An in situ study of inorganic nitrogen flow under different fertilization treatments on a wheat–maize rotation system surrounding Nansi Lake, China

Nitrogen (N) loss to the environment through water transport has been a serious challenge to agricultural practices for decades. Although much progress has been made in developed countries, suitable strategies for mitigating agricultural N loss in developing countries, such as China, are still required. In particular, studies that comprehensively measure the effect of fertilizer strategies on N loss through runoff and leaching would be beneficial in the design of fertilizer programs that meet the needs of the dominant cropping systems in China, while protecting nearby water bodies. In this study, agricultural plots (n = 21) in the Nansi Lake Watershed of Shandong District, China, were fitted with runoff and leachate collection devices to monitor the effect of different N fertilizer treatment strategies, including OPT, CRN, DMS, and STR for a wheat–maize rotation system on N loss through water transport. Runoff and leachates were collected at 10 typical growth stages of the consecutive wheat and maize seasons throughout a 2-year period. Yield and precipitation data for each plot were also collected. One of the main findings was the significant positive correlation between precipitation levels and the quantity of leachates and runoff. In addition, the amount of water collected in the various treatment was affected by agricultural practices, such as straw incorporation and tillage of surface soils. During the different growth stages, NO3−-N and NH4+-N concentrations varied in the leachate and runoff. The NO3−-N concentration had a greater impact on water quality during the R1 period of maize. However, NO3−-N concentrations were too low to cause ground water pollution. The total loss of inorganic N to leaching and runoff was 1.68–5.96 kg ha−1 among crops. Run off accounted for 63.4–73.8% of inorganic N loss. The amount of NO3−-N and NH4+-N lost through leaching and runoff was generally greater during the maize season compared to the wheat season. During the form of nitrogen loss, the ratio of NH4+-N and NO3−-N in the leachate was generally similar (the proportion of NO3−-N during the maize and wheat season was 65.2–70.9% and 46.0–54.6% respectively). However, there was a predominance of NO3−-N in the runoff (82.5–86.4% and 94.2–96.5% for the maize and wheat seasons, respectively). The NO3−-N and NH4+-N losses through total leachate and runoff were highest in the FP treatment, followed by OPT and DMS, and finally STR and CRN. To reduce N loss through water transport, in parallel to ensuring continued high agricultural production levels, CRN and OPT in conjunction with wheat straw incorporation should be encouraged during maize production, while CRN and OPT should be encouraged during wheat production.