Nitrate accumulation in soil profiles under seasonally open 'sunlight greenhouses' in northwest China and potential for leaching loss during summer fallow

Production of vegetables in greenhouses, having three solid walls and heated mainly or entirely by sunlight ('sunlight greenhouses'), has expanded greatly in the northern areas of China. Excessive applications of manure and fertilizers are common, leading to nitrate accumulations in soil. We surveyed nitrogen application rates in more than 130 commercial greenhouses in Shaanxi Province, northwest China. Average application of fertilizer N was 753 and 600 kg/ha in the Yangling and Xian areas, respectively. In addition, N added in organic form in 31 greenhouses surveyed in Yangling averaged 699 kg/ha. We also surveyed nitrate in the soil profile in 70 commercial greenhouses. In 33 greenhouses in Yangling after harvest, the average NO(3)--N accumulation to 200 cm depth was 737 kg/ha; in 43 greenhouses in Xian it was 506 kg/ha to a depth of 100 cm. Vegetables are mainly grown during the winter, and in summer the plastic is removed and the soil left fallow, in part to allow accumulated salts in the soil to be leached out during a rainy period. But this procedure also leads to nitrate leaching. Nitrate loss depends on rainfall during the fallow period. In a wet year (2007), average N loss below 100 cm was estimated to be 158 kg/ha; but in a dry year (2006) nitrate accumulated in the profile, with little loss. In a wet year, summer fallow and removal of plastic is beneficial because it decreases salt accumulation in the upper soil layers. How to balance this with the loss of nitrate is a challenge for greenhouse management in the study region.