Enhanced acidification in Chinese croplands as derived from element budgets in the period 1980-2010

Significant soil pH decrease has been reported in Chinese croplands in response to enhanced chemical fertilizer application and crop yields. However, the temporal and spatial variation of soil acidification rates across Chinese croplands is still unclear. We therefore assessed trends in soil acidification rates across provincial China for the period 1980-2010 by calculating inputs-outputs of major cations and anions in cropland systems. Nitrogen (N) induced proton production increased from 4.7keqH⁺/ha/yr in 1980 to a peak of 11.0keqH⁺/ha/yr in 1996 and remained nearly constant after 2000 at a rate of approximately 8.6keqH⁺/ha/yr. The proton production induced by crop removal increased from 1.2 to 2.3keqH⁺/ha/yr. The total proton production thus increased from 5.9 to 10.9keqH⁺/ha/yr in the 30years. As a result, the actual acidification rate, reflected by (base) cation losses, accelerated from 2.3 to 6.2keqH⁺/ha/yr and the potential acidification rate, reflected by phosphorus accumulation accelerated from 0.2 to 1.3keqH⁺/ha/yr. The national averaged total acidification rates were thus estimated to increase from 2.6 to 7.6keqH⁺/ha/yr in the past 30years. The highest soil acidification rate occurred in the Jiangsu Province with a rate of 17.9keqH⁺/ha/yr, which was due to both high N application rates and high base cation removals by crops and crop residues. The combination of elevated N inputs and decreased N use efficiency (NUE) in response to those N inputs, thus enhancing the nitrate discharge, were the main reasons for the accelerated acidification in Chinese croplands. Considering the expected growth of food demand in the future, and the linkage between grain production and fertilizer N consumption, a further acceleration of soil acidification can thus be expected, unless the N inputs is reduced and/or the NUE is increased substantially.