1. Introduction
Synthetic fertilizers are indispensable in feeding the rapidly increasing global population (Erisman et al., 2008) and dramatically changing the global biogeochemical cycles of nitrogen (N) and phosphorus (P) (Britz and Hertel, 2011; Steffen et al., 2015; Penuelas et al., 2020). For example, N inputs in global cropping systems increased fivefold from 1961 to 2015 (Galloway et al., 2008; Lassaletta et al., 2023). High nutrient surpluses (the differences between nutrient inputs and outputs) lead to a rapid decline in nutrient use efficiency (NUE) (Ma et al., 2012; Chadwick et al., 2015), resulting in a series of environmental problems, such as eutrophication, soil acidification, biodiversity loss, and global warming (Bouwman et al., 2009; Mu et al., 2016).
The application of mineral fertilizers increased rapidly in Europe and North America after World War II (Krausmann et al., 2013; Billen et al., 2021), which resulted in increased food production at the cost of polluting surface and groundwater (Haygarth et al., 2014; Ilampooranan et al., 2019). The adverse effects on water quality of the application of nutrients were first recognized in Western European countries (Rivett et al., 2008; Smith et al., 2017; Sabo et al., 2019; Sanchez et al., 2023). For example, 80% of European surface waters exceeded the European Commission’s drinking water standard (50 mg NO3L-1) (Molénat and Gascuel Odoux, 2002). In England and Wales, 28% of rivers exceeded the N concentration of 30 mg NO3 L-1, and 52% of the total river length exceeded the P concentration of 0.1 mg L-1(Cherry et al., 2008). To reduce nutrient surpluses and minimize environmental pollution, a series of regulations and laws have been introduced, such as the Clean Water Act in the United States (EPA, 1972), the Nitrate Directive (1991/676/EC) and the Water Framework Directive (2000/60/EC) in Europe (Grant and Blicher-Mathiesen, 2004; Zhao et al., 2022). Implementing these regulations and laws has successfully curbed net N input by reducing fertilizer usage and nutrient surpluses, increasing NUE (Lutz et al., 2022). The average gross soil N balance of the Tagus Nitrate Vulnerable Zone in Portugal decreased significantly from 125 kg ha-1 in 1989 to 63 kg ha-1 in 2016 (Cameira et al., 2019). However, implementing such mitigation regulations does not always lead to immediate or clear responses in improving surface water and groundwater quality (Tian et al., 2020; Nguyen et al., 2022). One of the reasons behind such failure is considered the legacy effect from historical nutrient accumulation in soils and aquifers. Simply managing yearly nutrient balances is unlikely to improve water quality; rather, many factors must be considered, including soil and groundwater storage capacity and gaseous loss pathways (Metson et al., 2020). Therefore, understanding the long-term trajectories of nutrient budgets in soils and their fates at the regional scales is key for managing nutrients efficiently and minimizing their negative environmental impacts.
Applying mineral fertilizer in emerging economies, such as China and India, is about two to three decades later than in developed countries (Lu and Tian, 2017; Yu et al., 2019; Bijay, 2023). As expected, the negative effects of nutrient losses from fertilizers, livestock, and domestic sources on water quality also occurred late (Zhang et al., 1995; Liu et al., 2001). It is reported that nitrate in more than half of the groundwater sampling sites in northern China Plain exceeded 50 mg L-1 in the 1990s (Zhang et al., 1996). Meanwhile, anomalously higher values of nitrate than the background in the Indian watershed (Pawar and Shaikh, 1995). In recent years, some emerging countries have taken measures to control environmental pollution caused by nitrate pollution (Xin et al., 2021). However, the mitigation regulations to control fertilizer usage and nutrient surpluses in emerging economies are still in the early stages. The population in developing countries is still high. Therefore, optimizing the beneficial role of nutrients in sustainable crop production to feed the population and minimizing environmental risks of excessive accumulation of soil nutrients is a major challenge.
Nutrient budgeting approaches is a useful and simple tool for assessing potential environmental risks from nutrient pollution by identifying primary sources of nutrient loading as a means to develop more targeted nutrient management practices (Chen et al., 2021; Kuestermann et al., 2010; Grieger and Harrison, 2021; Lim et al., 2021). Land use change (LUC) is one of the key forces driving the nutrient inputs (Zhi and Li, 2020; Suennemann et al., 2021). Dramatic LUCs have occurred across mainland China since the early 1990s (Zhang et al., 2009; Gao et al., 2021). Large areas of cereal lands have been converted into orchards because of lucrative economic benefits. Compared with cereals, the problem of excessive fertilization of orchards is particularly prominent, resulting in a large accumulation of soil nutrients (Gao et al., 2012; Zhou et al., 2016; Wang et al., 2017; Min et al., 2018). Soil nitrate storage within 6 m depth ranged from 5039-14,693 kg ha-1, 4.7-11.7-fold higher than under cereal on field scale on the Loess Plateau of China (Ren et al., 2023). The total nitrate stock in the 0-10 m soil profile of the orchards was 3.7 times higher than that of the croplands in the Guanzhong Plain (Niu et al., 2022). These studies focused on nutrient accumulation at the field scales. The nutrient accumulation at the county scale studies indicated that the nitrate accumulation in soil profiles is the main fate of N surplus at both Luochuan and Liquan counties dominated by orchards on the Loess Plateau (Zhu et al., 2021; Miao et al., 2023). However, no study has compared LUC from cereals to orchard systems at the county scale. Comparative research in different cropping systems at a regional scale will provide a clearer understanding of the impact of LUC on differences in nutrient surplus and their fates in different agricultural systems.
Therefore, we chose two neighboring counties (Wugong and Meixian) in the southern part of the Loess Plateau in Shaanxi, with different LUCs since the 1990s. The main crops in Wugong County are still cereals (winter wheat and summer maize), whereas land use in Meixian County has changed from cereals to kiwifruit orchards since the 1990s. This provides an ideal case study to compare the nutrient surpluses and their fates at the county scale between traditional cereal-dominant and horticultural-dominant counties. Our objectives were to (1) quantify the changes in the annual nutrient inputs and surpluses at the county scale, (2) determine the changes in soil surface nutrients under different land-use patterns, (3) investigate soil nitrate-N accumulation under different tree ages, and N fate under different land use patterns in two counties over the past decades.