Significant accumulation of nitrate in Chinese semi-humid croplands

Abstract

Soil nitrate is important for crop growth, but it can also leach to groundwater causing nitrate contamination, a threat to human health. Here, we report a significant accumulation of soil nitrate in Chinese semi-humid croplands based upon more than 7000 samples from 141 sites collected from 1994 to 2015. In the 0–4 meters depth of soil, total nitrate accumulation reaches 453 ± 39, 749 ± 75, 1191 ± 89, 1269 ± 114, 2155 ± 330 kg N ha−1 on average in wheat, maize, open-field vegetables (OFV), solar plastic-roofed greenhouse vegetables (GHV) and orchard fields, respectively. Surprisingly, there is also a comparable amount of nitrate accumulated in the vadose-zone deeper than 4 meters. Over-use of N fertilizer (and/or manure) and a declining groundwater table are the major causes for this huge nitrate reservoir in the vadose-zone of semi-humid croplands, where the nitrate cannot be denitrified due to the presence of oxygen and lack of carbon sources. Future climatic change with more extreme rainfall events would increase the risk of accumulated nitrate moving downwards and threatening groundwater nitrate contamination.

Legacy Nutrient Dynamics at the Watershed Scale: Principles, Modeling, and Implications

“…Costa et al (2002) also emphasized that intensive irrigations could lead to increasing N accumulations in cropland vadose zones. Accumulation of N in vadose zone is commonly observed in arid or semiarid region with high N surplus due to low carbon content, strong mineralization and nitrification abilities, and weak immobilization and denitrification abilities (Scanlon et al, 2008;Walvoord et al, 2003;Zhou et al, 2016). In many humid regions, nitrate would further (Puckett et al, 2011;Qi et al, 2011), Europe (39% of monitoring stations exceeded 25 mg L À1 ) (Angelopoulos et al, 2009;Worrall et al, 2015), India (average concentration exceeded 50 mg L À1 in northern India) (Suthar et al, 2009), and China (28% exceeded 10 mg L À1 ) (Gu et al, 2013;Ju et al, 2004).…”

“…Denitrification requires anoxic conditions and an electron donor (such as organic carbon, sulfide, or Fe 2+ , Slomp and Van Cappellen, 2004;Starr and Gillham, 1993;Tesoriero et al, 2000). Due to the oxic environment and lack of carbon sources, denitrification is usually limited in vadose zones, with particular in semiarid regions (Zhou et al, 2016). Although groundwater DO concentrations usually decrease with groundwater age (Daughney et al, 2010), denitrification is likely limited by the lack of a microbially labile organic carbon source in many regions (Pabich et al, 2001).…”

“…The N accumulation in vadose zone and groundwater is mainly facilitated by rapid infiltration and percolation (Hamilton, 2012). Therefore, legacy nitrate magnitudes in vadose zone and groundwater would be enhanced by high N application rate, high rainfall and frequent storm events, coarse soil texture, and intensive tillage and irrigation (McMahon and B€ ohlke, 2006;Zhou et al, 2016). A long-term (1998-2012) field experiment in the North China Plain indicated nitrate accumulations in vadose zone increased by $40-fold when applied N fertilizer rates increased from 0 to 600 kg N ha À1 year À1 (Zhou et al, 2016).…”

“…Due to the high solubility and mobility as well as long residence times, dissolved N (particular for nitrate) accumulating in the vadose zone/groundwater is liable to build up as a legacy N pool derived from soil surplus N. Legacy N pool in vadose zone/groundwater is usually occurring in many highly managed lands where the recharge zone receives excessive anthropogenic N inputs (i.e., croplands and residential lands, Costa et al, 2002;Scanlon et al, 2008;Walvoord et al, 2003;Zhou et al, 2016). The N accumulation in vadose zone and groundwater is mainly facilitated by rapid infiltration and percolation (Hamilton, 2012).…”

“…Therefore, legacy nitrate magnitudes in vadose zone and groundwater would be enhanced by high N application rate, high rainfall and frequent storm events, coarse soil texture, and intensive tillage and irrigation (McMahon and B€ ohlke, 2006;Zhou et al, 2016). A long-term (1998-2012) field experiment in the North China Plain indicated nitrate accumulations in vadose zone increased by $40-fold when applied N fertilizer rates increased from 0 to 600 kg N ha À1 year À1 (Zhou et al, 2016). Costa et al (2002) also emphasized that intensive irrigations could lead to increasing N accumulations in cropland vadose zones.…”