Quantification On Soil Acidification Under Different Nitrogen Managements And Cropping Systems At Field Scale

Human activities(e.g.over-fertilization,acid deposition and legumes planting),have strongly enhanced the natural soil acidification rates.Soil acidification can threaten terrestrial ecosystem functioning and services through altering biogeochemical cycles.Over fertilization with nitrogen(N)has caused major cropland acidification in China.However,the contribution of the causes of soil acidification to intensive croplands has seldom been quantified under field conditions.Apart from increased amount of N fertilization,there are various and changing N fertilizer types used and multiple cropping systems in China.This study systematically quantified soil acidification due to different N application rates(non-N fertilized,optimized N and over-used N)and fertilizer N forms(NH4Cl and Urea)in maize-wheat cropping system and/or three typical cropping systems(rice-fallow,rice-wheat and maize-wheat),based on the input-output budget of major elements.The main results are given as follows:1.In maize-wheat cropping system,fertilizers and atmospheric deposition were main nutrient input,especially for N,sulfate and base cations.The main output pathways of nutrients for the control plots were leaching and crop removal.Compared to the control plots,N addition significantly effected N cycling and enhanced nutrients uptaken by plants and leaching losses.N-induced impacts on element cycles were getlting stronger as N addition increased,which of NH4Cl application were stronger than Urea application.2.In maize-wheat cropping system above,soil acidification rate was 4.7 keq H+ ha-1 yr-1 for the control plots.Soil acidification rates increased as N addition,caused by N cycling and N-induced crop uptake.NH4Cl fertilizer had more serious soil acidification rates(30-53 keq H+ ha-1 yr-1)than Urea fertilizer(9-12 keq H+ ha-1 yr-1),dominated by N cycle accounting for 73-82%of the total H+production.Crop uptake was the primary driver of soil acidification for the control and urea treated plots,accounting for 71-77%of the total H+ production.Compared to the control plots,over-use Urea and NH4Cl treated plots led to a decline of 0.3,0.4 and 0.7 in topsoil pH,respectively,while corresponding soil base saturation decreased 3%,5%and 10%,respectively.3.In three cropping systems(rice-fallow,rice-wheat and maize-wheat),atmospheric deposition,soil mineralization,fertilizers and irrigation were important nutrient resouces.The main output pathways of nutrients for the control plots in three cropping systems were crop removal(accounting for 81-96%N,and 20-35%BCs of the total input)and leaching(accounting for 42-69%BCs and 31-80%AAs of the total input).For all cropping systems,optimized N addition enhanced crop uptake and leaching loss without changing soil exchangeable cations.Compared to optimized N addition,over N addition with increased P fertilization did not make differences in crop uptake and leaching loss for rice-fallow and rice-wheat cropping systems,further increasing crop uptake and leaching loss for maize-wheat cropping system.4.Soil acidification rates were significantly different between cropping systems and crops,showing maize-wheat(4.3-12.6 keq H+ ha-1 yr-1)>rice-wheat(5.0-7.9 keq H+ ha-1 yr-1)>rice-fallow(5.3-7.0 keq H+ ha-1 yr-1)and maize(2.7-8.9 keq H+ ha-1 yr-1)>rice(3.9-6.2 keq H+ ha-1 yr-1)>wheat(1.2-3.7 keq H+ ha-1 yr-1).Soil acidification rates increased as N addition increased for maize and wheat,being no different between optimized and over N addition for rice.For all cropping systems,soil acidification was mainly driven by crop removal,being 92-96%,88-98%and 76-84%of the total H+production for rice-fallow,rice-wheat and maize-wheat,respectively.Base cations uptake by straw contributed 88-93%of H+ production by crop uptake for all cropping systems,suggesting the importance of straw return into field against soil acidification via supplying consumed base cations in soil.In conclusion,this study systematically clarified the process of cropland soil acidification on an acid purple soil,revealed H+ production and the change in soil acid neutralizing capacity can be used to quantify the soil acidification rates on field conditions,found the soil acidification rates were strongly effected by N fertilizer forms and application rates,quantified the contribution of the key processes on farmland soil acidification where N cycle and crop uptake were the main drivers of soil acidification depending on N fertilizer forms,and pointed out that optimizing N management,e.g.applying urea rather than NH4+-N and rationalizing N application rates,in combined with straw return would be effective approaches to mitigate intensive cropland acidification.