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.