Using nitrification inhibitors to mitigate agricultural N2O emission: a double‐edged sword?

Nitrification inhibitors show promise in decreasing nitrous oxide (N₂O) emission from agricultural systems worldwide, but they may be much less effective than previously thought when both direct and indirect emissions are taken into account. Whilst nitrification inhibitors are effective at decreasing direct N₂O emission and nitrate (NO₃^–) leaching, limited studies suggest that they may increase ammonia (NH₃) volatilization and, subsequently, indirect N₂O emission. These dual effects are typically not considered when evaluating the inhibitors as a climate change mitigation tool. Here, we collate results from the literature that simultaneously examined the effects of nitrification inhibitors on N₂O and NH₃ emissions. We found that nitrification inhibitors decreased direct N₂O emission by 0.2–4.5 kg N₂O‐N ha^?1 (8–57%), but generally increased NH₃ emission by 0.2–18.7 kg NH₃‐N ha^?1 (3–65%). Taking into account the estimated indirect N₂O emission from deposited NH₃, the overall impact of nitrification inhibitors ranged from ?4.5 (reduction) to +0.5 (increase) kg N₂O‐N ha^?1. Our results suggest that the beneficial effect of nitrification inhibitors in decreasing direct N₂O emission can be undermined or even outweighed by an increase in NH₃ volatilization.     

中国土壤甲烷吸收时空格局及其驱动因素

土壤中甲烷氧化菌驱动的甲烷氧化过程是大气甲烷汇的重要途径,准确量化土壤甲烷的吸收对全球及区域甲烷收支评估具有重要意义。研究基于实测数据对土壤甲烷扩散-反应模型(R99)关键过程进行改进,利用该改进模型对我国1990-2020年不同生态系统土壤甲烷吸收进行了模拟计算,并解析其时空格局关键驱动因子。结果表明:(1)1990年至2020年全国年均土壤甲烷吸收总量约为(2.95±0.09) Tg CH₄/a,裸地、耕地、森林、草地和灌丛生态系统土壤甲烷吸收量均值分别为0.53、0.43、0.79、1.00、0.20 Tg CH₄/a,且各生态系统土壤甲烷吸收量均呈现出夏季高冬季低的季节性特征。(2)1990年至2020年间全国土壤甲烷年吸收速率南方呈现增长的趋势,北方呈现降低的趋势,全国总体上呈上升趋势,空间上南方土壤甲烷吸收速率高于北方。(3)土壤甲烷吸收速率的空间差异主要受温度、降水、潜在蒸散量和土壤含水量的影响;南方地区土壤甲烷吸收速率的增加主要受温度和潜在蒸散量增加的影响,北方地区土壤甲烷吸收速率的降低主要受降雨量和土壤含水量减少的影响。其中,青藏高原地区受土壤含水量增加和潜在蒸散量减少的影响,土壤甲烷吸收速率呈现显著增加趋势。(4)不同生态系统土壤甲烷吸收速率的主要影响因子不同,草地和裸地主要受土壤含水量影响;森林和灌丛主要受温度影响,耕地主要受耕作强度的影响。研究对我国土壤甲烷吸收及其驱动因素进行了定量评估,可以为我国甲烷收支计算提供数据与方法支撑。     

Which cropland greenhouse gas mitigation options give the greatest benefits in different world regions? Climate and soil‐specific predictions from integrated empirical models

Major sources of greenhouse gas (GHG) emissions from agricultural crop production are nitrous oxide (N₂O) emissions resulting from the application of mineral and organic fertilizer, and carbon dioxide (CO₂) emissions from soil carbon losses. Consequently, choice of fertilizer type, optimizing fertilizer application rates and timing, reducing microbial denitrification and improving soil carbon management are focus areas for mitigation. We have integrated separate models derived from global data on fertilizer‐induced soil N₂O emissions, soil nitrification inhibitors, and the effects of tillage and soil inputs of soil C stocks into a single model to determine optimal mitigation options as a function of soil type, climate, and fertilization rates. After Monte Carlo sampling of input variables, we aggregated the outputs according to climate, soil and fertilizer factors to consider the benefits of several possible emissions mitigation strategies, and identified the most beneficial option for each factor class on a per‐hectare basis. The optimal mitigation for each soil‐climate‐region was then mapped to propose geographically specific optimal GHG mitigation strategies for crops with varying N requirements. The use of empirical models reduces the requirements for validation (as they are calibrated on globally or continentally observed phenomena). However, as they are relatively simple in structure, they may not be applicable for accurate site‐specific prediction of GHG emissions. The value of this modelling approach is for initial screening and ranking of potential agricultural mitigation options and to explore the potential impact of regional agricultural GHG abatement policies. Given the clear association between management practice and crop productivity, it is essential to incorporate characterization of the yield effect on a given crop before recommending any mitigation practice.     

全球气候变化对我国西北地区农业的影响

在比较系统总结全球气候变化对我国西北地区农业影响的主要研究成果的基础上,揭示了我国西北地区现代气候变化对全球气候变暖响应的基本特征,阐述了现代气候变化对土壤水分、地表蒸发和作物气候生产力的影响规律;并且比较全面地概括了西北地区冬、春小麦、玉米、马铃薯、冬油菜、棉花、胡麻、牧草、葡萄等9种主要农作物的生长发育、病虫害、种植面积、气候产量以及畜牧业活动等对气候变化的响应特征,发现气候变化对农业生产过程的影响利弊皆存,而且不同农作物对气候变化的响应特征差异较大.研究对西北地区农业生产具有比较重要的科学指导意见.     

Can biochar reduce soil greenhouse gas emissions from a Miscanthus bioenergy crop?

Energy production from bioenergy crops may significantly reduce greenhouse gas (GHG) emissions through substitution of fossil fuels. Biochar amendment to soil may further decrease the net climate forcing of bioenergy crop production, however, this has not yet been assessed under field conditions. Significant suppression of soil nitrous oxide (N₂O) and carbon dioxide (CO₂) emissions following biochar amendment has been demonstrated in short‐term laboratory incubations by a number of authors, yet evidence from long‐term field trials has been contradictory. This study investigated whether biochar amendment could suppress soil GHG emissions under field and controlled conditions in a Miscanthus × Giganteus crop and whether suppression would be sustained during the first 2 years following amendment. In the field, biochar amendment suppressed soil CO₂ emissions by 33% and annual net soil CO₂ equivalent (eq.) emissions (CO₂, N₂O and methane, CH₄) by 37% over 2 years. In the laboratory, under controlled temperature and equalised gravimetric water content, biochar amendment suppressed soil CO₂ emissions by 53% and net soil CO₂ eq. emissions by 55%. Soil N₂O emissions were not significantly suppressed with biochar amendment, although they were generally low. Soil CH₄ fluxes were below minimum detectable limits in both experiments. These findings demonstrate that biochar amendment has the potential to suppress net soil CO₂ eq. emissions in bioenergy crop systems for up to 2 years after addition, primarily through reduced CO₂ emissions. Suppression of soil CO₂ emissions may be due to a combined effect of reduced enzymatic activity, the increased carbon‐use efficiency from the co‐location of soil microbes, soil organic matter and nutrients and the precipitation of CO₂ onto the biochar surface. We conclude that hardwood biochar has the potential to improve the GHG balance of bioenergy crops through reductions in net soil CO₂ eq. emissions.     

Immediate and long-term nitrogen oxide emissions from tropical forest soils exposed to elevated nitrogen input

Tropical nitrogen (N) deposition is projected to increase substantially within the coming decades. Increases in soil emissions of the climate‐relevant trace gases NO and N₂O are expected, but few studies address this possibility. We used N addition experiments to achieve N‐enriched conditions in contrasting montane and lowland forests and assessed changes in the timing and magnitude of soil N‐oxide emissions. We evaluated transitory effects, which occurred immediately after N addition, and long‐term effects measured at least 6 weeks after N addition. In the montane forest where stem growth was N limited, the first‐time N additions caused rapid increases in soil N‐oxide emissions. During the first 2 years of N addition, annual N‐oxide emissions were five times (transitory effect) and two times (long‐term effect) larger than controls. This contradicts the current assumption that N‐limited tropical montane forests will respond to N additions with only small and delayed increases in soil N‐oxide emissions. We attribute this fast and large response of soil N‐oxide emissions to the presence of an organic layer (a characteristic feature of this forest type) in which nitrification increased substantially following N addition. In the lowland forest where stem growth was neither N nor phosphorus (P) limited, the first‐time N additions caused only gradual and minimal increases in soil N‐oxide emissions. These first N additions were completed at the beginning of the wet season, and low soil water content may have limited nitrification. In contrast, the 9‐ and 10‐year N‐addition plots displayed instantaneous and large soil N‐oxide emissions. Annual N‐oxide emissions under chronic N addition were seven times (transitory effect) and four times (long‐term effect) larger than controls. Seasonal changes in soil water content also caused seasonal changes in soil N‐oxide emissions from the 9‐ and 10‐year N‐addition plots. This suggests that climate change scenarios, where rainfall quantity and seasonality change, will alter the relative importance of soil NO and N₂O emissions from tropical forests exposed to elevated N deposition.