Characteristics And Mechanisms For Soil Acidification Under Long-term Fertilization In Chinese Croplands

The aims of this study were to characterize the spatial and temporal evolution of soil pHunder long-term fertilizations, and further to understand the mechanisms and effectivemethods against soil acidification in croplands. We collected historical data on ten18–30years long-term fertilization experiments in six typical Chinese cropland soils (red soil atQiyang and Jinxian, paddy soil at Jinxian and Wangcheng, purple soil at Chongqing, alluvialsoil at Zhengzhou and Changping, black soil at Gongzhuling and Harbin and dark brown soilat Heihe). We compared the differences in topsoil pH among experimental stages of six yearsand fertilization treatments, i.e., no-fertilizer control (CK), sole chemical nitrogen fertilizer(N), chemical nitrogen, phosphorus and potassium fertilizers (NPK), manure amendmentswith NPK fertilizers (NPKM) and sole manure (M), calculated soil acidification rates (SAR)and analyzed how the characteristics of soil acidification were affected by natural factorsincluding regional climate and the initial soil properties. On the other hand, we expoundedthe mechanisms for soil acidification in croplands from apparent nitrogen balance, protonbudgets in ecosystem, the changes in soil acid-neutralizing capacity (ANC) andwater-extractable components, respectively.1) There were significant differences in topsoil pH among treatments at the experimentalsites of red, purple and black soils but not at the experimental site of purple, alluvial and darkbrown soil. The SARs in the six long-term fertilization experiments were significantlydifferent among treatments (p<0.05or0.01by Paired T-test), and decreased in the order of N> NPK>NPKM≈CK≈M. The SARs under the N and NPK treatments were0.066and0.046pH/yr, and were4.9and3.4times higher than that under the CK treatment, respectively.Moreover, sole chemical P and K fertilizers also significantly decreased topsoil pH andincreased SAR compared with control. Therefore, the evolution of pH in the topsoil wascharacterized as a common response to fertilization with regional differences.2) The factors dominated by average annual temperature were significantly and negativelycorrelated with the decrease in topsoil pH at the first stage of six years. The factors dominatedby precipitation were significantly correlated with SAR. The SARs averagely decreased 0.02~0.04pH/yr over one unit increase in the ratio of evaporation to precipitation (E/P). Therelative SAR under sole PK fertilization averagely increased0.024pH/yr over one unitincrease in altitude of100meters. The factor on the initial available phosphorous content inthe topsoil was significantly and positively correlated with the decrease of topsoil pH at thefirst stage of six years, especially under the N and NPK treatments. The factors on the initialsoil pH and clay content significantly affected SAR. The SARs increased with the decrease ofinitial pH (5.5~8.5) and with the increase of clay content (5~40%). Therefore, the effects ofregional climate and the initial soil properties at each site on the characteristics of soilacidification were significant.3) The increasing responses on fertilization with chemical N, P and K nutrients decreasedin the order of available phosphorous (168~599%)> available potassium (16~189%)>available nitrogen (9~33%) in the topsoil. Soil acidification under long-tem application withchemical N, P or K fertilizers in croplands was effectively alleviated while the correspondingnutrient was deficient in ecosystem, but largely accelerated while the corresponding nutrientwas surplus in ecosystem.4) In the cropland soils, the excess of N fertilization over the N in biomass harvested plusthe N in soil accumulated resulted in the N surplus in ecosystem. The increase of apparent Nbalance may accelerate soil acidification in croplands. The linear correlation between SARand apparent N balance were different among experimental sites, and the slope of linearfitting equation decreased in the order of red soil at Qiyang, black soil at Gongzhuling, highfertilization rate in black soil at Harbin, dark brown soil at Heihe, purple soil at Chongqing,conventional fertilization rate in black soil at Harbin and paddy soil at Wangcheng. In red soilat Qiyang, the SAR increased0.01pH/yr over one unit increase in apparent N balance of10kg/ha/yr.5) By the methods for quantifing soil acidification, total proton budgets under differentfertilization treatments were-3.58~19.84、-2.52~9.96、0.75~13.95和-0.86~10.74kmol/ha/yrin red soil at Qiyang and Jinxian, black soil at Harbin and dark brown soil at Heihe,respectively. The net acid addition rate (NAAR) was significantly correlated with the rate ofsoil pH decline relative to the initial value. However, this correlation was not significant inthe purple soil at Chongqing and black soil at Gongzhuling. Therefore, the SAR underlong-term fertilization was affected by four major factors, i.e., fertilization with excess basecations over acid anions, biomass removal, apparent N balance in ecosystem and thedecomposition of organic carbon.6) Under laboratory conditions, the process of soil protonation was considered as thepseudo-order reaction kinetics controlled by the proton diffusion in soil particles. At each long-term experiment, there were significant correlation between ANC and pH in the topsoil.Soil ANC was more sensitive to soil acid-base status rather than pH, but not to soil organicmatter (SOM). Significant differences in soil ANC among long-term fertilizations at theexperimental sites were in the order of inorganic nitrogen <non-nitrogen≈organic plusinorganic nitrogen with variations at each site.7) In black soil at Harbin, the decrease in C-related anions percentage and the increase innitrate percentage were along with the decrease in topsoil pH under the control, conventionaland high rate of mineral N treatments. The accumulations of nitrate and bivalent base cationspresented both at the top (<5cm) and sub (>20cm) soil depths under high rate of mineral Nfertilization. In the water-extractable components, the percentages of C-related anions andnitrate may indicate the percentages of proton budgets from carbon and nitrogen cycles inecosystem, respectively, and so as the components of dissolved organic N and inorganic N.8) The excess output of base cations over acid anions via soil leaching and biomass harvestproduced the net proton budgets in agricultural ecosystem. As the companied anions withbase cations, the output fluxes of bicarbonate, organic acid anions and nitrate were the mainperformances of proton budgets in ecosystem, and constituted the effects of carbon andnitrogen cycles for driving soil acidification, respectively. To prevent soil acidification incroplands in the long-term scales, the limitations of proton budgets included total amountbeing less than3%of soil ANC and the percentage of nitrogen cycle effects ranging10~50%.9) The general principle against soil acidification in croplands was summarized that thebiological cycle of nutrients in ecosystem should be accelerated but the geochemical cycle ofnutrients in ecosystem should be alleviated under the suitable mode of fertilization, fieldmanagements and biomass harvest. The effective methods against soil acidification incroplands were summed up from five aspects: suitable amount of chemical N fertilizer tokeep apparent N balance in ecosystem; suitable amount of chemical P and K fertilizers tooffset the output via biomass harvest; suitable amount of organic manure to keep or enhancesoil basic fertility gradually; suitable water managements to reduce the leaching losses of soilnutrients; suitable field managements to improve crop production.