Effects Of Litter Composition Diversity On Soil Aggregate Stability And Its Mechanism Analysis

At present,there are divergent conclusions about the effect of litter diversity on soil carbon stability.Soil aggregates are the basic units of soil structure and play an important role in the formation and protection of soil organic carbon.In response to global climate change,exploring the effects and mechanisms of litter diversity on soil aggregate stability is of guiding significance for forest management and afforestation tree species selection.In this study,leaf litter was collected from eight dominant tree species in the northeastern region of China,and a one-year laboratory litter culture experiment was conducted to explore the effects and mechanisms of tree species richness of litter(1,2,4,8)(1,2,4 and 8 kinds of litter were mixed with soil)and arbuscular mycorrhizal(AM)tree fraction of leaf litter(0,25%,50%,75%,100%)on soil aggregate stability.The main research results are as follows:(1)Tree species richness and AM tree fraction of litter have significant effects on the particle size distribution and stability of soil aggregates.With the increase of tree species richness of litter,the proportion of macroaggregates increased from 29.83% to 40.64%,the proportion of microaggregates(0.053-0.25 mm)increased from 43.91% to 48.98%,and the proportion of silt and clay(< 0.053 mm)decreased from 21.23% to 11.98%.Increase in mean weight diameter(MWD)from 0.40 mm to 0.53 mm and geometric weight diameter(GWD)from 0.19 mm to 0.28 mm.With the increase of AM tree fraction of litter,the proportion of macroaggregates increased from 29.33% to 33.74%,with no effect on the proportion of microaggregates,and the proportion of silt and clay decreased from 20.17% to 15.32%;The MWD increased from 0.41 mm to 0.46 mm and the GWD from 0.20 mm to 0.23 mm.(2)Tree species richness and AM tree fraction of litter significantly affected the content of glomalin-related soil protein(GRSP)in different aggregate sizes.The content of easily extractable GRSP(EEG)and total GRSP(TG)was different in different aggregate distributions,and in general,the content was the highest in the macroaggregates,followed by the microaggregate,and the silt and clay content was the lowest.(3)Tree species richness and AM tree fraction of litter had different effects on the soil organic carbon(SOC)and total nitrogen(TN)contents in different aggregate particle sizes.The SOC and TN content of macroaggregates increased with the increase of tree species richness and AM tree fraction of litter.When the tree species richness of litter was 8,the SOC and TN contents reached the maximum(44.44 g/kg and 2.04 g/kg,respectively).When AM tree fraction of litter was 100%,the SOC content reached the maximum of 39.09 g/kg,and in the treatment with AM tree fraction of leaf litter being 75%,the TN content reached the maximum of 0.89 g/kg.Tree species richness and AM tree fraction of litter had weak effects on microaggregate SOC and TN content.With the increase of tree species richness of litter,the SOC content of silt and clay decreased from 10.84 g/kg to 8.06 g/kg,and the TN content of silt and clay decreased from 0.86 g/kg to 0.76g/kg.However,with the increase of AM tree fraction of litter,the SOC content of silt and clay was gradually reduced,and the TN content was gradually increased.(4)Structural equation modeling(SEM)results showed that tree species richness of litter,AM tree fraction of leaf litter,GRSP,SOC,and TN played important roles in stabilizing soil aggregates.Although the direct effect of tree species richness of litter on aggregate stability was-0.212,the 28% direct effect could be cancelled out by directly affecting SOC(0.182),TN(0.338),TG(0.331),and EEG(0.203),thereby improving aggregate stability.AM tree fraction of litter had a weak direct effect on aggregate stability,but it could affect aggregate stability by indirectly affecting TG(0.253)and SOC(0.321).The species richness and AM tree fraction of litter promoted the accumulation of SOC,TN and GRSP in the macroaggregates,and thus improved the stability of the aggregates.In future forest management,mixed forest with various tree species and planting of AM tree species will effectively improve soil structure and aggregate stability.