Mechanisms Of Cropland Soil Acidification Induced By The Long-term Fertilization And Mitigation Strategy

In the past decades,Chinese croplands were significantly acidified by over-application of nitrogen fertilizer,which poses a great risk to agricultural production in the future.In non-agricultural soil,enormous studies on acidification of natural ecosystems have shown that quantification of soil acidification process not only provides a theoretical basis for elucidating acidification mechanism,but also provides policy recommendations for acidification mitigation in the future.However in agricultural soil,quantifying the impacts of fertilization on acidification is largely unknown.To understand the cropland soil acidification processes,it is necessary to quantify the contribution of the main processes to soil acidification,reveal the impacts of soil acidity on soil nitrogen transformation and then on H+ production of fertilizer input,clarify improvement of farmland management mitigating soil acidification,and validate the effects of effective mitigation strategies on soil acidification reduction based on results of the VSD+ model.Here,the relationship between proton generation and soil acidification was revealed,and the impacts of soil pH on soil nitrogen cycling,and then on proton productions were quantified by laboratory experiment.The main drivers of cropland soil acidification due to long-term fertilization were quantified using soil acidification model(VSD+ model),and the efficient mitigation managements of soil acidification were developed through combining the results of modelling and three years' field experiment.The main results and conclusions drawn from this study were listed as follows:1.Proton productions of different types of nitrogen fertilizer,and the relationships between proton production and actual soil acidification were quantified.The rate of proton(H+)production in ammonium nitrogen fertilizer treatment is 3.8 times of that in urea treatment,which is higher than that the theoretical value.The main reason is that the amount of acid production by fertilizer is partly dependent on the nitrification capacity of soil.In non-calcareous soil,there was a significant 1:1 positive correlation between the amount of acid produced by N and HCO3 cycles with the reduction of soil base cation pool,while positive correlation was found in calcareous soil.Therefore,the H+ production can be used to quantify the soil acid neutralizing capacity(ANC)change,i.e.the actual soil acidification rate.2.The effects of soil pH on nitrogen cycling and then on H+ production by N fertilizer input were revealed.The rates of nitrification,denitrification and ammonia volatilization decreased with decreasing soil pH while the ratio of ammonium and nitrate in leached solution increased significantly.The amount of H+ produced in the nitrogen cycle was also significantly decreased when soil pH decreased.These effects were sharply inhibited when soil nitrification rate is very low.Under the same amount of nitrogen fertilizer input,the actual acidification rate of the neutral soil and alkaline soil is similar,which is significantly higher than that of the acidic soil.The acidification of netural soil should be more concerned due to relatively low acid buffering capacity.3.The contribution of the main processes to soil acidification was quantified using validated VSD+ model.The soil acidification model VSD+ was further developed for intensive agricultural systems and validated against observed data from three long-term fertilization experiments in China,The model well simulated changes in soil pH and base saturation over the last 20 years.The net nitrate leaching and ammonium input accounted for 80%of the proton production under N application,whereas one third of acid was produced by base cations uptake when N was not applied.4.It was proved that the increased nitrogen use efficiency(NUE)could significantly reduce the soil acidification rate.Effective mitigation strategies of soil acidification in intensive farming cropland system include a reducing NH4+ input and NO3-leaching by optimizing N management,and maintaining the soil BC balance by applying manure and reducing straw removal.In addition,three years of field experiments have confirmed that NUE increases by 20%resulted in 23%reduction of the amount of H+ production by fertilizers.In conclusion,this study systematically clarified the process of cropland soil acidification,revealed the 1:1 positive relationship between the H+ production by fertilizers input and the actual soil acidification in non-calcarous soil,clarified the acid sensitivity of neutral soil through quantitative study of the H+ production by fertilizers input,quantified contribution of the key processes on farmland soil acidification,and pointed out that reducing ammonium input and nitrate leaching by optimizing N management and increasing crop NUE,and maintaining the soil BC pool balance by applying manure,lime and reducing straw removal are effective approaches to mitigating soil acidification in intensive farming cropland systems.