, G.M.-L., Z.N., and N.G.; investigation, G.M.-L. and
B.T.; resources, G.M.-L., Z.N., and S.M.S.; data curation, G.M.-L.; writing—original draft preparation, G.M.-L.;
writing—review and editing, all authors.
Funding: This research was funded by a postdoctoral fellowship to G. Maltais-Landry from the Natural Sciences
and Engineering Research Council of Canada (NSERC).
Acknowledgments: We thank the UBC farm staff for logistical support while establishing the experiment; Spencer
Bailey, Alice Chang, Winnie Cheung, Li-Yun Hsu, Alfred Ke and Katarina Neufeld for assistance in the field and
in the lab.
Conflicts of Interest: The authors declare no conflicts of interest.

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