Crop management options to reduce nitrogen pollution in Liangzihu lake basin, Central China

Abstract

In Central China, high mineral nitrogen (N) application rates lead to low N recovery and high N losses. Large amounts of the nitrate-N leached from agricultural soil end up in aquatic ecosystems, which negatively affects both ecosystem and human health. Such effects are particularly pronounced in the Liangzihu Lake basin, Central China, where application of mineral N to the predominating maize-wheat rotation systems on coarse-textured soils can exceed 300 kg ha^-1. We hypothesize that improved crop management can reduce the current nitrate-N pollution while enhancing system performance. The present study initially identified the main drivers of excessive N use by household surveys. Subsequent field experiments between 2012 and 2013 evaluated the effects of modified fertilizer-N management and the use of N-catching cover crops on soil-N dynamics, N-use efficiency, yield of maize and wheat, and nitrate-N leaching. Finally, the field trial data were used to parameterize the Environmental Policy Integrated Climate (EPIC) model to estimate N leaching losses under current and alternative crop and N management.<br /> Current N application rates average 229 kg N ha^-1 season^-1, which is higher than the cereal crop requirements of 150-180 kg N ha^-1. The main reasons for the excessive use of mineral N are related to low farmland productivity (r = -0.184, p = 0.003), small farm size (r = -0.168, p = 0.006), a high share of off-farm income (coefficient = 25.94, p = 0.003), and a low education level of the household head (coefficient = -11.20, p = 0.034).<br /> The field experiment could show that cultivating a cover crop combined with a reduced application rate (290 kg N ha^-1 in 3 splits) and multiple splitting of mineral N fertilizer can achieve similar yields (6.4-6.9 Mg ha^-1) to those obtained with current management (470 kg N ha^-1 in 2 splits). In addition, this alternative crop and fertilizer management increased the agronomic N-use efficiency by 7 kg grain kg^-1 N applied in both wheat and maize, and enhanced the N-fertilizer recovery by 15% in wheat and 20% in maize. In addition, nitrate-N leaching was reduced by 15 kg N ha^-1 in both the first-year maize and wheat crops.<br /> Once calibrated with the data from the field experiment, the EPIC model was able to predict crop biomass and the soil water content under moderate (long-term mean) climate conditions with a determination coefficient higher than 0.5 and a model bias of less than 3%. However, the model underestimated the soil water content in the drought seasons with a bias of >36%. Moreover, it tended to slightly overestimate nitrate-N leaching with 13-181 kg N ha^-1 for the entire experimental period and in both 1 m and 1.8 m soil depths. <br /> It is concluded that (1) the current N application rate in the study area is excessive because of insufficient awareness and the easy and low-cost availability of mineral-N fertilizers, (2) the currently high N losses from crop fields can be substantially reduced by reducing application rates and by replacing bare fallow periods with legume cover crops without negative trade-offs in crop yields, and (3) the calibrated EPIC model can be used to predict the aboveground crop biomass and the soil water content, but it tends to overestimate nitrate-N leaching. Consequently, there is a need to inform farmers about the negative effects of excessive N use, popularize alternative agronomic management options, and adapt the existing EPIC model to improve the prediction of nitrate pollution in Central China.