Significant Acidification in Major Chinese Croplands

Cropland Acidification in China China is experiencing increasing problems with acid rain, groundwater pollution, and nitrous oxide emissions. Rapid development of industry and transportation has accelerated nitrate (N) emissions to the atmosphere. Consequently, soil degradation, water shortage, and pollution, in addition to atmospheric quality decline are becoming major public concerns across China. Since the 1990s, China has become both the largest consumer of chemical N fertilizers and the highest cereal producer in the world, which has consequences for arable soil acidification. Guo et al. (p. 1008, published online 11 February) present a meta-analysis of a regional acidification phenomenon in Chinese arable soils that is largely associated with higher N fertilization and higher crop production. Such large-scale soil acidification is likely to threaten the sustainability of agriculture and affect the biogeochemical cycles of nutrients and also toxic elements in soils. Intensifying agriculture in China in the past 30 years is the major contributor to soil acidification at the regional scale. Soil acidification is a major problem in soils of intensive Chinese agricultural systems. We used two nationwide surveys, paired comparisons in numerous individual sites, and several long-term monitoring-field data sets to evaluate changes in soil acidity. Soil pH declined significantly (P < 0.001) from the 1980s to the 2000s in the major Chinese crop-production areas. Processes related to nitrogen cycling released 20 to 221 kilomoles of hydrogen ion (H+) per hectare per year, and base cations uptake contributed a further 15 to 20 kilomoles of H+ per hectare per year to soil acidification in four widespread cropping systems. In comparison, acid deposition (0.4 to 2.0 kilomoles of H+ per hectare per year) made a small contribution to the acidification of agricultural soils across China.