Mechanism Of Cattle Manure Addition In Mitigating Acidification Of Mollisols Farmland

Acidification is one of the most important signs for Mollisols degradation d ue to long-term excessive chemical fertilizer application.Although organic manure is well known for its role in mitigating soil acidification,few studies ha ve been conducted on its mechanism.A long-term field experiment was initiated from 2012 in the Gu angrong Village,Hailun city,Heilongjiang Province,China.The experiment was implemented under a soybean[Glycine max（Merrill.）L.]and maize（Zea mays L.）rotation in Mollisols farmland.The fertilization treatments comprised（1）CK:the non-fertilizer control,（2）F:chemical fertilizer（NP K）,non-manure application,（3）F+M:chemical fertilizer combined with 15 Mg ha^-1 of cattle manure and（4）F+2M:chemical fertilizer combined with 30 Mg ha^-1 of cattle manure.After10 years of long-term treatment,soil samples were collected from soil lay ers of 0-10 cm,10-20 cm,20-30 cm,and 30-40 cm after crop harvest,respectively.This study focused on the effects of long-term cattle manure addition on soil pH,soil cation exchange capacity,soil salt-based ions concentrations,soil pH buffer capacity,soil available nutrients concentrations,soil microbial activity and crop yield.The findings could provide scientific basis for rational fertilization,acid mitigation and high yield crop production,and thus sustainable agricultural development in blac k soil region.The main results are as follows:Long-term application of chemical fertilizers reduced soil pH in the 0-20 cm soil layer of Mollisols.Soil pH in the 0-10 cm and 10-20 cm layers of F treatment was 5.13 and 5.43,respectively,which was 0.23 and 0.02 units lower than the No F treatment.Long-term cattle manure addition could significantly alleviate soil acidification of Mollisols.As compared to the F treatment,the soil pH value of 0-40 cm soil layer increased by an average of 0.6 unit under the continuous F+M treatment,while it increased by an average of 1.07 unit under the continuous F+2M treatment after 10 years.In terms of soil pH buffering capacity,different treatments showed the following trends:F+2M>F+M>No F>F,suggesting that c hemical fertilizers reduce soil pH buffering capacity.The soil pH buffer capacity was 46.51-48.78 mmol kg^-1 pH^-1 in F treatment,which was lower than No F treatment with 48.31-52.08 mmol kg^-1 pH^-1.Long-term application of cattle manure can enhance the soil pH buffering capacity in the plough layer,which was51.81-56.50 mmol kg^-1 pH^-1 in F+M treatment and 58.14-60.61 mmol kg^-1 pH^-1 in F+2M treatment.The soil pH buffer capacity decreased with the soil layer,but no s ignificant differences were found among treatments.There was a significant impact of long-term cattle manure addition on soil cation exchange capacity（CEC）in the 0-10 cm soil layer,but no difference in CEC values among fertilizer treatments in the 10-40 cm soil layer（P>0.05）.As compared to F treatment,soil salt-based ions concentrations increased by 2.0%under F+M treatment and increased with the rate of cattle manure addition.Cattle manure addition increased exchangeable potassium,calcium,and magnesium contents in the 0-20 cm soil layer,with average increase of 7.0%-16%,7.5%-10%and 12%-23%.F treatment had the highest soil exchangeable acidity content in the 0-20 cm soil layer,while cattle manure addition reduced it significantly.The soil e xchangeable acidity content under F+M and F+2M treatments was 24%-25%and 22%-24%lower than under the F treatment,respectively.Similar response of the soil exchangeable Al ³⁺content in the 0-20 cm soil to cattle manure addition was found as to the trend of soil exchangeable acidity content.In contrast,the exchangeable H⁺content was relatively insensitive to cattle manure additionCattle manure addition significantly increased soil organic matter and available nutrients content.As compared to the F treatment,the F+M treatment increased so il organic matter,available phosphorus,and available potassium by 6.3%-11%,17%-177%and 25%-37%,while that of the F+2M treatment increased by 18%-19%,84%-338%and 68%-109%,in the0-40 cm soil layer respectively.Soil pH buffer capacity was positively correlated（P<0.05）with soil microbial biomass carbon/nitrogen concentration and soil respiration.The long-term application of cattle manure increased soil microbial biomass carbon/nitrogen concentration and soil respiration,showing an improvement in soil microbial activity.Compared to the F treatment,the F+M treatment increased soybean and corn yields by34%and 12%,and aboveground biomass by 32%and 11%,respectively.While F+2M treatment increased soybean and corn yields by 57%and 18%,and above ground biomass by49%and 17%.The structural equation model showed that the increase in soil pH value with cattle manure application could directly affect soybean yield,and the increases in soil pH value with cattle manure application could affect maize yields indirectly through changing soil CEC,exchangeable acids,microbial mass carbon contents and protons production in crop.