Response Of Soil Organic Carbon And Microbial Properties Within Aggregates To High-input Fertilizer Practices Under An Intensive Agricultural System

Soil organic carbon(SOC)in agro-ecosystem represents the most important soil property because of its significance to soil fertility and soil quality.The physical protection of organic carbon in soil aggregate is the most important stabilization mechanism of organic carbon.Many previous studies have reported the effects of fertilization on the distribution of soil aggregates and organic carbon within aggregates.However,microbial communities and carbon mineralization within aggregates under different fertilization are still unclear.Moreover,the winter wheat-summer maize double cropping is the principal cropping system in northern China.Annual organic C and mineral nitrogen(N)inputs under this cropping system can be multifold higher than single cereal-based rotations.However,it is not clear at all how such high-input fertilizer practices affect soil aggregation,microorganisms and functions relevant to SOC.The objectives of this study were(1)to determine the distribution of soil organic carbon fractions,microbial community composition and enzymes activities,carbon mineralization within aggregates under an intensive agricultural system with high carbon and nitrogen inputs;(2)to evaluate the relationships among soil organic carbon,microbial properties and carbon mneralization.The fertilization field trial with a winter wheat and summer maize double-cropping system had been conducted in 2007.Treatments included:(1)CK,unfertilized control;(2)F,inorganic N and P fertilizers;(3)S,maize straw;(4)M,chicken manure;(5)SF,maize straw in combination with chemical fertilizers;(6)MF,chicken manure in combination with chemical fertilizers and(7)SMF,both maize straw and chicken manure in combination with mineral fertilizers.The soil type is silty clay loam.Soil samples were collected in 0-10 cm layer in March 2016.We applied a dry sieving method to separate three aggregates sizes,including>2 mm,0.25-2 mm and<0.25 mm.We used a particle density and size fractionation procedure to obtain physical fractions including free and occluded particulate organic matter(fPOM and oPOM)and mineral associated fractions(M>53 and M<53)within three aggregate sizes.We tested the microbial community composition using phospholipid fatty acid profiles technology,soil enzymes including P-Glucosidase,?-Xylosidase,N-Acetyl-glucosidase and L-leucine aminopeptidase by microplate fluorimetric assay,and carbon mineralization from a 124-day incubation experiment.The main results were as follows:(1)The application of organic fertilizers with inorganic fertilizers increased organic C and N contents across soil aggregates under intensive agricultural ecosystem.Nine years of mineral N and P fertilizer application at high doses on its own did not significantly increase the bulk SOC and N levels.The application of organic fertilizers(manure and/or straw)in combination with inorganic fertilizers,especially the SMF treatment,significantly(P<0.05)increased the SOC and N contents.The 0.25-2 mm macroaggregates contributed the most(54-59%)to the total dry-sieving aggregates.Fertilization did not significantly influence the prportions of different dry-sieving aggregates.We assumed the main binding agent for dry aggregates might be CaCO3.The organic C in 0.25-2 mm macroaggregates contributed above 50%to the total soil C.The application of organic fertilizers with inorganic fertilizers increased the organic C and N contents in all aggregates,particularly in 0.25-2 mm macroaggregates.(2)The application of organic fertilizers with inorganic fertilizers increased organic C and N contents of fPOM,oPOM and M>53 within all three aggregates,especially the fPOM fraction.The M<53 fraction was the largest C fraction,however,organic C of M<53 was not affected by different fertilization,indicating that the M<53 fraction became C-saturated or incorporation of fresh inputs into these fractions might take longer than 10 years.In>0.25 mm macroaggregates,fPOM and oPOM fractions stored the larger parts of organic C and N than mineral fractions.In contrast,most C was stored in M>53 and M<53 in<0.25 mm microaggregates.The C and N were predominately stored in oPOM of 0.25-2 mm macroaggregates under high-input fertilization in the intensive agricultural system.(3)The application of organic fertilizers with inorganic fertilizers significantly increased microbial communities' abundances and soil enzymes' activities.The abundances of total PLFAs,Gram-positive bacteria(Gram+),Gram-negative bacteria(Gram-),actinomycetes,saprophytic fungi and arbuscular mycorrhizal fungi(AMF)were higher in 0.25-2 mm macroaggregates.Organic fertilization significantly increased the amounts of microbial communities,but had no indluences on the composition of microbial communities.Activities of ?-Xylosidase,N-Acetyl-glucosidase and L-leucine aminopeptidase were higher in 0.25-2 mm macroaggregates,while ?-Glucosidase activities were higher in<0.25 mm microaggregates.The SOC and N contents were the two main factors to influence microbial communities and soil enzymes.In>2 mm macroaggregates,fPOM and oPOM were the main factors to affect microbial communities and soil enzymes.In 0.25-2 mm macroaggregates,oPOM was the main factor to affect microbial properties.In<0.25 mm microaggregates,oPOM was the main factor to affect the microbial communities,while M>53 was the main factor to affect the soil enzymes'activities.(4)The application of organic fertilizers with inorganic fertilizers significantly increased the cumulative mineralization C(Cmin)within aggregates,especially in 0.25-2 mm macroaggregates.The mineralization rate of organic C in aggregates decreased quickly in the early period of incubation,and remained stable in the late period.The application of organic fertilizers with inorganic fertilizers decreased the ratios of Cmin/SOC,indicating increased SOC accumulation.The Cmin and Cmin/SOC increased as aggregates sizes decreasing.Increases in microbial communities' abundances and enzymes' activies resulted in more mineralization C.The oPOM is the main source of mineralizable C in macroaggregates,while M<53 was the main source of mineralizable C in microaggregates.Thus,we concluded that the application of high level organic fertilizers with inorganic fertilizers increased the SOC level.The C was mostly stored in occluded POM fractions in 0.25-2 mm macroaggregates at medium-long(decadal)time scales,which would be easily lost again by microbial communities under intensive tillage management.These have to be carefully taken into account when setting realistic and effective goals for long-term sequestration of SOC in intensively agricultural ecosystems.