Mineral nitrogen content and accumulation in soil profiles under sunlight greenhouse cultivation

Mineral nitrogen(NO3--N and NH4+-N) content,accumulation and their distribution in 0100 cm or 0200 cm soil profiles over the growth period of tomato under the sunlight greenhouse cultivation in the suburbs of Xi'an city and Yangling,Shaanxi was studied.It was found that NO3--N content at each layer stepwise decreased with tomato growing;however,the magnitude of decrease in NO3--N varied with the depth of soil layers.NH4+-N in March was the highest compared with that in November or May.After the harvest of tomato,the residual mineral nitrogen in soil profile mainly remained in the form of NO3--N,accounting for about 80% to 90% of mineral nitrogen in the soil profiles.The distribution of residual NH4+-N in the profiles of soil was rather identical;but higher residual NO3--N in soil profiles in the upper layers was observed.The residual NO3--N in the profiles of soil under greenhouse cultivation was higher than that under open-field vegetable cultivation or fertile arable fields.It implied that reducing NO3--N leaching,optimizing application of nitrogen fertilizer during the growth period of vegetable crops,preventing the leaching of residual NO3-N in greenhouse by catch-crop after crop harvest during the rainy season need further study.