Denitrification and associated soil N2O emissions due to agricultural activities in a changing climate

Human activities have accelerated global nitrogen cycling by approx. a factor of two. Also under future environmental conditions, agricultural nitrogen use is expected to remain the leading cause of reactive nitrogen (N-r) release to the environment. The main process to remove N-r from the environment is microbial denitrification. Here we summarize potential mechanisms that may affect denitrification and associated nitrous oxide (N2O) emissions in/from agricultural systems under future environmental conditions. Though changes in climate, specifically in temperature and precipitation, are likely to directly affect denitrification rates and N2O emissions, we identified several indirect mechanisms of global change that may potentially override direct effects. Among these are a) landscape scale changes of hotspots of denitrification: while the importance of non-hydromorphic upland soils for denitrification may decrease owing to limitations in soil moisture the importance of riparian areas as denitrification hotspots may further increase owing to the increased likeliness of flooding events leading to more frequent occurrences of aerobic-anaerobic cycles in riparian areas and, thus, increased denitrification, b) increased provision of labile carbon substrates via plant root exudation in the rhizosphere under elevated atmospheric carbon dioxide (CO2) concentrations, leading to increased microbial activity and higher denitrification rates in agricultural subsoils, thereby potentially reducing rates of nitrate leaching from agricultural soils and c) increased ammonia (NH3) volatilization from agricultural systems leading to increased denitrification rates and N2O emissions downwind from NH3 emission sources. Obviously, under future environmental conditions the mentioned mechanisms would further strengthen the regional disjunction of areas of N-r application from those of N-r removal by denitrlfication, thereby calling for a reappraisal of the importance of indirect emissions of N2O from agricultural N-r use. It remains unclear, to which extent climate change mitigation options such as the introduction of no-till systems or the increasing use of slow release fertilizers in conjunction with nitrification inhibitors or the adaptations of agricultural management practices to climate change such as altered timing of cultivation, choice of crop varieties and adaptation of water saving production systems may finally override direct and indirect climate change effects on denitrification and N2O emissions from agricultural systems.