Ranking factors affecting emissions of GHG from incubated agricultural soils

Greenhouse gas emissions and C costs of N release associated with cover crop decomposition are plant specific and depend on soil moisture: A microcosm study

Cover cropping is used to improve soil quality and increase N inputs in agricultural systems, but it also may enhance greenhouse gases (GHG) emissions. Here, a 47-d incubation study was conducted to track the decomposition process and evaluate GHG emissions and its drivers and to calculate the C costs of residue-derived N released following the addition of residues from cover crops (pigeon pea, cowpea, lablab bean, vetch, and black oat) and maize under two water-filled pore space (WFPS) levels (40 and 70%). For both WFPS levels, the increase in cumulative CO₂ fluxes in plots that received residues is mainly related with the increment of potentially mineralizable C. Crop residues increased the global warming potential (GWP) under both WFPS levels, with CO₂ emissions accounting for ≥98% of the GWP at 40% WFPS. At 70% WFPS, the GPW increment was driven by a notable increase in N₂ O emissions. The contribution of CH₄ in the GWP emissions was negligible for all the crop residues evaluated. Principal component analysis highlighted that the optimal conditions for production and release are specific for each GHG. The cleaner N source was cowpea at 40% WFPS, which produced only 17.7 kg CO₂ -eq kg^-1 N mineralized, compared with vetch residues, which produced 233 kg CO₂ -eq kg^-1 N mineralized. To integrate agronomic and climate change mitigation perspectives, we suggest considering the C costs of the residue-N released when choosing a cover crop.

Apparent Nitrogen Recovery in Milk and Early Dry Season Nitrous Oxide Emission Factors for Urine Deposited by Dual-Purpose Cattle on Different Soil Types

Pasture conditions influence the nutrients use efficiency and nitrogen (N) losses from deposited excreta. Part of the N is lost as nitrous oxide (N2O), a potent greenhouse gas. The objective of this study was to characterize apparent N recovery in milk of dual-purpose cattle and to quantify N2O emissions from the urine they deposit following grazing on Megathyrsus maximus cv. Mombasa. The N content in the grass and the milk produced by the cattle and the milk urea N (MUN) content were quantified in two contrasting regions of Colombia (Casanare and Atlántico). Dry matter intake (DMI) by the cattle was estimated using the Cornell Net Carbohydrate and Protein System. We used a closed static chamber technique to measure N2O emissions from soils in areas with and without urine patches (21 days in Atlántico and 35 Days in Casanare). Estimated DMI values were 11.5 and 11.6 kg DM day−1, milk production was 6.5 and 5.9 L day−1, apparent N recovery in milk was 24 and 23%, and the MUN content was 4.4 and 17.2 mg N dl−1 in Casanare and Atlántico, respectively. N applied to soil in the form of urine corresponded at rates of 20 and 64 g N m−2 and net cumulative N2O emissions were 350 and 20 mg N2O-N m−2 in Casanare and Atlántico, respectively. Despite low digestibility of offered diet, N recovery in milk was above the values reported at dairy cattle in tropical conditions. High urine-N inputs at Atlántico site did not result in high N2O emissions suggesting that the default Tier 1 emission factor (EF) which is based on N inputs would have overestimated urine-based N2O emissions in Atlántico. Comparing previous studies conducted in Colombia, we observed inter-regional differences by urine-based N2O emissions. This observation suggests that to increase certainty in estimating urine-based N2O emissions, Colombia needs to move toward more region-specific Tier 2 EF and reduce its dependence on the default IPCC Tier 1 EF. In addition, the adoption of Tier 2 EF in the cattle sector will facilitate accounting for the effect of animal diets on N2O inventories.

Reduction of Nitrogen Fertilizer Requirements and Nitrous Oxide Emissions Using Legume Cover Crops in a No-Tillage Sorghum Production System

Nitrous oxide (N2O) emission from denitrification in agricultural soils often increases with nitrogen (N) fertilizer and soil nitrate (NO3−) concentrations. Our hypothesis is that legume cover crops can improve efficiency of N fertilizer and can decrease N2O emissions compared to non–cover crop systems. The objectives of this study were to (a) evaluate the performance of summer leguminous cover crops in terms of N uptake and carbon (C) accumulation following winter wheat and (b) to quantify the effects of summer leguminous cover crops and N fertilizer rates on N2O emissions and grain yield of the subsequent grain sorghum crop. Field experiments were conducted in the context of a wheat-sorghum rotation for two seasons in Kansas. Treatments consisted of double-cropped leguminous cover crops following winter wheat harvest with no fertilizer applied to the following grain sorghum or no cover crop after wheat harvest and N fertilizer rates applied to the grain sorghum. The cover crops were cowpea (Vigna unguiculata L. Walp.), pigeon pea (Cajanus cajan L. Millsp.), and sunn hemp (Crotalaria juncea L.). The three N treatments (were 0, 90, and 180 kg·N·ha−1). Fallow systems with 90 and 180 kg·N·ha−1 produced significantly greater N2O emissions compared with cropping systems that received no N fertilizer. Emissions of N2O were similar for various cover crops and fallow systems with 0 kg·N·ha−1. Among cover crops, pigeon pea and cowpea had greater C accumulation and N uptake than sunn hemp. Grain yield of sorghum following different cover crops was similar and significantly higher than fallow systems with 0 kg·N·ha−1. Although fallow systems with 90 and 180 kg·N·ha−1 produced maximum sorghum grain yields, N2O emissions per unit of grain yield decreased as the amount of N fertilizer was reduced. We conclude that including leguminous cover crops can decrease N fertilizer requirements for a subsequent sorghum crop, potentially reducing N2O emissions per unit grain yield and providing options for adaptation to and mitigation of climate change.

Multi-objective optimization as a tool to identify possibilities for future agricultural landscapes

Agricultural landscapes provide many functions simultaneously including food production, regulation of water and regulation of greenhouse gases. Thus, it is challenging to make land management decisions, particularly transformative changes, that improve on one function without unintended consequences for other functions. To make informed decisions the trade-offs between different landscape functions must be considered. Here, we use a multi-objective optimization algorithm with a model of crop production that also simulates environmental effects such as nitrous oxide emissions to identify trade-off frontiers and associated possibilities for agricultural management. Trade-offs are identified in three soil types, using wheat production in the UK as an example, then the trade-off for combined management of the three soils is considered. The optimization algorithm identifies trade-offs between different objectives and allows them to be visualised. For example, we observed a highly non-linear trade-off between wheat yield and nitrous oxide emissions, illustrating where small changes might have a large impact. We used a cluster analysis to identify distinct management strategies with similar management actions and use these clusters to link the trade-off curves to possibilities for management. There were more possible strategies for achieving desirable environmental outcomes and remaining profitable when the management of different soil types was considered together. Interestingly, it was on the soil capable of the highest potential profit that lower profit strategies were identified as useful for combined management. Meanwhile, to maintain average profitability across the soils, it was necessary to maximise the profit from the soil with the lowest potential profit. These results are somewhat counterintuitive and so the range of strategies supplied by the model could be used to stimulate discussion amongst stakeholders. In particular, as some key objectives can be met in different ways, stakeholders could discuss the impact of these management strategies on other objectives not quantified by the model.

Effect of N dose on soil GHG emissions from a drip-fertigated olive (Olea europaea L.) orchard

Agronomic practices may mitigate greenhouse gas emissions (GHG) from crops. Appropriate nitrogen (N) and irrigation management provide the potential to reduce nitrous oxide (N₂O) and methane (CH₄) emissions. However, there is little information about the combination of both practices on the GHG emissions from olive orchards. This four-year study was conducted to qualitatively compare the effect of N doses applied through two drip irrigation strategies on N₂O and CH₄ emissions in a super-intensive (1010 trees ha^-1) olive orchard. The design (randomised blocks) was asymmetric: 0, 50 and 100 kg N ha^-1 yr^-1 were tested with full irrigation (FI; 2013 to 2016), but only 0 and 50 kg N ha^-1 yr^-1 were tested with regulated deficit irrigation (RDI; 2014 to 2016). The study shows that the soil acted as a main sink of N₂O and CH₄, regardless of the soil water content. Methane oxidation increased with N dose in the FI strategy (significant in 2013 and 2015). Overall, there was a tendency of yield to increase with the N dose without increasing emissions and without depending of the irrigation strategy. However, these results were not significant. Further confirmation of this tendency is necessary; particularly comparing FI + N100 (most promising treatment in terms of profitability) with the RDI + N100 (not available in this study) water-saving strategy.

Nitrogen use efficiency and nitrous oxide emissions from five UK fertilised grasslands

Intensification of grasslands is necessary to meet the increasing demand of livestock products. The application of nitrogen (N) on grasslands affects the N balance therefore the nitrogen use efficiency (NUE). Emissions of nitrous oxide (N₂O) are produced due to N fertilisation and low NUE. These emissions depend on the type and rates of N applied. In this study we have compiled data from 5 UK N fertilised grassland sites (Crichton, Drayton, North Wyke, Hillsborough and Pwllpeiran) covering a range of soil types and climates. The experiments evaluated the effect of increasing rates of inorganic N fertiliser provided as ammonium nitrate (AN) or calcium ammonium nitrate (CAN). The following fertiliser strategies were also explored for a rate of 320 kg N ha^-1: using the nitrification inhibitor dicyandiamide (DCD), changing to urea as an N source and splitting fertiliser applications. We measured N₂O emissions for a full year in each experiment, as well as soil mineral N, climate data, pasture yield and N offtake. N₂O emissions were greater at Crichton and North Wyke whereas Drayton, Hillsborough and Pwllpeiran had the smallest emissions. The resulting average emission factor (EF) of 1.12% total N applied showed a range of values for all the sites between 0.6 and 2.08%. NUE depended on the site and for an application rate of 320 kg N ha^-1, N surplus was on average higher than 80 kg N ha^-1, which is proposed as a maximum by the EU Nitrogen Expert Panel. N₂O emissions tended to be lower when urea was applied instead of AN or CAN, and were particularly reduced when using urea with DCD. Finally, correlations between the factors studied showed that total N input was related to Nofftake and Nexcess; while cumulative emissions and EF were related to yield scaled emissions.

N2O emission contributions by different pathways and associated microbial community dynamics in a typical calcareous vegetable soil

Nitrous oxide, one of the powerful long-lived greenhouse gases, is emitted mainly through biological processes, especially from fertilized soil. It is critical to partition the contribution of different pathways to N₂O emissions and the relevant characteristics of microbial communities to identify the key N₂O processes. An microcosm was conducted to partition the N₂O emissions from different pathways, and the changes in soil mineral nitrogen and various nitrifiers (amoA bacteria and amoA archaea) and denitrifiers (nirS, nirK, and nosZ) were also determined using qPCR and high-throughput sequencing methods. Different gas inhibitor combinations (i.e., 0.06% acetylene, pure oxygen, 0.06% acetylene in pure oxygen, and pure helium) were used to partition the N₂O pathways. A 5% oxygen treatment, with and without acetylene, was also included so that the N₂O emissions could be measured under lower oxygen partial pressure. Results showed that ammonia-oxidation (AO) and successive nitrifier denitrification (NiD) were the main pathways contributing to N₂O emissions at the earlier period after ammonium sulfate application with the cumulative N₂O emissions accounting for 30.9% and 59.2% of the total N₂O emissions, respectively. The higher NiD N₂O contributions occurred when the soil nitrite concentration appeared higher, especially under the lower oxygen conditions. Higher N₂O emissions from AO and NiD were associated with the compositional proportion of some dominant AOB species. Denitrification contributed more N₂O (63.6%-69.3%) in the later period during incubation, coinciding with the following characteristics for denitrifiers: a) lower nosZ/(nirS + nirK) ratio, b) more diversity in nirS, and c) different proportions of some dominant species in nirK. Our results demonstrated that higher AO and successive NiD were the main N₂O emission pathways, suggesting that controlling the ammonium content and weakening the AO are critical in decreasing N₂O emissions.