Nitrous oxide emissions from soils under traditional cropland and apple orchard in the semi-arid Loess Plateau of China

The conversions of cropland to forest, or other tree-based systems, are considered to be important processes affecting regional and global greenhouse gas budgets, especially for nitrous oxide (N2O). From April 2007 to March 2009, in the rain-fed semi-arid climate of the Loess Plateau, China, soil N2O emissions were measured using static chambers from a winter wheat field and an apple orchard, which had been established in part of the wheat field 23 years earlier. Annual average N2O emissions from the apple orchard (2.40 kg N2O ha(-1)yr(-1)) were 12.15% higher than those in the wheat field (2.14 kg N2O ha(-1)yr(-1)). Seasonal rainfall, in combination with higher nitrogen fertilization, had a promoting effect on N2O emissions in the apple orchard compared with the wheat field. The amounts and patterns of summer rainfall and winter snowfall were the principal controllers of seasonal and annual N2O fluxes in these rain-fed semi-arid regions, likely through their influence on soil moisture content. Since there may be a moisture threshold associated with summer rainfall and winter snowfall triggering higher N2O emissions, climatic regimes should be taken into account when assessing the effects of land use on N2O emissions in the Loess Plateau.