Effectiveness And Mechanisms Of Organic Materials Affecting On Red Soil Acidification From Chemical N Fertilizer

Red soil acidification has been accelerated by overuse of chemical nitrogen?N?fertilizer in the last30 years,and resulted in aluminum?Al?toxicity,which is a major cause for soil degradation and limiting crop growth in the red soil region of Southern China.Organic materials,such as manure and crop straws,as amendments have been commonly used to alleviate acidification and improve soil fertility.However,manure could effectively alleviate red soil acidification while crop straws showed less effective in a long-term field experiment,Qiyang,Hunan Province,and the mechanisms involved are not fully understood.The objective of this research was to examine the mechanisms for the effectiveness of different organic materials?manure and crop straws?in alleviating red soil acidification from chemical N fertilizers.The effects of 25-,7 and 7-year fertilization treatments?CK,chemical fertilizers,manure,and crop straws?on soil acidification indicators were examined in soils from 3 long-term feild experiments located at the Hengyang Red Soil Research Station of Chinese Academy of Agricultural Sciences.Laboratory incubation experiments were conducted to determine the effects of organic materials in ameliorating acidity of red soils,the effects of treatments with different ratios of organic-N to chemical N fertilizer and composition?C/N ratio,salt of organic acid?of fertilizers on soil N transformation,nitrification potential and rate,and soil acidity.The main results and conclusion were showed below:?1?Chemical fertilizers combined with swine manure could effectively ameliorate soil acidification,while crop straws showed less effective.Our results indicated that at 70%or more total N source(300kg N ha^-1 yr^-1)from manure could effectively maintain or increased soil pH by 1.08 units,significantly decreased soil exchangeable acidity,and increased soil exchangeable base cations.While soil pH decreased sharply from an initial pH of 5.7 and then stabilized after 8 to 10 years of fertilization in the NPK and NPKS1 treatments with a final pH of 4.2-4.5,thereafter the increase in soil exchangeable acidity was dominated by exchangeable aluminum.?2?Manure application rate required to prevent red soil acidification is increasing as total N application rate increasing.Soil pH decreased significantly from initial value in NPKM0 and NPKM20treatments at total N application rate of 225 kg N ha^-1 yr^-1,with increased soil exchangeable acidity and aluminum.In contrast,at manure incorporation rate of 40%or 60%of total N?NPKM40 or NPKM60?either no change or a significant increase in soil pH,decreased exchangeable acidity and increased exchangeable base cations.While 70%or more total N source from manure would be needed to maintain soil pH at total N application rate of 300 kg N ha^-1 yr^-1,which indicated manure application rate required to prevent red soil acidification was increasing as total N application rate increasing.?3?Swine manure and crop straws rich in ash alkalinity could effectively neutralize soil acidity,and crop straws with high C/N ratio reduced nitrate concentration,consumed H⁺,and increased soil pH.All three organic materials significantly decreased soil acidity?dominated by aluminum?as the application rate increased.Soybean straw was as effective?sometimes more effective?as swine manure in raising pH in both soils.Maize straw was less effective than soybean straw in affecting soil pH and the acidity.The exchangeable aluminum decreased at a rate of 4.48 cmol_?+?kg^-1 per pH unit increase for both straws compared to 6.25 cmol_?+?kg^-1 per pH unit from the manure.The NO₃^-+NO₂^-concentration in soil increased significantly for swine manure amendment,but decreased markedly for straw treatments.The high C/N ratio in the straws led to N immobilization and pH increase.?4?Organic materials reduced nitrification rates,production of NO₃^--N+NO₂^--N and H⁺via increasing the ratios of organic-N to urea-N and C/N?>30?.At the same N application rate(150 mg N kg^-1),chemical N combined application with manure or maize straw could significantly reduce nitrification rate and H⁺production,and therefore prevent decrease in soil pH;as swine manure-N increased 1%,the productions of NO₃^--N+NO₂^--N and H⁺were reduced by 0.08-1.18 mg N kg^-1 and0.01-0.08 mmol kg^-1,respectively.As 50%or more total N source as maize straw application can fully prevent or reverse red soil acidification process.Adjusted C/N ratio above 30 could effectively reduce nitrate concentration in soil and H⁺production,and thus increase soil pH.?5?Decarboxylation of organic anions to alleviate acidification of red soils from urea application.All three organic anions significantly increased pH of NPK and NPKM soils and the effectiveness was positively correlated with application rate.The organic anion effectiveness in decreasing soil acidity was in the order oxalate>malate?citrate.The change in total exchangeable soil acidity was dominated by aluminum concentration in the NPK soil,but by proton concentration in the NPKM soil.The change in soil pH was positively correlated with total CO₂ production,and the ratio of nitrate concentration to total CO₂ production,suggesting that the proton released from nitrification was neutralized by the process of decarboxylation.Addition of organic anions led to a lower nitrification rate in NPKM soil relative to the NPK soil.The lower soil pH?4.36?limited nitrification process but had no effect on decarboxylation.While swine manure rich in ash alkalinity continues to be effective for ameliorating soil acidity,and the ratio of swine manure to chemical N increased as N application rate increasing to maintain soil pH.Organic materials released alkalinity and reduced production of NO₃^--N+NO₂^--N and H⁺via increasing the ratios of organic-N to urea-N and C/N?>30?,which are the main mechanisms for alleviating soil acidification.Crop straw amendment has a good potential to ameliorate the acidity of the red soil.Thus,after harvest,straws should preferably not be removed from the field,but mixed with the soil to decelerate acidification.