Effects Of Long-term Different Fertilization On Soil Carbon Pool And Its Microbial Mechanism

Long-term different fertilization patterns significantly change soil structure,chemical and biological properties,and shape different microhabitats such as nutrients,aeration and water status,which directly affect the transformation and stabilization of farmland soil organic carbon.In addition,long-term experiment represents the long-term and stable trend of farmland ecosystem,and long-term positioning experiment is very necessary to study the long-term process,dynamics and ecosystem mechanism of soil carbon cycle.This research is based on the long-term positioning test and biomarker method of the typical fluvo-aquic soil in North China Plain since 2007,was investigated: 1)the change law of soil organic carbon components and soil carbon pool under long-term different fertilization modes;2)the stabilization mechanism of soil organic carbon aggregates under different fertilization modes;3)the mechanism of regulating soil carbon sequestration by fertilization modes was explored from the perspective of microbial metabolism.This experiment set up 5 long-term fertilization,specifically: Control treatment with no-fertilizer(CK);Chemical fertilizer alone(NPK);Chemical fertilizer combined with straw(NPKS);Chemical fertilizer combined with manure(NPKM);Total replacement of chemical fertilizer with manure(100%M).The main results and conclusions are as follows:(1)The effects of long-term different fertilization modes on soil organic carbon components and soil carbon pool were clarified.Compared with NPK,the application of organic fertilizer or supporting straw returning measures(NPKS,NPKM,and 100%M)significantly increased the content of various organic carbon components(easily oxidized organic carbon,particulate organic carbon,microbial biomass carbon,dissolved organic carbon,mineral-associated organic carbon,and non-easily oxidizable carbon),and the soil organic carbon storage,carbon sequestration rate and soil carbon pool management index increased by 126.3%-206.6%,112.0%-358.8% and 224.0%-375.5%.However,the application of organic fertilizer or supporting straw returning measures(NPKS,NPKM,100%M)also increased the contribution rate of active organic carbon components(dissolved organic carbon,easily oxidized organic carbon,particulate organic carbon)to total soil organic carbon and compared with CK reduced the proportion of mineral component organic carbon in soil organic carbon by 14.1%-23.9%.It can be seen from the above results that the application of organic fertilizer or supporting straw returning measures increased soil organic carbon,mainly particulate organic carbon,which increased soil organic carbon storage and reduced the stability of soil carbon pool.(2)The effects of long-term different fertilization patterns on soil aggregate stability and aggregate organic carbon storage were discussed.Compared with the other four fertilization modes,under organic fertilizer total substitution the average weight diameter of soil aggregates increased by 15.0%-65.9%,10.8%-44.0%,and 14.6%-66.7%,respectively.Linear analysis showed that the stability of soil aggregates might be related to organic cementing substances such as soil organic carbon,polysaccharide and saccinomycin.Compared with CK,the organic carbon storage on each particle size aggregate increased significantly by applying organic fertilizer or straw-returning measures(NPKS,NPKM,100%M)and 100% M has the most significant effect which increased 58.8%-305.4% and the proportion of organic carbon on aggregates >0.25 mm was increased which indicated that the organic carbon stored by adding organic fertilizer or straw-returning measures was mainly protected by physical methods.It was also found that there was a negative correlation between organic carbon storage and carbon-nitrogen ratio in aggregates of each particle size,which indicated that organic fertilizer or straw returning(NPKS,NPKM,100%M)increased organic carbon storage in aggregates of each particle size,but reduced the stability of corresponding carbon pool.(3)The difference in carbon metabolic activity of aggregates with different particle sizes under long-term different fertilization modes was explored.The biomass carbon content and the activities of enzymes related to carbon metabolism(β-glucosidase,sucrase,urease,and catalase)in the aggregates of 0.053-0.25 mm and <0.053 mm under different fertilization patterns were significantly higher than those of other aggregates,and they were active areas for carbon metabolism of microbial communities.Compared with other fertilization treatments,the microbial biomass carbon on each particle size aggregate increased by16.6%-134.6%,16.5%-126.0%,17.5%-120.6%,and 17.3%-114.4% under 100%M treatment,in addition 100%M increase enzymes related to carbon metabolism on aggregates of each particle size,and increased the carbon metabolism rate of aggregates of each particle size.Compared with CK,under 100%M the content of polysaccharide and saccinomycin in aggregates of each particle size was increased by 235.6%-279.5% and 41.7%-62.7%,which increased the carbon deposition of microbial metabolites in soil,promoted the formation of water-stable aggregates,and provided more places for organic carbon to be stored.(4)The relative contribution of microbial residue carbon to soil total organic carbon under long-term different fertilization modes was quantified.Fungal carbon is the main source of soil microbial residues in farmland.Fungal carbon contributes 10.0%-63.0% of soil organic carbon,which is 4.85-31.50 times bacterial carbon.The relative contribution of fungus residue carbon and bacterial residue carbon to total soil organic carbon was 39.5%-63.0% and 7.3%-12.7%,respectively.After applying organic fertilizer or straw(NPKS,NPKM,100%M),the relative contribution of fungus residue carbon and bacterial residue carbon to soil total organic carbon decreased by 125.0%-523.8% and 101.6%-370.4%,respectively,among which the total amount of 100%M had the largest decline.Combined with the change of soil organic carbon composition,the application of organic fertilizer or straw(NPKS,NPKM,100%M)reduced the relative contribution of microbial residues to total soil organic carbon,and increased the relative contribution of plant-derived carbon represented by particulate organic carbon.In summary,adding organic fertilizer or returning straw(NPKS,NPKM,100%M)to the field significantly increased soil organic carbon storage,and the organic carbon mainly exists in large aggregates,and its stability mechanism is mainly physical protection,but the proportion of active organic carbon components in soil organic carbon increased,which reduced the stability of soil carbon pool.From the perspective of microbial metabolism,it is found that micro-aggregates and sticky powder are the "hot areas" for microbial carbon metabolism.100%M to the field significantly increased the content of soil polysaccharide and saccinomycin,which promoted the formation and stability of aggregates and provided more places for organic carbon to be stored.The application of organic manure or straw(NPKS,NPKM,100%M)returning treatment decreased the content of microbial residue carbon and its contribution to organic carbon,and increased the relative contribution of plant-derived carbon represented by granular carbon.