Catch Crop Residues Stimulate N2O Emissions During Spring, Without Affecting the Genetic Potential for Nitrite and N2O Reduction

Straw return and nitrogen fertilization regulate soil greenhouse gas emissions and global warming potential in dual maize cropping system

Abundant nitrogen (N) fertilization is needed for maize (Zea mays L.) production in China because of its huge residual biomass return. However, excessive N fertilization has a negative impact on the soil ecosystem and environment, which contributes to climate change. Soil incorporation of maize residues is a well-known practice for reducing chemical N fertilization without compromising maize yield and soil fertility. Thus, residues incorporation has the capacity to minimize N fertilization uses and hence mitigate soil greenhouse gas emissions by improving plant N uptake and use efficiency. There is still a research gap regarding the effects of maize residues incorporation on maize yield, soil fertility, greenhouse gas emissions, and plant N and carbon (C) contents. Therefore, we conducted a field experiment during spring and autumn involving four different N fertilization rates (N0, N200, N250, and N300 kg N ha^-1), with and without maize residues incorporation, to evaluate grain yield, soil fertility, plant N and C contents, and greenhouse gas emissions (GHGs). Compared to N0, N fertilizer application at 300 kg N ha^-1 with residues incorporation significantly increased area-scaled global warming potential (GWP) compared to other N fertilization rates in both spring and autumn seasons, but soil nutrient contents and plant N and C contents were not statistically different from the N250 treatment. In contrast, the N recovery use efficiency (NRUE), physiological N use efficiency (PNUE), and agronomic N use efficiency (ANUE) were significantly lower in the N300 treatment than in the lower N treatment groups. Nitrous oxide (N₂O) and carbon dioxide (CO₂) fluxes, area-scaled GWP, and greenhouse gas intensity (GHGI) were significantly lower in the N200 treatment with straw incorporation than the N250 and N300 treatments of the traditional planting system. Thus, we concluded that N200 treatment with residues incorporation is optimal for improving grain yield, soil fertility, plant N uptake, and mitigating greenhouse gas emissions.

Potential for the adoption of measures to reduce N2O emissions from crop residues in Denmark

Crop residues represent a climate change dilemma: they can promote carbon (C) sequestration, but they may also stimulate emissions of the powerful greenhouse gas nitrous oxide (N₂O). Although there are crop residue management measures to reduce N₂O emissions, N₂O reductions achieved at national scale with these measures have been seldom studied, and how farmers' willingness to accept the measures constrains their potential remains largely unknown. Using Denmark as a case study, we combined a survey (completed by 592 farmers) and national data to assess the practical potential and obstacles for the successful implementation of management strategies to reduce N₂O emissions from crop residues. Crop residue removal (particularly from vegetables and cover crops) and nitrification inhibitors were identified as effective in reducing N₂O emissions from a biophysical perspective. If all aboveground crop residues from vegetables and cover crops were removed, N₂O emissions could be reduced by 0.181 Gg N₂ON, corresponding to 11% of the total N₂O emissions from crop residues nationally. However, a low percentage of farmers would be willing to remove crop residues from the field, especially for vegetables and cover crops (25%), in connection to the possible short- to medium-term reduction in C sequestration. Similarly, use of nitrification inhibitors would reduce emissions by 0.247 Gg N₂ON, corresponding to 15% of the total residue N₂O emissions, and only 37% of all farmers would accept their use. Our results highlight that farmer' preferences for the adoption of measures can constrain the use of the few available effective mitigation options. Better knowledge dissemination and advisory services are crucial to address this challenge; farmers may be motivated to remove aboveground crop residues by highlighting the proportionally more important contribution of belowground residues to C sequestration, and that aboveground residues may have commercial value (biorefining, biogas, biofuel), although these options need further development.

Linking meta-omics to the kinetics of denitrification intermediates reveals pH-dependent causes of N2O emissions and nitrite accumulation in soil

Soil pH is a key controller of denitrification. We analysed the metagenomics/transcriptomics and phenomics of two soils from a long-term liming experiment, SoilN (pH 6.8) and un-limed SoilA (pH 3.8). SoilA had severely delayed N₂O reduction despite early transcription of nosZ (mainly clade I), encoding N₂O reductase, by diverse denitrifiers. This shows that post-transcriptionally hampered maturation of the NosZ apo-protein at low pH is a generic phenomenon. Identification of transcript reads of several accessory genes in the nos cluster indicated that enzymes for NosZ maturation were present across a range of organisms, eliminating their absence as an explanation for the failure to produce a functional enzyme. nir transcript abundances (for NO₂⁻ reductase) in SoilA suggest that low NO₂⁻ concentrations in acidic soils, often ascribed to abiotic degradation, are primarily due to biological activity. The accumulation of NO₂⁻ in neutral soil was ascribed to high nar expression (nitrate reductase). The -omics results revealed dominance of nirK over nirS in both soils while qPCR showed the opposite, demonstrating that standard primer pairs only capture a fraction of the nirK pool. qnor encoding NO reductase was strongly expressed in SoilA, implying an important role in controlling NO. Production of HONO, for which some studies claim higher, others lower, emissions from NO₂⁻ accumulating soil, was estimated to be ten times higher from SoilA than from SoilN. The study extends our understanding of denitrification-driven gas emissions and the diversity of bacteria involved and demonstrates that gene and transcript quantifications cannot always reliably predict community phenotypes.

Exploring Temperature-Related Effects in Catch Crop Net N Mineralization Outside of First-Order Kinetics

Catch crops are an effective method for reducing nitrogen (N) leaching in agriculture, but the mineralization of incorporated catch crop residue N is difficult to predict and model. We conducted a five-month incubation experiment using fresh residue from three catch crops (hairy vetch, fodder radish and ryegrass) with three temperature treatments (2 °C, 15 °C and 2–15 °C variable temperature) and two termination methods (glyphosate and untreated). Mineral N (ammonium and nitrate) in soil was quantified at 0, 1, 2, 4, 8 and 20 weeks of incubation. Ammonium accumulation from residue decomposition showed a lag at low and variable temperature, but subsequent nitrification of the ammonium did not. Mineral N accumulation over time changed from exponential to sigmoidal mode at low and variable temperature. Incubation temperature significantly affected mineralization rates in a first-order kinetics (FOK) model, while plant type and termination method did not. Plant type alone had a significant effect on the final mineralized fraction of added catch crop N. FOK models modified to accommodate an initial lag were fitted to the incubation results and produced better goodness-of-fit statistics than simple FOK. We suggest that initial lags in residue decomposition should be investigated for the benefit of mineralization predictions in cropping models.

Nitrous oxide emissions from oilseed rape cultivation were unaffected by flash pyrolysis biochar of different type, rate and field ageing

Nitrous oxide (N₂O) emission from winter oilseed rape (WOSR) cultivation may compromise the sustainability of oilseed rape biodiesel. Typically, greenhouse gas budgets of WOSR cultivation assume an N₂O emission factor (EF) of 1% of the N added in fertilizer and crop residues. Management options to reduce direct soil emissions of N₂O include the application of biochar, but efficacy and mechanisms of N₂O suppression are elusive. We measured N₂O emissions in a WOSR field trial on a sandy loam soil in Denmark over 402 days in 2017-2018, comparing biochar applications from two feedstocks (wheat straw and pig manure fibers), two application rates (1.5 and 15 Mg ha^-1) and field ageing of up to three years. Further, a controlled incubation experiment was performed to examine the effect of biochar dose and ageing on N₂O production and consumption by denitrification. Biochar treatments had no significant effects on cumulative N₂O emissions (1.71-2.78 kg N ha^-1 yr^-1). Likewise, no significant effects were found on crop yield, yield-scaled N₂O emission, soil mineral N content, gravimetric soil moisture or pH. The fertilizer induced EF was 0.51% which is well below the IPCC Tier 1 EF of 1%. High doses of fresh, but not field-aged biochar suppressed N₂O production under anoxic conditions ex situ, suggesting that biochar with sufficient liming capacity could mitigate N₂O emissions from denitrification also under field conditions. Yet, rates of up to 15 Mg ha^-1 flash pyrolysis biochar in the current in situ study, which comprised a pronounced summer drought, showed no significant N₂O mitigation. This highlights the need for selecting dedicated biochars and doses and test them in multi-year studies to conclude on their N₂O mitigating effect. Yet, in relation to sustainability of WOSR cultivation for biodiesel, the current study suggests that C sequestration by biochar is not compromised by increased N₂O emissions.

Nitrate leaching and nitrous oxide emissions from maize after grass-clover on a coarse sandy soil: Mitigation potentials of 3,4-dimethylpyrazole phosphate (DMPP)

Cropping of maize (Zea mays L.) on sandy soil in wet climates involves a significant risk for nitrogen (N) losses, since nitrate added in fertilizers or produced from residues and manure may be lost outside the period with active crop N uptake. This one-year lysimeter experiment investigated the potential of Vizura®, a formulation for liquid manure (slurry) with the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP), to mitigate nitrous oxide (N₂O) emissions and nitrate (NO₃^-) leaching from a coarse sandy soil cropped with maize. Maize followed grass-clover (Lolium perenne L.-Trifolium pratense L.) with spring incorporation and was fertilised with cattle slurry. A total of 12 treatments in triplicate were included in a factorial experiment with 1 m² large and 1.4 m deep lysimeters: 1) with or without spraying the above-ground biomass of grass-clover with DMPP before incorporation; 2) application of cattle manure with or without DMPP, or no fertilization; and 3) natural rainfall or extra rain events to represent wet spring conditions, which were simulated with an automated and programmable irrigation system. Around 20 kg N ha^-1 was returned to the soil in grass-clover above-ground biomass, and 145 kg N ha^-1 in cattle manure. Cumulative annual N₂O emissions ranged from 0.4 to 1.3 kg N ha^-1, with between 49 and 86% of emissions occurring during spring. Manure application increased N₂O emissions, while extra rainfall had no effect. The mitigation of N₂O emissions by DMPP ranged from 46 to 67% under natural, and from 44 to 48% under high rainfall conditions. Total annual NO₃^- leaching ranged from 65 to 162 kg N ha^-1. The extent of NO₃^- leaching to 1.4 m depth during spring was low, and instead most (72-83%) of total annual NO₃^--N leaching was recorded during autumn before harvest. The extra rainfall during spring increased NO₃^--N leaching in the pre-harvest period, but it is not clear to what extent this was associated with the N in grass-clover residues or manure applied in spring, or from N mineralisation below the root zone. Despite evidence for a reduction of NO₃^- leaching in three of four scenarios, overall this effect was not significant. No DMPP was detected in leachates. In conclusion, DMPP significantly reduced N₂O emissions from cattle manure on this sandy loam soil independent of rainfall, while there was no significant effect on NO₃^- leaching. The results indicate that N₂O emissions and NO₃^--N leaching were partly derived from below-ground sources of N not affected by DMPP, which should be further investigated to better predict the mitigation potential of nitrification inhibitors.

Valorization of agricultural wastes could improve soil fertility and mitigate soil direct N2O emissions

The emerging need for sustainable management of the increasing quantities of urban and industrial organic wastes creates opportunities for the development of alternative strategies for the improvement of degraded soils. The current study was performed to examine the effects of agricultural wastes application on soil bacterial community as well as CO₂ and N₂O direct gas emissions. Untreated soils were compared with soils, which received the same amount of N (100 μg/g soil) in the form of ammonium nitrate and organic agricultural waste. In particular, soils were incubated with three different organic agricultural wastes, orange (OP), mandarin (MP) and banana peels (BP) and ammonium nitrate (F) after adjusting soil water at 70% of its holding capacity. In the current study, soil chemical characteristics, quantitative PCR of denitrifiers (nirK, nirS, nosZI and nosZII) and16s rRNA amplicon sequencing were assessed to examine the links between the soil microbial communities and short-term soil direct N₂O emissions when treated with agricultural wastes. The highest soil direct N₂O emissions were recorded in soils received ammonium nitrate while soils received agricultural wastes exhibited substantially lower soil direct N₂O emissions. On the contrary, agricultural wastes stimulated CO₂ accumulation as well as the growth of copiotrophic bacterial groups like Proteobacteria and Firmicutes. Interestingly, direct soil N₂O emissions were decoupled from the density of denitrifier community while agricultural wastes caused a substantial reduction of the relative abundance of bacterial taxa associated with N₂O emissions in the soil. This study proves evidence that agricultural wastes could be integrated in a waste management strategy, which inter alia includes their direct use in agricultural ecosystems resulting in reduced N₂O emissions.

The effects of different soil nutrient management schemes in nitrogen cycling

It is imperative for sustainable agriculture to explore practices and inputs creating low N₂O emission capacity without reducing the productivity of the agricultural system. To evaluate different nutrient management schemes, a microcosm study was conducted to assess the direct N₂O emission from soil. Four different treatments were used to provide a preliminary assessment of N₂O emissions, as well as the concentrations of nitrates (NO₃^-) and ammonium (NH₄⁺) produced in soil: compost (derived from green plant residues), chickpea residues (green manure) in two different N concentrations (2.6% and 5.5%, respectively) and ammonium nitrate (fertilizer). The soil was thoroughly mixed with the organic amendments and ammonium nitrate and incubated for 31 days. The emissions of N₂O were higher in green manure with high-N content, as a source of nitrogen in the soil, and were similar to the emissions measured from the chemically fertilized soil. In particular, chickpea residues, with high-N content, exhibited cumulative N₂O emissions, equal to 266.17 μg N/m², whereas in fertilized soil the emissions were 267.10 μg N/m². On the contrary, the incorporation of chickpea plant residues with low-N content can be an efficient way to minimize the N₂O emissions at 21.63 μg N/m². The emissions of N₂O when compost was applied, remained relatively low, equal to 5.47 μg N/m², and in comparison to soil without any treatment. Overall, a positive association between NH₄⁺, NO₃^- in soil and N₂O emissions were observed. However, this response was treatment depended, and the significant positive correlation between NH₄⁺ and N₂O emissions were noticed in soils treated with ammonium nitrate, chickpea residues with low N content, as well as untreated controls. On the contrary, the positive correlation observed between NO₃^- and N₂O emissions in soils receiving compost and high N chickpea residues, suggest that the different treatments are differentially affecting the processes that are contributing to N₂O emissions in agricultural soils. These findings, emphasize that the different nutrient management schemes are differentially affecting the main process contributing to N₂O emissions in agricultural soils.