Regulatory effects of soil properties on background N2O emissions from agricultural soils in China

The background nitrous oxide (N2O) emission (BNE) from agricultural soils originates from microbial nitrification and denitrification processes of soil nitrogen supplies, excluding emissions from nitrogen fertilizers applied in the current year. It is of great necessity to quantify BNE accurately at various spatial scales since BNE contributes considerably to the overall N2O emissions from croplands. Annual BNE rates across various soil/climate regions and major cropping systems of China were determined by network observations during 2002-2006 using the static chamber technique. The observations show BNE rates ranging from 0.1 to 3.67 kg N ha(-1) year(-1), with a mean of 1.35 kg N ha(-1) year(-1). Empirical functions are derived for cultivated mineral soils and describe the dependences of annual BNE rates upon soil total nitrogen (TN) content, soil organic carbon (SOC) content, bulk density (BD) and clay fraction (CF), separately or collectively. These empirical functions provide simple approaches to scale up estimated national/regional BNE inventories using available database of soil properties surveys and cropland area statistics. The national BNE of China is estimated to be 0.114-0.184 Tg (1 Tg = 10(12) g) N year(-1) in 2000, with the range being due to the use of different approaches. However, the available observations of annual BNE rates do not cover the entire range of soil properties on a national scale. Further work is needed to verify the empirical models for a complete range of soil types. In addition, a predictive empirical relationship between annual BNE rates and TN or SOC is established for cultivated mineral soil at the global scale. However, the empirical models could not accurately predict the BNE rates of cultivated organic soils. Further studies are required to understand the regulatory effects of soil properties on annual BNE rates of cultivated organic soils.