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.
Highlights
Soil nitrate is important for crop growth, but it can leach to groundwater causing nitrate contamination, a threat to human health
We collected more than 7000 analyses from 141 sites reported in literature from 1994 to 2015 for the nitrate accumulation in 0–4 meters depth of the soil profile across Chinese uplands
We cataloged the data into five cropping systems, i.e. wheat, maize, open-field vegetables (OFV), solar plastic-roofed greenhouse vegetables (GHV) and orchard, mainly based on their different N inputs (Table S1) and management practices
Summary
Soil nitrate is important for crop growth, but it can leach to groundwater causing nitrate contamination, a threat to human health. Applications of synthetic nitrogen (N) fertilizer (derived from the HBNF) or manure can increase soil N supply and substantially increase crop yields[2] In some areas such as Africa people use far too little N for adequate food production on existing lands, while other areas, such as China, use far too much – both extremes damage the environment and threaten human well-being[3]. We are conscious of more and more reports of large accumulations of residual nitrate in different depths of soil in Chinese uplands[10,12,13], sometimes remaining in the vadose-zone[11] and sometimes leaching to groundwater causing nitrate contamination during the last three decades[8,14]. We are interested in determining the overall magnitude and the mechanism of nitrate accumulation in the soil profile, the impacts of declining groundwater levels, and threats to groundwater contamination under future climate change
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