Effects Of Long-term Fertilization On Soil Organic Matter Fractions,microbial Community Structure And Enzyme Activities In A Red Upland Soil

Soil organic matter(SOM)and microorganisms play critical roles in maintaining the function and stability of agroecosystem.Fertilization is an important agricultural management practice.It is necessary to better understand how long-term fertilization influence the soil organic matter fractions,microbial community structure and function,which can provide theoretical basis and practical significance for improving the crop yield,maintaining the soil fertility and quality and realizing the sustainable utilization and development of agricultural soils.Based on a long-term field experiment of red upland soil in South China,the SOM was divided into six groups:unprotected(cPOM and fPOM),pure physically protected(iPOM),physical-biochemically protected(NH-μSilt and NH-μClay),physical-chemically protected(H-μSilt and H-μClay),chemically protected(H-dSilt and H-dClay)and biochemically protected(NH-dSilt and NH-dClay)fractions,using physical-chemical fractionation method.And then the influence of long-term fertilization treatments on the mass proportion and soil organic carbon(SOC)content of SOM fractions were analyzed.Furthermore,the microbial community structure and enzyme activities were respectively determined by phospholipid fatty acid(PLFA)analysis and a microplate fluorimetric assay method.And the response of microbial community structure and enzyme activities to long-term fertilization was studied.The main results were as follows:(1)Compared with the non-fertilization(CK)treatment,the combined application of mineral and manure fertilizers(NPKM)significantly increased the SOC and total nitrogen(TN)contents by 70.6%and 57.5%respectively;the sole mineral application(N,NPK)significantly reduced the soil pH value,indicating that long-term mineral fertilizers addition can result in the soil acidification.Compared with CK treatment,the SOC content in six functional SOM fractions(except for H-dClay and NH-dClay)showed a significant increase and the highest increase was SOC content in cPOM and iPOM fractions under long-term NPKM treatment.This suggested that the SOC content in unprotected and pure physically protected SOM fractions were more sensitive to the fertilization management practice.SOC content in SOM fractions varied with total SOC.With the increase of total SOC,the SOC content in iPOM fraction showed a highest linear increase trend.This indicated that the stabilization mechanisms of SOC in different SOM fractions under long-term fertilization were different.This study proves that the long-term combined application of mineral and manure fertilizers is an effective management practice to improve soil fertility in agroecosystem.(2)The microbial community structure was significantly altered by long-term fertilization.The total microbial biomass and biomass of bacteria,fungi,actinomycetes,Gram-positive bacteria(G~+)and Gram-negative bacteria(G~–)under long-term NPKM treatment were the highest among all treatments.Compared with the other three treatments(CK,NPK,NPKM),the biomass of the above-mentioned microbial communities was the lowest under sole N fertilizer addition.Compared with CK treatment,NPK treatment only significantly affected the fungal biomass.These results indicated that long-term combined application of mineral and manure fertilizers improved the soil environment,which might favor the microbial growth.In addition,this study found that the correlation between soil microbial biomass and soil physical and chemical properties was significant.And the correlation between soil microbial biomass and soil pH was stronger,suggesting that soil pH was the main factors affecting soil microbial community biomass under long-term fertilization conditions.(3)Long-term fertilization also showed a significant effect on soil enzyme activities.The activities ofβ-1,4-glucosidase(βG),β-1,4-N-acetylglucosaminidase(NAG)and acid phosphatase(AP)were the highest under long-term NPKM treatment.Both SOC and TN had extremely significant positive correlation with theβG、NAG and AP activities,indicating that SOC and TN were the major environmental factors affecting soil enzyme activities under long-term fertilization conditions.