4.3 Implications and uncertainties
Our study showed that long-term heavy fertilization caused high nutrient surpluses in fields and nitrate accumulation in vadose zones, especially in intensive horticultural cropping regions. Fertilizer inputs account for more than 80% of total inputs in the region, while nutrient output is only 10% of inputs (Lu et al., 2016). This calls for the establishment and education effective nutrient management practices for the horticultural production regions in China. Some strategies may include using the 4R (right time, right place, right, right source, and right rate) approach to fertilization, selecting farmers or cooperatives with large planting areas for promotion and demonstration, and upgrading smallholder education. The experiences of Europe and North America on nutrient management since the 1980s indicate that comprehensive measures combining the technological, economic, and social aspects are needed to reduce nutrient surpluses and improve NUE (Billen and Garnier, 2021; Christensen et al., 2021; van Grinsven et al., 2012). Such an approach is urgently needed in agricultural production in China to curb high nutrient surpluses in soils to protect the environment.
Excessive nitrate accumulation in soil threatens groundwater quality (Ascott et al., 2016). To address the N related problems, the Nitrates Directive has been implemented across Europe, resulting in the designation of Nitrate Vulnerable Zones (NVZs) (Lutz et al., 2022). However, there is still no similar regulation in China to control N surplus in intensive agricultural regions. Although different water quality mitigation measures (such as the EU Water Framework Directive (Directive2000/60/EC)) have been implemented in the EU, it still observes nitrate levels above the regulatory threshold in several water-bodies across Europe due to the presence of N legacies (Batool et al., 2022). Therefore, appropriate measures and models combining climatic, soil and anthropogenic management factors must be developed to fully understand the potential fate of nitrate in deep soils to avoid future problems in intensive agricultural systems.
The research compared long-term nutrient surpluses and their fates in regions with different LUCs. Despite considerable efforts to provide reliable results, some uncertainties remained in this study. First, to minimize the uncertainty of our analysis, we used official statistics when calculating nutrient input and output because official statistics are widely recognized as the best available data sources in China. However, there is uncertainty in estimating nutrient budgets at the county scale due to the large number of data sources over a long period. Second, although we tried our best to select parameters used for calculation nutrient budgets from the same region and our previous studies, many parameters were still obtained mainly from literatures. Thirdly, the complexity of the landform types in this area, close to the Qinling Mountains and the Wei River, resulted in differences in accumulation at different sampling sites. Although it was impossible to sample every area, we collected as many soil samples as possible to minimize uncertainties. A more systematic study is needed to reveal the differences in nutrient accumulation at different sampling sites and the factors affecting them in the future.