Soil Organic Carbon Distribution In Soil Aggregates And Its Microbial Influencing Mechanisms In Orchards Of Different Ages

Soil is the largest organic carbon pool of the global terrestrial ecosystem,containing more than 1502 Pg C to the first meter,which is approximately 2-3 times that of the atmospheric and terrestrial vegetation carbon pool.Slight changes in soil organic carbon(SOC)will cause dramatic changes in atmospheric CO₂ concentration.Anthropogenic activities in recent decades have increased the amount of greenhouse gases in the atmosphere,leading to a continuous increase in the greenhouse effect.Regulating SOC pool in agricultural ecosystems to reduce greenhouse gas emissions is an important way to mitigate climate change.Moreover,SOC abundance is linked with ecosystem functions and services such as agricultural production,soil biodiversity,and climate change mitigation.Recently,the“4 per 1000”initiative intended to increase SOC storage in global agricultural soils at a yearly rate of 4‰to secure food production and to mitigate greenhouse gas emissions.Physical protection offered by soil aggregates underlies the SOC stabilization mechanism.The SOC content generally varies in different size fractions of soil aggregates.Macroaggregates(>0.25 mm)roughly have higher SOC contents,while in microaggregates,the SOC content increases as the aggregate size decreases.Furthermore,soil aggregates are perceived as microbial hotspots.Different sized soil aggregates provide microorganisms and extracellular enzyme activities with stratified habitats.Therefore,it is essential to investigate the relationship between microbial activities and SOC changes in different sized soil aggregates for a holistic understanding of SOC stabilization mechanism.In this study,terraced slope citrus orchards of different ages in the red soil region of southern China were chose.We aim to(1)quantify the SOC content,storage and percentage contribution in different soil aggregates in different orchards,(2)determine extracellular enzyme activities,microbial diversity,microbial community composition in different soil aggregates in different orchards,(3)reveal the SOC sequestration in different soil aggregates mechanism mediated by soil microbial organisms.The main conclusions of this study were as follows:(1)Long-term orchard management promoted the increase of the SOC content in different soil aggregates.In orchards of different ages,the SOC content of macroaggregates(>0.25 mm)was 48.83%and 25.68%higher than that of microaggregates(0.053-0.25 mm)and the silt and clay fractions(<0.053mm).The SOC content in the silt and clay fractions was about 17.54%higher than that of microaggregates.The one-way ANOVA showed that with the increasing age of orchards the percentage of macroaggregates increased significantly and the percentage of microaggregate and the silt and clay fractions decreased significantly,resulting in a higher SOC storage and contribution in macroaggregates than microaggregates and the silt and clay fractions.This indicated that macroaggregates play an important role in the SOC accumulation during the long-term orchard management.(2)The microbial biomass carbon(MBC)and extracellular enzyme activities in different soil aggregates increased with the orchard age.The MBC content of macroaggregates was approximately 40.26%and 14.88%higher than that of microaggregates and the silt and clay fractions.The?-glucosidase,?-glucosidase and sucrase activities decreased with the increase of the soil aggregate size.In the newly built,7-year orchard,the percentage contribution of?-glucosidase,?-glucosidase and sucrase activities increased with the decrease of soil aggregate size,while the percentage contribution of enzyme activity in 20-year,40-year orchard increased with the soil aggregate size decreasing.This indicated that with the increase of orchard age,SOC in macroaggregates was more vulnerable to the degradation of extracellular enzymes,compared with that in microaggregates and the silt and clay fractions.Pearson correlation analysis showed that MBC extracellular enzyme activities in different soil aggregates were significantly correlated with SOC.This demonstrated that the mineralization of SOC in different sized soil aggregates accelerated during the long-term orchard management.(3)Changes in microbial diversity and community composition affect the SOC distribution in different soil aggregates.The diversity and abundance of bacteria and fungi decreased with the increase of soil aggregate size,the uniformity of bacteria decreased with the increase of aggregate size,and the uniformity of fungi increased with the increase of aggregate size.Principal coordinate analysis showed that there were significant differences in bacterial and fungal community composition among different sized aggregates in orchards of different ages.In different orchards,with the increasing soil aggregate size,the relative abundance of Chloroflexi increased,and the relative abundance of Actinobacteria,Proteobacteria and Acidobacteria decreased.In 40-year orchards,the relative abundance of Ascomycota fungi decreased,and the relative abundance of Basidiomycota fungi increased with the increase of soil aggregate size.The increase in the relative abundance of Ascomycota fungi can help to form mycorrhizas,bind the soil particles into aggregates,and promote the accumulation of SOC.(4)The microbial properties mediated SOC in different sized soil aggregates under long-term orchard management.Although long-term orchard management has promoted the increase of extracellular enzyme activities and MBC content in different soil aggregates,and accelerated the SOC decomposition in different sized aggregates,the content and storage of SOC were both increased in different soil aggregates.In addition,the macroaggregates can accumulate more SOC than the microaggregates and the silt and clay fractions.Redundancy analysis indicated that the percentages of soil aggregates of 2-5 mm and<0.053 mm were significantly positively correlated and negatively correlated with SOC content and storage,respectively.This indicated that the percentages of these soil aggregate sizes can be effective indicator of SOC storage potential.