Effects of Straw Mulching and Nitrogen Reduction on the Distribution of Soil Nitrogen and Groundwater Nitrogen Pollution

Abstract

To explore the effects of straw mulching and reduced nitrogen fertilization on the temporal and spatial patterns of soil nitrogen, groundwater nitrogen pollution, and summer maize yield, field experiments were carried out in the Hetao irrigation district in 2017 and 2018. The experiment involved the following seven treatments:a control (CK) treatment involving conventional fertilization and traditional tillage, and conventional nitrogen applications reduced by 30% (N1), 20% (N2), and 10% (N3) coupled with either straw surface covering (B) or deep straw burial (S). The results showed that the distribution of soil nitrogen in the CK treatment varied depending on soil depth, with an overall decreasing trend. In the 0-20 cm soil layer under straw surface covering (B) treatments, soil nitrogen was superficially accumulated. NO3--N and NH4+-N content increased by an average of 22.2% and 42.7% compared to the CK treatment, respectively, which decreased significantly at first and then increased slightly with depth. In the 20-40 cm deep soil layer under the deep straw burial (S) treatments, soil nitrogen accumulated and the content of NO3--N and NH4+-N increased by an average of 29.8% and 48.1%, respectively, compared to the CK treatment. Nitrogen accumulation first and then decreased significantly with depth. Nitrogen accumulation under the different straw mulching regimes increased with an increase in the application of reduced nitrogen. After the harvest of summer maize, the accumulation of NO3--N and NH4+-N in the >80 cm soil layer under the B treatments was 19.9%-58.2% and 31.1%-61.7% lower than that of the CK treatment, respectively. This compared to reductions of 36.7%-70.9% and 82.6%-89.2% for the S treatments, respectively. Only the BN3 treatment increased accumulation compared with CK by 0.4% on average, while the SN2 treatment resulted in a 9.3% increase. Summer maize yield and relative indexes were also improved relative to the other treatments. Nonlinear fitting of yield and application reduction showed that deep straw burial was better than surface covering at increasing summer maize production. The effect of deep straw burial and 14%-20% application reduction was better. Straw mulching with reduced nitrogen fertilization can limit nitrogen leaching and thereby reduce the risk of groundwater pollution. After the harvest, groundwater quality was classified in the Ⅱ class, with the risk of nitrogen contamination being lowest under deep straw burial with>20% reduced nitrogen fertilization. These observations show that deep straw deep alongside 14%-20% application reduction could effectively alleviate nitrogen leaching and reduce the risk of nitrogen pollution in groundwater. This approach can help improve the ecological environment and summer maize yields in the Hetao irrigation district.