Dynamic changes of soil NO3--N as affected by different N fertilizer application and weed treatments in semi-humid winter wheat field

Application of nitrogen (N) fertilizer is one of the most important measures to increase grain yield and improve grain quality of winter wheat. However, it is common that excessive N fertilizer is applied in fertile field, which results in higher amount of nitrate (NO3--N) accumulation in the soil, bringing potential risks to the environment. Nitrogen (N) application strategies were evaluated to determine the extent to which these treatments could reduce nitrate (NO3--N) leaching. With normal density treatments, six fertilizer treatments (P0N0, PN0, PN45, PN90, PN135 and PN180) were applied, and with lower density treatments, four levels of fertilizer P0N0, PN0, PN135 and P0N135 were applied. Within each of these treatment combinations, different weed treatments were imposed. This experiment was carried out in Eum-Orthic Anthrosol located in the semi-humid farmland ecological system. Soil samples of 0-100cm layer were collected during different growth stages of winter wheat to study the dynamic changes of NO3--N as affected by different fertilizer and weed treatments in semi-humid winter wheat（Triticum aestivum）field. The results showed that NO3--N accumulation in soil profile remarkably increased with N application rate, and NO3--N accumulation of 0-100cm soil profile was linearly related with N rate. The correlation coefficients at over-wintering, regreening and jointing stage were 0.995, 0.971 and 0.949, respectively. 2) During the whole growth stage of winter wheat, soil NO3--N content decreased first, and then increased. NO3--N content at jointing stage was the lowest. NO3--N resulted from organic nitrogen mineralization at maturity was a major contributor to soil residual NO3--N after harvesting. 3) During over-winter, regreening, jointing stage and after harvest, NO3--N of P application (PN135) was reduced by N 61.4(P0.05), 26.9(P0.05), 36.6(P0.05) and 5.5 kg/hm2, compared with P0N135. 4) In different weed treatments of low level N (PN45, PN90), NO3--N accumulation was not significantly different, but there was significant difference at high level N (PN180) (P0.05) among different weed treatments. After removing weeds in the over-winter stage, the NO3--N content of PN135 was remarkably higher than other weed treatments at jointing stage.