Agricultural soil acidification in China is known to be caused by the over-application of nitrogen (N) fertilizers, but the long-term impacts of different fertilization practices on intensive cropland soil acidification are largely unknown. Here, we further developed the soil acidification model VSD+ for intensive agricultural systems and validated it against observed data from three long-term fertilization experiments in China. The model simulated well the changes in soil pH and base saturation over the last 20 years. The validated model was adopted to quantify the contribution of N and base cation (BC) fluxes to soil acidification. The net NO leaching and NOinput accounted for 80% of the proton production under N application, whereas one-third of acid was produced by BC uptake when N was not applied. The simulated long-term (1990-2050) effects of different fertilizations on soil acidification showed that balanced N application combined with manure application avoids reduction of both soil pH and base saturation, while application of calcium nitrate and liming increases these two soil properties. Reducing NH input and NO leaching by optimizing N management and increasing BC inputs by manure application thus already seem to be effective approaches to mitigating soil acidification in intensive cropland systems.

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.

Nitrogen (N) deposition can profoundly alter soil N cycling of grassland ecosystems.Substrates and soil acidification are expected to modify soil N transformations in response to elevated N deposition. Here, we carried out 15 N tracing studies to test the effects of N addition rates (low: 30 kg N ha -1 and high: 90/120 kg N ha -1 ) and soil acidification on gross N transformation rates using two typical Chinese grassland soils, an alpine calcareous soil and a temperate neutral soil. We found that N addition significantly increased the ratio of gross nitrification rate to gross ammonia immobilization rate (N/I) in both soils, but gross N transformation rates changed differently as a function of N addition rates and soil types. In the calcareous soil, N addition increased soil gross N transformations, largely due to mineral N substrates, SOC, TN and fungal dominance. In contrast, low N addition did not affect gross N transformation rates in the neutral soil, but high N addition significantly decreased gross N transformation rates. Although both SOC and TN were increased with N addition in the neutral soil, N-induced soil pH decline decreased gross N transformation rates. Our results indicate that the effects of N addition on grassland soil gross N transformations are highly dependent on mineral N substrates, SOC and TN. Soil acidification played a more important role than SOC and TN in gross N transformation rate changes in response to elevated N deposition. These findings suggest that the different changes of gross N transformation rates in response to N deposition and soil properties (e.g. SOC, TN and soil pH) should be integrated into biogeochemical models to better predict grassland ecosystem N cycling in the future scenarios of N deposition.

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