Intensive vegetable production results in high nitrate accumulation in deep soil profiles in China☆
Graphical abstract
Introduction
The vadose zone is the Earth's terrestrial subsurface that connects the upper soil and groundwater (Holden and Fierer, 2005). The excessive and repeated input of N into agricultural land results in high nitrate accumulation in the vadose zone (Ascott et al., 2017). As one of the most important potential sources of groundwater nitrate pollution and nitrous oxide (N2O) emission, nitrate accumulation in the vadose zone has received widespread attention in recent years (Lu et al., 2019; Peterson et al., 2013).
The N cycle of the soil–plant system mainly occurs owing to the biological activity in the root zone (0–100 cm depth) (Xin et al., 2019). The surplus nitrate leaching out of the root zone is considered as nitrate leaching loss. However, the leached nitrate may accumulate in the deep vadose zone (>100 cm soil profile), especially for the land with thick vadose zone. The deep vadose zone is an important storage location for nitrate. The result of previous studies highlighted that the nitrate located in the deep vadose zone was a long-term source of groundwater pollution (Huang et al., 2018; Rashti et al., 2017; Torres-Martínez et al., 2020; Torres-Martínez et al., 2021). A case study in Mississippi River basin revealed that the legacy nitrate in vadose zone could influence water quality for decades (Van Meter et al., 2018). Therefore, it is essential to quantify and understand the behaviour of nitrate in the vadose zone (Ju et al., 2007; Wang et al., 2016).
Vegetable production is a very intensive system characterized by many rotations, high N inputs, and frequent irrigation (Scheer et al., 2014). Consequently, a build-up of nitrate in the vadose zone can be expected. For instance, Di and Cameron (2002) reported that a comparison of land use systems in temperate regions, including forests, cut grasslands, grazed pastures, arable cropland, and vegetable farms, showed that vegetable production resulted in the highest amount of nitrate accumulation and leaching losses. However, this problem may be even worse in China, where more N fertiliser and irrigation water are input owing to the dominance of small-holder farms (Han et al., 2010; Jiang et al., 2013; Zhang et al., 2017a, 2017b). In a recent study, Wang et al. (2021) reported that China's vegetable production taken up 13% of the harvesting areas and consumed 25% of the chemical fertiliser in the country, accounting for 2% of the global harvest area of crops and 8% of the chemical fertiliser, respectively. Therefore, vegetable production in China as a whole, with high N rates as well as N surplus, represented as a super-hotspot of global N losses and accumulation. Accurately predicting nitrate accumulation is very helpful to specify measures to solve this problem (Li et al., 2018a). Many studies have studied nitrate accumulation in vegetable production at the field scale (Bai et al., 2020; Gao et al., 2019; Han et al., 2010; Li et al., 2018a). However, no study has characterized the nitrate accumulation at the national scale, due to the difficulty, tediousness and expensive cost of the borehole drilling method. Ti et al. (2015) conducted a meta-analysis of characteristics of nitrogen balances in vegetable production of China, and concluded that 19–31% of the annual nitrogen inputs accumulated in soil, which were equivalent to 1.2–1.9 Tg N yr−1. However, this study did not distinguish the specific nitrate accumulation and failed to reveal the key factors affecting nitrate accumulation in the vadose zone.
Nitrate accumulation in the vadose zone is influenced by N and water inputs as well as soil properties. Excessive ammonium-based N fertiliser and manure provide substrates for nitrate accumulation in the soil profile (Chen et al., 2016). The previous studies have highlighted the significantly linear or exponential relationships between N inputs and nitrate accumulation (Gao et al., 2019; Zhou et al., 2016). Moreover, water inputs including irrigation water and precipitation is another key factor affecting nitrate accumulation. First, the N uptakes are affected by soil water status (Li et al., 2007). Second, water application significantly influences the solute-transport process and therefore influences nitrate accumulation in soil profiles (Fan et al., 2014; Hebbar et al., 2004; Liu et al., 2019). Third, soil aeration conditions depend on soil water content which controls nitrate production by nitrification and consumption by denitrification (Li et al., 2013; Smith, 2017). Similarly, soil properties can regulate nitrate accumulation. For instance, soil organic carbon regulates N availability between ammonium N immobilisation and nitrification (Trinsoutrot et al., 2000). Soil pH is another essential factor for nitrification and nitrate accumulation. The nitrification rate is relatively low in acidic soils because the availability of the substrate ammonia and the activity of ammonia nitrifiers are limited below pH 5.5 (De Boer and Kowalchuk, 2001; Li et al., 2018b). In addition, soil clay content controls the nitrate migration process and air diffusion in soil which significantly influences soil aeration conditions and nitrate consumption by denitrification (Zhu et al., 2013). Additionally, the uptake of N by vegetables is affected by other nutrition (e.g., available phosphorus). Although these factors have been individually evaluated, it is difficult to comprehensively understand soil nitrate accumulation in vegetable production, without clarifying the relative roles of the influencing factors.
To address these knowledge gaps, this study was designed to estimate the nitrate accumulation characteristics vegetable production in China. We tried to address two questions in our study: (1) What are the quantities and national patterns for nitrate accumulation? (2) How do N and water inputs and soil properties influence national nitrate accumulation?
Section snippets
Data compilation and overview
Nitrate accumulation is referred to nitrate storage in soil profile for this study. The dataset of soil nitrate accumulation was constructed by obtaining data from peer-reviewed articles. Initially, we searched for articles using terms including nitrate, nitrate accumulation, and nitrate storage in the ISI Web of Science (Thomson Reuters, New York, NY, USA), Google Scholar (Google Inc., Mountain View, CA, USA), and China National Knowledge Infrastructure database (CNKI) up to 30 April 2020.
Nitrate accumulation in soil of vegetable farms
Nitrate accumulation in soil decreased with soil depth (PG, R2 = 0.967, p < 0.001; OF, R2 = 0.467, p < 0.001; Fig. 2). The average values of nitrate accumulation in soil depths of 0–100 cm, 100–200 cm, 200–300 cm, and >300 cm for PG were 504 ± 188, 390 ± 149, 349 ± 146, and 244 ± 141 kg N ha−1, respectively (Fig. 2a). Nitrate accumulation in OF soil was lower than that of PG, with mean values of 264 ± 187, 217 ± 149, 228 ± 146, and 242 ± 111 kg N ha−1 in soil depths of 0–100 cm, 100–200 cm,
High nitrate accumulation in vegetable farms
Previous studies have highlighted significant amounts of nitrate accumulation in the vadose zone worldwide (Ascott et al., 2017). The vegetable production system is the hotspot of nitrate accumulation (Table S6). For example, Soto et al. (2015) reported that the storage of nitrate was 484–490 kg N ha−1 in 0–100 cm soil for PG tomato production in Spain. Similarly, Wang et al. (2019) revealed that nitrate accumulation was 295–837 kg N ha−1 in 0–100 cm for OF in China. A five-year study in
Conclusions
In this study, we comprehensively evaluated nitrate accumulation characteristics in vegetable systems in China. Nitrate accumulation in 0–400 cm soil was 1487 and 950 kg N ha−1 for PG and OF vegetables, accounting for 13% and 17% of accumulated N inputs, respectively. Of which, it was approximate 65–70% storage below the rooting zone (0–100 cm depth). The nitrate accumulation rates were 16–62 kg N ha−1 yr−1 and 10–26 kg N ha−1 yr−1 for PG and OF vegetables, respectively. N input rates, soil
Credit author statement
Xinlu Bai, Zhujun Chen and Jianbin Zhou designed the research. Xinlu Bai, Yun Jiang, Shaoqi Xue and Hongzhi Miao analysed the data. Xinlu Bai, Yun Jiang, Hongzhi Miao, Shaoqi Xue, Zhujun Chen, Jianbin Zhou wrote the manuscript. All authors read and approved the final manuscript.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key Research and Development Program of China (2017YFD0200106), the National Natural Science Foundation of China (41671295), and the 111 Project (No. B12007).
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This paper has been recommended for acceptance by Dr. Yong Sik Ok.