Effects of Pig Manure Compost Application Timing (Spring/Autumn) on N₂O Emissions and Maize Yields in Northeast China

Abstract

Animal manure application is widely recognized for its agronomic benefits in enhancing soil fertility and crop productivity through organic matter enrichment and nutrient supply, but the critical application time governing its greenhouse gas emission trade-offs remains unresolved. The objective of this study was to investigate the effects of pig manure compost application timing on nitrous oxide (N₂O) emissions and maize yields in Northeast China through a four-year field experiment. The treatments included: (1) inorganic fertilizers (NPK); (2) NPK plus pig manure compost applied in spring (NPK-MS); and (3) NPK plus pig manure compost applied in autumn (NPK-MA). The N₂O fluxes, NH₄⁺-N contents, NO₃⁻-N contents, and maize yields were analyzed. The results showed that compared with NPK, NPK-MA increased N₂O emissions by 44.4%. Applying pig manure compost in autumn promotes N₂O emissions during the freeze–thaw period. However, there was no significant effect of NPK-MS on N₂O emissions compared with NPK (p > 0.05). Spring-applied manure compost (NPK-MS) resulted in an 11.9% increase in maize yield compared to NPK. In contrast, autumn-applied manure compost (NPK-MA) did not significantly affect maize yield (p > 0.05). Furthermore, yield-scaled N₂O emissions were significantly increased in NPK-MA (p < 0.05). Overall, spring application of pig manure compost is recommended for increasing maize yield without significantly increasing N₂O emissions while in Northeast China.

1. Introduction

Animal manure application not only improves soil fertility, but also increases crop yield [1]. In China, manure is highly recommended for application to crop fields as low soil organic carbon contents restrict crop yields [2]. However, studies have indicated that animal manure application could significantly increase soil nitrous oxide (N₂O) emissions [3,4,5,6]. N₂O is the third most important greenhouse gas, which also influences atmospheric chemistry by being involved in the destruction of the stratospheric ozone layer [7]. N₂O is primarily produced in soils through the microbial process of nitrification and denitrification [8]. The application of manure influences soil N₂O emissions by affecting several factors regulating these processes, including the availability of carbon and nitrogen, and soil environmental factors such as soil temperature and moisture, etc. [9,10].

Manure compost is generally applied in autumn after maize harvest or in the spring before maize is planted in Northeast China. Inorganic N is always applied in spring when maize is sown. However, N₂O emission rates may be the highest when the availability of N to microorganisms exceeds the supply of carbon, as is often the case when fertilizers are applied to the soil [11]. Manure and inorganic fertilizers are applied separately in different seasons, which may reduce the magnitude of the N₂O flux peaks and avoid providing more available carbon and inorganic nitrogen simultaneously to the microorganisms performing denitrification [12]. However, applying manure compost in autumn may lead to additional nitrogen losses, due to the absence of plants for immediate uptake. Hence, the timing of pig manure compost application could potentially be one of the crucial factors influencing N₂O emissions and nitrogen utilization by crops.

Different fertilization timings significantly influence N₂O emissions, as variations in environmental factors (such as soil temperature, moisture, and crop growth) affect the transformation of carbon and nitrogen in organic fertilizers, thereby modulating the N₂O production [13]. Autumn applications are frequently associated with potential environmental contamination due to the augmented risk of nutrient losses [14,15,16,17]. Moreover, manure applied in the autumn can trigger significant N₂O emissions during snow melt in the following spring [18,19]. Hernandez-Ramirez et al. [20] found that spring application of liquid manure resulted in 1.8 times higher N₂O emissions than autumn application, a finding supported by Lin et al. [21], who also reported elevated N₂O emissions following spring manure injections. The authors attributed the increased emissions to wet and cold soil conditions during the spring thaw. However, at the same experimental site, Lin et al. [22] observed higher N₂O emissions with autumn manure application compared to spring application, attributing the discrepancy to spring wetness and drought conditions during the growing season. Furthermore, manure management practices—such as injection versus broadcasting—and manure type (liquid vs. solid) have been shown to influence N₂O emissions [23,24,25]. Additionally, the method and timing of manure application significantly affect ammonia (NH₃) volatilization and nitrogen (N) leaching, which subsequently influence soil N₂O emissions [6,23,26]. Notably, previous studies on fertilizer application timing and its impact on N₂O emissions have primarily focused on regions such as the United States and Canada. Therefore, further investigation is needed to evaluate the effects of pig manure application timing on N₂O emissions under local soil and climatic conditions.

It is essential to highlight that N₂O emission fluxes during freezing-thawing periods should not be overlooked in quantitative measurement studies. A number of studies have demonstrated that N₂O emissions during freeze–thaw cycles can account for over 50% of annual emissions [27,28]. In contrast, other research has reported relatively low N₂O emissions during such periods [29]. Our previous investigation, however, observed significantly elevated N₂O emissions during the freeze–thaw period [30]. Therefore, when manure compost is applied in autumn, the potential for increased N₂O emissions during subsequent spring freeze–thaw cycles must be taken into account. Accordingly, year-round monitoring of N₂O fluxes is particularly critical in regions frequently affected by freeze–thawing processes.

Northeast China represents a critical maize-producing agroecosystem in the country [31]. Although previous research has investigated the effects of manure application on N₂O emissions from agricultural soils in this region, limited empirical evidence exists on how the timing of pig manure compost influences N₂O fluxes and maize productivity. To address this knowledge gap, a four-year field experiment was conducted in Northeast China to assess the effects of different manure compost application timings on maize yield and annual N₂O emissions. Based on previous studies, we hypothesize that the application of pig manure compost in autumn results in higher N₂O emissions compared to its application in spring. The objectives of this study were: (1) to determine the influence of manure compost application timing on maize yield, (2) to evaluate its effect on annual N₂O emission fluxes, and (3) to recommend the optimal timing for manure compost application in Northeast China by integrating crop productivity and environmental sustainability. The results of this study will contribute to the formulation and adoption of effective agricultural management strategies for manure utilization in regional maize production systems.

2. Materials and Methods

2.1. Study Area and Soil Properties

A field experiment was established in May 2012 at Shenyang Agro-Ecological Station (41°31′ N, 123°22′ E) of Institute of Applied Ecology, Chinese Academy of Sciences, Northeast China. This region has a warm-temperate continental monsoon climate. The mean annual air temperature and annual precipitation are 7.5 °C and 680 mm, respectively. The soil is classified as Luvisol (FAO classification). The top 20 cm of soil, with a silt loam texture (20.4% clay, 50.1% silt, and 28.9% sand), had the following physicochemical properties: soil bulk density, 1.25 g cm⁻³; soil pH, 5.80; total N content, 0.90 g kg⁻¹; organic carbon content, 9.00 g kg⁻¹; Olsen-P, 38.50 mg kg⁻¹; and NH₄OAc-K, 97.90 mg kg⁻¹, Available NH₄⁺-N = 1.18 mg kg⁻¹; available NO₃⁻-N = 9.04 mg kg⁻¹.

Meteorological data, including precipitation and air temperature, were obtained from the meteorological station at the Shenyang Agro-Ecological Station. As illustrated in Figure 1, the total annual precipitation for the four consecutive monitoring years (2012/2013 to 2015/2016) exhibited considerable interannual variation, measuring 911.9 mm, 621.7 mm, 485.7 mm, and 585.3 mm, respectively. A significant proportion of this rainfall occurred during the maize-growing season, accounting for 72.3%, 75.5%, 66.5%, and 73.0% of the annual totals in the respective years. The mean annual air temperatures were 7.7 °C (range: −21.2 to 27.5 °C), 8.1 °C (range: −22.7 to 28.3 °C), 9.5 °C (range: −21.7 to 28.2 °C), and 9.3 °C (range: −17.1 to 27.0 °C) for each corresponding year.

2.2. Field Experiment

Three treatments were established in this experiment: (1) inorganic fertilizers (NPK); (2) NPK plus pig manure compost applied in spring (NPK-MS); and (3) NPK plus pig manure compost applied in autumn (NPK-MA); the manure compost application rate was 15 Mg ha⁻¹ yr⁻¹ base on dry weight. The treatments were applied following a randomized design across three replicate field plots (4 m × 5 m). Each year, the composted pig manure was broadcasted evenly onto the plots a few days before maize planting in spring and a few days later after maize harvest in autumn, and ploughed to a depth of 20 cm by machine. we collected pig manure from same farm to ensure the uniformity of “animal manure” throughout the 4 years. In each year, the composted manure applied in spring was identical to that applied in autumn. The total nitrogen and carbon contents of the composted pig manure were: 19.9% and 1.6% in 2012, 23.9% and 2.2% in 2013, 21.1% and 2.5% in 2014, and 20.6% and 2.8% in 2015, respectively. For the respective treatments, urea (North huajin chemical industries group, Panjin, China), calcium superphosphate (Hebei fanshan phosphorite Co., Ltd., Zhangjiakou, China) and potassium chloride (Sinochem Group, Beijing, China) were applied at a rate of 220 kg N ha⁻¹ yr⁻¹, 110 kg P₂O₅ ha⁻¹ yr⁻¹ and 110 kg K₂O ha⁻¹ yr⁻¹ on the same day as maize (Zea mays L., cultivar was Fuyou 9, Liaoning Fuyou Seed Co., Ltd., Shenyang, China) was planted. The row spacing and plant spacing were 60 cm and 37 cm, respectively, resulting in a planting density of 45,000 plants per hectare. Insecticides were applied at key growth stages of maize. No topdressing fertilization was conducted throughout the entire growing season. The detailed sowing time of maize was 3 May 2012, 3 May 2013, 6 May 2014, 10 May 2015, and the harvesting time during the four years was 13 September 2012, 29 September 2013, 29 September 2014, 29 September 2015. To mitigate border effects, two rows of unfertilized maize were planted around each plot as protective borders. The aboveground biomass and grain yield were determined by manually harvesting all maize plants within the inner area of each plot at physiological maturity. After harvest, grain and straw samples were sun-dried to a constant moisture content of 14%.

2.3. Gas Sampling and N₂O Analysis

The gas was sampled using a static closed chamber system as described by Dong et al. [32]. In short, a stainless-steel chamber was inserted into each test plot (56 cm in length × 28 cm in width × 10 cm in depth), with its long side perpendicular to the corn rows. The top chamber (also made of stainless-steel) was 56 cm in length, 28 cm in width, and 20 cm in height. Gas samples were collected between 9:00 and 11:00 in the morning. Gas samples were collected using a 50 mL polypropylene syringe with a three-way plug valve at 0, 20, and 40 min after chambers were sealed. Measurements of N₂O fluxes were conducted at 2–6 day intervals during growing seasons and 7–15 day intervals during non-growing seasons. A gas chromatograph (Agilent 7890A, Shanghai, China) was used to analyze N₂O concentrations of gas samples. N₂O flux values were derived from the following Equation:

F = M × V × A⁻¹ × (Δc/Δt) × [273/(273 + T)] × (1/22.4) × 1000

In this equation, F represents the N₂O flux (μg N₂O-N m⁻² h⁻¹), M denotes the molecular weight of N₂O-N, V indicates the volume of the chamber (m³), A corresponds to the base area of the chamber (m²), Δc/Δt refers to the rate of N₂O concentration increase inside the chamber (ppbv N₂O–N h⁻¹), and T signifies the air temperature within the chamber (°C).

Cumulative N₂O emissions were determined by integrating the flux measurements over time using the following formula:

$$\mathrm{C}\mathrm{u}\mathrm{m}\mathrm{u}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{e} \mathrm{e}\mathrm{m}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}={\sum }_{\mathrm{i}=1}^{\mathrm{n}}{\displaystyle \frac{{(\mathrm{F}}_{\mathrm{i}}+{\mathrm{F}}_{\mathrm{i}+1})}{2}}\times {(\mathrm{t}}_{\mathrm{i}+1}-{\mathrm{t}}_{\mathrm{i}})\times 24$$

where F denotes the N₂O flux (μg N₂O-N m⁻² h⁻¹), i represents the sequential measurement index, (t_i+1 − t_i) corresponds to the time interval (in days) between consecutive measurements, and n indicates the total number of measurements.

2.4. Collection and Analysis of Soil Samples

The soil samples were randomly selected from five points (0–20 cm) in each plot, and mixed into one soil sample. Upon removal of visible roots, the collected soil samples were stored at −20 °C after passing through a 2-mm sieve. The soil NH₄⁺-N and NO₃⁻-N were extracted by adding potassium chloride solution (2 M) at a soil-to-water ratio of 1:2.5 (m/v). After oscillation and filtering, the extracted samples were measured with a continuous flow injection analyzer (Futura, Alliance, France). Gravimetric determination of soil water content was performed by oven-drying the samples at 105 °C for 24 h.

2.5. Data Analysis

Statistical analyses were performed using GraphPad Prism software version 8.0 (GraphPad Software Inc., San Diego, CA, USA) and SPSS 13.0 (SPSS Inc., Chicago, IL, USA). The differences in cumulative N₂O emissions and maize yields within a year, and other factors among treatments were assessed using one-way Analysis of Variance (ANOVA) with Duncan tests and a 95% confidence limit, after verified each data’s normality and homoscedasticity. The effects of different treatments, years, and their interactions on N₂O emission, maize yield and aboveground biomass were examined using two-way ANOVA. Pearson correlation analysis was used to analyze the relationships between N₂O emissions and soil available nitrogen content.

3. Results

3.1. Soil Mineral N

The contents of soil NH₄⁺-N and NO₃⁻-N increased after fertilization, and gradually decreased after reaching the maximum value (Figure 2 and Figure 3). In 2013, soil NH₄⁺-N concentrations were higher in NPK-MS than that in the NPK-MA on 31 May, 16 June, 5 July and 27 October. In 2012 and 2015, there was no significant difference in soil NH₄⁺-N among the three treatments.

Soil NO₃⁻-N contents of the NPK-MA treatment on 15 August, 26 August and 8 September of 2013 were significantly higher than those of the NPK and NPK-MS treatments (Figure 3). In 2015, soil NO₃⁻-N contents of composted manure applied (NPK-MS and NPK-MA) were higher than that in the NPK treatment on 20 May and 31 May.

3.2. Maize Grain Yields and Aboveground Biomasses

Compared with NPK treatment, the grain yields were significantly increased in NPK-MS treatment in 2014 and 2015, respectively (Figure 4). However, there was no significant difference in maize grain yield between the NPK and NPK-MA treatments. Across the four-year observation period, the average maize grain yield of the NPK-MS treatment was significantly increased by 11.9% compared to NPK treatment (p < 0.05, Figure 4), while the average maize grain yield in the NPK-MA treatment was not significantly different relative to that of NPK.

In terms of maize straw yield, there was no significant difference among the three treatments in 2012, 2013 and 2015. However, the straw yield was increased by 39% in the NPK-MS treatment, compared with that in the NPK treatment in 2014 (Figure 5, p < 0.05).

Across the four-year observation, the above-ground biomass in NPK-MS increased by 12.0% compared with that in NPK (p < 0.05).

3.3. N₂O Emissions

Soil N₂O fluxes are shown in Figure 6. The peaks of N₂O fluxes typically occurred after inorganic fertilizer application. However, the highest N₂O flux (305.2 μg m⁻² h⁻¹) was observed from the NPK-MA treatment on 6 March 2013, while it was significantly lower in the NPK and NPK-MS treatments (106.1 and 146.7 μg m⁻² h⁻¹).

In different years, we found that the fertilizer treatments had different effects on soil N₂O emissions. In the years 2012, 2013 and 2015, there was no significant difference in cumulative N₂O emissions among the three treatments (Figure 7). In 2014, however, NPK-MS and NPK-MA significantly increased N₂O emission by 63.0% and 58.5%, compared with NPK, respectively (p < 0.05). Across the four-year observation period, the N₂O emissions significantly increased by 44.4% in NPK-MA treatment, compared with the NPK treatment (p < 0.05). However, no significant difference in annual N₂O emissions was observed between the NPK and NPK-MS treatments (p > 0.05).

Correlation analysis showed that N₂O emission fluxes had a significant positive correlation with soil NH₄⁺-N (r = 0.330) and NO₃⁻-N contents (r = 0.349), respectively.

3.4. Yield-Scaled N₂O Emissions

In 2012, 2014 and 2015, there was no significant difference among the three treatments in yield-scaled N₂O emissions (Figure 8). In 2013, yield-scaled N₂O emission in the NPK-MA treatment was significantly higher than that in the NPK-MS treatment (p < 0.05). In terms of different years, yield-scaled N₂O emissions varied considerably from year to year, which was mainly due to the large differences in cumulative N₂O emissions in different years. Two-way ANOVA results showed that NPK-MA had significantly higher yield-scaled N₂O emissions than NPK treatment (p < 0.05). However, the difference in yield-scaled N₂O emissions was not significant between the NPK and NPK-MS treatments (p > 0.05).

4. Discussion

4.1. The Effect of Composted Manure Application Timing on N₂O Emissions

Our four-year field study revealed that annual N₂O emissions were significantly higher (by 44.4%) under autumn-applied pig manure compost (NPK-MA) compared to inorganic fertilization alone (NPK), whereas spring application (NPK-MS) did not result in a significant increase (Figure 7). These findings are consistent with those of Lin et al. [22], who observed elevated N₂O emissions following autumn manure application, attributed to N₂O emission during early spring thaw. The increased emissions in NPK-MA likely result from three interrelated mechanisms: First, autumn-applied manure undergoes mineralization during winter, releasing labile nitrogen (NH₄⁺-N and NO₃⁻-N), which is susceptible to leaching and denitrification [16,33]. In Northeast China, autumn application coincides with post-harvest soil conditions characterized by high water-filled pore space—due to lower evaporation—and abundant carbon from manure, creating favorable anaerobic environments for denitrifying microbes [9]. In our study, correlation analysis linking N₂O fluxes to soil NH₄⁺-N and NO₃⁻-N contents suggests that elevated availability of mineral N in NPK-MA (e.g., the higher NO₃⁻-N levels in August 2013) was a key driver of increased emissions. This aligns with Kurganova and Lopes de Gerenyu [9], who emphasized N availability as a critical factor influencing denitrification. Second, spring freeze–thaw cycles (FTCs) in Northeast China can physically release trapped N₂O and stimulate microbial activity upon soil thawing [27]. The pronounced N₂O flux peak observed in our study during the 2013 spring FTC period in NPK-MA (305.2 μg m⁻² h⁻¹) supports the role of FTCs in mobilizing mineral N derived from autumn-applied manure, as previously reported by Hansen et al. [16]. Third, the lack of plant N uptake during autumn and winter leaves mineral N vulnerable to denitrification under high soil moisture conditions [13]. Spring application avoids this issue, as manure decomposition synchronizes with maize’s nitrogen demand, thereby reducing the accumulation of mineral N.

Our study quantified N₂O emissions under a fixed manure compost application rate (15 Mg ha⁻¹). Future research should investigate the effects of varying application rates and compare liquid and solid manure forms [34]. Additionally, mechanistic studies integrating real-time N₂O isotopomer measurements with microbial functional gene analyses (e.g., nirS, nosZ) during FTCs could provide deeper insight into emission pathways. Regional modeling efforts incorporating climate projections should also assess long-term emission trends under changing freeze–thaw dynamics.

4.2. The Effect of Manure Compost Application Timing on Maize Yields

Spring manure compost application (NPK-MS) enhanced average maize yield by 11.9% and aboveground biomass by 12.0% over a four-year period compared to inorganic fertilization alone (NPK) (Figure 4 and Figure 5), whereas autumn application (NPK-MA) did not show significant improvements. These results are consistent with Isbell et al. [35], who reported that manure application synchronized with crop nutrient demand enhances nutrient use efficiency. Spring application coincides with maize planting, allowing manure-derived nutrients (N, P, organic matter) to mineralize during the growing season, thereby aligning with peak crop uptake. The mineralization of spring-applied manure occurs concurrently with crop demand, providing a steady supply of nitrogen that complements inorganic fertilizer inputs [6]. Elevated NH₄⁺-N availability in NPK-MS during critical growth phases (May–July 2013; Figure 2) likely contributed to sustained grain filling. In contrast, autumn-applied manure is susceptible to nitrogen losses via overwinter leaching and volatilization before planting, which reduces N availability during early crop growth [36].

The transient increase in straw yield under NPK-MS (39% in 2014) further suggests improved nutrient uptake, consistent with the observed increase in aboveground biomass. This aligns with Hansen et al. [16], who also found that spring manure application improved crop yield. Our study focused on grain yield without differentiating the sources of nitrogen in terms of nitrogen use efficiency (NUE). Future research should incorporate ¹⁵N tracing techniques to quantify the uptake of manure-derived nitrogen. Soil health indicators (e.g., aggregate stability, microbial biomass) under different application timings should be evaluated to better understand the mechanisms underlying yield stability.

4.3. The Effect of Manure Application Timing on Yield-Scaled N₂O Emissions

Yield-scaled N₂O emissions (YSNE)—a key indicator for assessing the trade-off between agricultural productivity and environmental impact [37]—were significantly greater under autumn-applied pig manure (NPK-MA) compared to inorganic fertilization alone (NPK), whereas spring application (NPK-MS) showed no significant difference (Figure 8). This finding underscores the dual disadvantage of autumn application: it leads to elevated emissions without corresponding yield benefits. The increase in YSNE under NPK-MA results from both higher cumulative N₂O emissions (Figure 7) and unchanged grain yield (Figure 4), indicating suboptimal nitrogen use efficiency. In contrast, NPK-MS achieved lower YSNE by increasing yield without a proportional rise in N₂O emissions. The positive correlation between N₂O fluxes and soil mineral nitrogen (NH₄⁺-N and NO₃⁻-N) observed in this study further supports the conclusion that spring application, by minimizing soil nitrogen surplus, can effectively reduce emission hotspots.

The interannual variability in yield-scaled emissions—such as the significant differences observed in 2013 but not in 2012—demonstrates the influence of climatic factors, particularly precipitation during freeze–thaw periods, on both N₂O emissions and crop performance. Therefore, future research should prioritize comprehensive investigations into the relationship between climatic factors—such as temperature, rainfall amount, and precipitation timing—and soil N₂O emissions to support the development of effective mitigation strategies.

The four-year timeframe of our study is inadequate for evaluating the long-term effects of manure application on soil N₂O emissions. This is because the soil’s carbon sequestration potential may become saturated under decadal-scale manure carbon inputs, whereas N₂O emissions would remain driven by enhanced nitrogen availability. Consequently, the sustainability of benefits from long-term manure application requires further investigation through extended monitoring.

In summary, this study systematically evaluated how the timing of pig manure compost application affects both N₂O emissions and maize productivity in Northeast China—a key grain-producing region. Our four-year field experiment provides evidence that pig manure compost applied in spring significantly enhances maize yield without increasing annual N₂O emissions, whereas its autumn application results in markedly higher N₂O emissions without offering any yield advantage. These findings have important implications for the development of region-specific manure management strategies that effectively balance productivity objectives with environmental sustainability.

5. Conclusions

Pig manure compost application time affected N₂O emissions as well as maize yield. Over the four-year study period, compared with the NPK treatment, manure compost applied in autumn (NPK-MA) increased soil N₂O emission and yield-scaled N₂O emissions, whereas maize yield was not significantly increased. In contrast, pig manure compost applied in spring (NPK-MS) did not increase N₂O emissions and yield-scaled N₂O emissions significantly, while maize yield and above-ground biomass were increased significantly in NPK-MS treatment. Therefore, as a fertilization strategy that delivers both economic and environmental benefits, pig manure compost is strongly recommended applied in spring for maize planting in Northeast China.