Response Of Microbial Community To Nutrient Limitation In Soil Aggregates During Conversion Of Cropland Into Grassland

Returning cropland to grassland is an effective strategy for mitigating soil erosion and promoting vegetation recovery in arid and semi-arid regions.A systematic understanding of soil biogeochemical processes during this process can enhance land use efficiency and facilitate ecosystem restoration and development.Soil aggregates play a critical role in soil structure,with many soil microorganisms residing in environments closely associated with them.The metabolic activities of these microorganisms have a direct impact on soil material and energy flow.However,the metabolic characteristics of soil microorganisms in aggregates and their response to nutrient limitations during the cropland-to-grassland conversion process remain unclear.In this study,soil aggregates were fractionated of different sizes through both dry and fresh soil samples collected at various stages of the cropland-to-grassland conversion process on the Loess Plateau.Then,this study investigated changes in the water stability of soil aggregates,as well as total and available nutrients,extracellular enzyme activities related to the C,N,and P cycles,and microbial community changes in four grassland(GL)restoration years(7years,18 years,35years and 45years)and one cropland(CL).The study also aimed to investigate the driving mechanisms behind microbial metabolic restrictions and community structure in different aggregate sizes,thereby providing a foundation for understanding nutrient cycling processes of carbon,nitrogen,and phosphorus that microorganisms participate in at the soil aggregate scale during vegetation restoration.The main results were as follows:(1)Returning cropland to grassland on the Loess Plateau is a beneficial practice that enhances water stability in soil aggregates and promotes the transformation of microaggregates(<0.25 mm)to macroaggregates(>0.25 mm).The soil aggregate waterstability indices in the restored grassland were significantly higher than those in cropland(CL,P<0.05),and the mean weight diameter(MWD)and geometric mean diameter(GMD)decreased as follows: GL35 > GL45 > GL18 > GL7 > CL,indicating improvements in soil structure.(2)At the early stage of recovery(GL7),the soil aggregates nutrient content(i.e.,C and N)decreased,while it gradually increased at the middle stage of recovery(GL18,GL35)and the late stage of recovery(GL45).Enzyme activities related to the C and N cycles,microbial biomass(i.e.,C and N),and nutrient content exhibited a similar trend over time.However,the activity of enzymes related to the P cycle increased with the recovery period,indicating that phosphorus in the soil was being consumed faster than its supplement amount,leading to the secretion of P acquisition enzymes by microorganisms to maintain their own growth and metabolism.Grassland restoration leads to soil nutrients migrating from largemacroaggregates(5-2 mm)to small-macroaggregates(2-0.25 mm)and microaggregates(<0.25 mm),promoting nutrient accumulation and protection.The lower microbial biomass and extracellular enzyme activity were mainly distributed in microaggregates(<0.25 mm).The interaction between soil p H and water content,extracellular enzyme activity,and microbial biomass regulated the nutrient content of soil aggregates(i.e.,C,N,and P).Furthermore,extracellular enzyme activity and microbial biomass were found to be the main factors affecting the nutrient cycle in soil aggregates during conversion of cropland to grassland.(3)The soil aggregate microorganisms were limited by relative C,N,and P during the process of returning cropland to grassland.During vegetation restoration,the relative C and N limitations of microorganisms gradually weakened,and the microbial community in GL45 had the lowest C limitation.The relative P limitations of microorganisms gradually increased in the early and middle stages(GL7,GL18,and GL35),but decreased in the late stage(GL45),indicating that the long-term conversion of cropland to grassland improved the nutrient environment for microbial life.Aggregate nutrient content,microbial activity,and diversity were the main driving factors for microbial metabolic restriction.Additionally,the microbial C limitation in microaggregates(<0.25 mm)was significantly higher in the early and middle stages(GL7 and GL18)than in the large-macroaggregates(5-2 mm)and smallmacroaggregates(2-0.25 mm)(P<0.05).Furthermore,the enhanced ability of aggregates to protect soil carbon effectively alleviates the relative C limitation of microorganisms.The C limitation in microaggregates(<0.25 mm)was significantly reduced in the middle and late stages(GL35 and GL45,P<0.05).(4)The conversion of cropland to grassland had a profound impact on the species abundance and composition of bacterial and fungal communities in soil aggregates(P<0.05).This promoted the transformation of bacterial communities from an r-strategy to a K-strategy,with an increase in the abundance of Actinobaciota and Acidobacterium in soil aggregates(oligotrophic bacteria,r-strategy),and a reduction in the abundance of Proteobacteria(copiotrophic bacteria,K-strategy).The r-strategy group of bacteria tending to inhabit microaggregates(<0.25 mm).Grassland restoration also caused a decrease in bacterial α-diversity and an increase in fungal α-diversity,with the late stage of vegetation restoration(GL45)including the most species of bacteria and fungi with relative differences in abundance.The α-diversity of soil aggregate bacterial and fungal communities was found to be relatively high in small-macroaggregates(2-0.25 mm)and microaggregates(<0.25 mm).Furthermore,returning cropland to grassland significantly affected the community structure of bacteria and fungi in aggregates(β-diversity)(P<0.05),while having no significant impact on microbial community succession with respect to aggregate size fractions(P>0.05).The response of aggregate fungal communities to environmental and nutrient limitation was found to be more sensitive than that seen in bacterial communities.The association between bacterial and fungal communities and environmental factors(i.e.,soil moisture,p H,nutrients,and microbial biomass)was found to be closer to small-macroaggregates(2-0.25 mm)and microaggregates(<0.25 mm).The supply of resources and nutrient limitation in soil aggregates promoted changes in microbial communities.The impact of nutrient limitation on community diversity was more pronounced with soil aggregate size reduction,while community diversity regulated the secretion of extracellular enzymes.