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We combine field observations, microcosm, stoichiometry, and molecular and stable isotope techniques to quantify N2O generation processes in an intensively managed low carbon calcareous fluvo-aquic soil. All the evidence points to ammonia oxidation and linked nitrifier denitrification (ND) being the major processes generating N2O. When NH4+-based fertilizers are applied the soil will produce high N2O peaks which are inhibited almost completely by adding nitrification inhibitors. During ammonia oxidation with high NH4+ concentrations (>80 mg N kg−1) the soil matrix will actively consume oxygen and accumulate high concentrations of NO2−, leading to suboxic conditions inducing ND. Calculated N2O isotopomer data show that nitrification and ND accounted for 35–53% and 44–58% of total N2O emissions, respectively. We propose that slowing down nitrification and avoiding high ammonium concentrations in the soil matrix are important measures to reduce N2O emissions per unit of NH4+-based N input from this type of intensively managed soil globally.

Abstract. The effects of nitrogen and straw management on global warming potential (GWP) and greenhouse gas intensity (GHGI) in a winter wheat-summer maize doublecropping system on the North China Plain were investigated. We measured nitrous oxide (N 2 O) emissions and studied net GWP (NGWP) and GHGI by calculating the net exchange of CO 2 equivalent (CO 2 -eq) from greenhouse gas emissions, agricultural inputs and management practices, as well as changes in soil organic carbon (SOC), based on a long-term field experiment established in 2006. The field experiment includes six treatments with three fertilizer N levels (zero N (control), optimum and conventional N) and straw removal (i.e. N 0 , N opt and N con ) or return (i.e. SN 0 , SN opt and SN con ). Optimum N management (N opt , SN opt ) saved roughly half of the fertilizer N compared to conventional agricultural practice (N con , SN con ), with no significant effect on grain yields. Annual mean N 2 O emissions reached 3.90 kg N 2 O-N ha −1 in N con and SN con , and N 2 O emissions were reduced by 46.9 % by optimizing N management of N opt and SN opt . Straw return increased annual mean N 2 O emissions by 27.9 %. Annual SOC sequestration was 0.40-1.44 Mg C ha −1 yr −1 in plots with N application and/or straw return. Compared to the conventional N treatments the optimum N treatments reduced NGWP by 51 %, comprising 25 % from decreasing N 2 O emissions and 75 % from reducing N fertilizer application rates. Straw return treatments reduced NGWP by 30 % compared to no straw return because the GWP from increments of SOC offset the GWP from higher emissions of N 2 O, N fertilizer and fuel after straw return. The GHGI trends from the different nitrogen and straw management practices were similar to the NGWP. In conclusion, optimum N and straw return significantly reduced NGWP and GHGI and concomitantly achieved relatively high grain yields in this important winter wheat-summer maize double-cropping system.

Soil carbon sequestration is being considered as a potential pathway to mitigate climate change. Cropland soils could provide a sink for carbon that can be modified by farming practices; however, they can also act as a source of greenhouse gases (GHG), including not only nitrous oxide (N O) and methane (CH ), but also the upstream carbon dioxide (CO ) emissions associated with agronomic management. These latter emissions are also sometimes termed "hidden" or "embedded" CO . In this paper, we estimated the net GHG balance for Chinese cropping systems by considering the balance of soil carbon sequestration, N O and CH emissions, and the upstream CO emissions of agronomic management from a life cycle perspective during 2000-2017. Results showed that although soil organic carbon (SOC) increased by 23.2 ± 8.6 Tg C per year, the soil N O and CH emissions plus upstream CO emissions arising from agronomic management added 269.5 ± 21.1 Tg C-eq per year to the atmosphere. These findings demonstrate that Chinese cropping systems are a net source of GHG emissions and that total GHG emissions are about 12 times larger than carbon uptake by soil sequestration. There were large variations between different cropping systems in the net GHG balance ranging from 328 to 7,567 kg C-eq ha year , but all systems act as a net GHG source to the atmosphere. The main sources of total GHG emissions are nitrogen fertilization (emissions during production and application), power use for irrigation, and soil N O and CH emissions. Optimizing agronomic management practices, especially fertilization, irrigation, plastic mulching, and crop residues to reduce total GHG emissions from the whole chain is urgently required in order to develop a low-carbon future for Chinese crop production.

Nitrate leaching is one of the most important pathways of nitrogen (N) loss which leads to groundwater contamination or surface water eutrophication. Clarifying the rates, controlling factors and characteristics of nitrate leaching is the pre-requisite for proposing effective mitigation strategies. We investigated the effects of interactions among chemical N fertilizer, straw and manure applications on nitrogen leaching in an intensively managed calcareous Fluvo-aquic soil with winter wheat-summer maize cropping rotations on the North China Plain from October 2010 to September 2013 using ceramic suction cups and seepage water calculations based on a long-term field experiment. Annual nitrate leaching reached 38–60 kg N ha−1 from conventional N managements, but declined by 32–71% due to optimum N, compost manure or municipal waste treatments, respectively. Nitrate leaching concentrated in the summer maize season, and fewer leaching events with high amounts are the characteristics of nitrate leaching in this region. Overuse of chemical N fertilizers, high net mineralization and nitrification, together with predominance of rainfall in the summer season with light soil texture are the main controlling factors responsible for the high nitrate leaching loss in this soil-crop-climatic system.