Nitrogen leaching mitigation in fluvo-aquic soil in the North China Plain

The North China Plain is a grain production region with fluvo-aquic soil and has seen rapid agricultural development over the past four decades. Excessive fertilization and frequent irrigation have increased nitrogen (N) leaching and nonpoint source pollution since the 1990s. This study screened published nitrogen leaching data on the North China Plain grain farmlands to identify the relationship between fertilization and irrigation with N leaching and to evaluate the primary N leaching mitigation measures. The results showed that regional groundwater during the 1970s was shallow and then deeper. During the 2010s, the regional cropping system changed from one to two crops per annum, and the annual N fertilizer rapidly increased to 600 kg(N)·hm^-2·a^-1 but then slowly decreased to 500–550 kg(N)·hm^-2·a^-1. Since the 1990s, irrigation increased from zero (rainfed during the 1980s) to 150–400 mm per annum, crop straw had gradually been incorporated into farmlands, and the fertilizer synergist technology had been accepted. The soil organic matter and total N improved by 38%–47%, pH decreased by 0.5 units, and available potassium decreased slightly. Fertilization and irrigation were the main influencing factors of N leaching, and the exponential relationship between N leaching and the N fertilizer balance (N fertilizer rate - crop above-ground N uptake) was better than the relationship between N leaching and N fertilizer rate. Random forest (RF) regression modeling based on machine learning was used to determine the relationship between N leaching and impacting factors such as irrigation, soil properties, and climate; the prediction results were satisfactory. At the same rate of N fertilization, organic fertilization combined with chemical fertilization significantly decreased N leaching because the N supply and crop demand were synchronized. Fertilizer synergists, such as control-release fertilizers, ureases, and nitrification inhibitors, mitigated N leaching by 1/3 and should be used in the North China Plain. Crop straw incorporation microbially improved N fertilizer in the short term and increased the long-term soil total N stock and inorganic N buffering capacity and reducing N leaching by 10%. The no-tillage mitigation effects were low and variable among farmlands. Fallow farmland and rotation/intercropping of deep root and shallow root crops, leguminous crops with cereal crops, and grains with vegetable crops were effective at reducing N leaching, but the crop yields also reduced. Therefore, these techniques required careful examination during technical dissemination. Governmental support, technical training, and proper planning should be implemented during the 14^th Five-Year Plan of China to prevent and mitigate N pollution. Ecological compensation and an agricultural sector water use charge could also be used to encourage farmer participation.