Effect Of Drought On Soil Aggregates Stability And Its Biological Regulation Mechanism

As soil is the largest carbon(C)pool in terrestrial ecosystem,soil aggregate plays a vital role in maintaining C balance.The decline in stability of soil aggregate may induce soil structure degradation,severe disturbance on C cycle process,which further modifies C exchange between terrestrial ecosystem and atmosphere.Plant roots,microbes,soil C and soil nitrogen(N)were the main important factors that affecte soil aggregate stability by directly or indirectly regulate aggregate formation and evolution process.Over past decades,global change significantly enhanced the frequency and intensity of extreme drought,resulting in important impacts on plant roots,microbes,soil C and N cycle process as well as the dynamic development of soil aggregates.Therefore,understanding the effects of drouht on soil aggregate and explore its potential mechanisms has important ecological significances for us to accurately forecast and evaluate future global C budget balance.This study based on Tiantong national forest ecosystem observation and research station(Tiantong national forest park,Ningbo,Zhejiang Province),selected secondary forest aged from 50 to 70 as research objects.We simulated extreme drought via PVC transparent plastic isolation for natural precipitation isolation and evaluated plant root function by trenching method.The paremeters including soil aggregate mean mass diameter,plant root biomass and morphological structure,microbial PLFAs,soil C and soil N content were simultaneously measured.Based on the above method,we analyzed and discussed regulation process of plant roots on aggregate under drought.The main conclusions were as follows:(1)Drought dramatically influenced root biomass,morphological structure and soil N.Drought significantly increased total and fine root(<2mm)biomass,specific root length,specific root area and root N content,but decreased root tissue density.Meanwhile,drought had no significant effect on fine root mean diameter and C content.Drought evidently reduced aggregate mean mass diameter of trenching soil,but had slight influence on soil outside trenching.Drought had no effect on soil C and N both inside and outside trenching,but soil C and N in outside trenching was significantly higher than these in trenching soil under both drought and control treatments.Correlation analysis showed that macroaggregate(>2mm)had positive correlation with soil C and N,fine root biomass and specific root length,but no correlation between aaggregate(0.25-2mm,<0.25mm)and these parameters were observed.These results demonstrated that fine root biomass and morphological structure were the primary factors in regulating soil aggregate stability under drought condition.(2)For soil inside trenching,drought treatment didn't alter total microbial PLFAs,but significantly reduced arbuscular mycorrhizal fungi(AMF),and increased the ratio of gram-negative(G-)bacteria and gram-positive(G+)bacteria.Drought had no effect on the ratio of fungi and bacteria(F:B).For soil outside trenching,drought had no significant effects on total microbial PLFAs and community structure.Drought treatment significantly increased microbial stress index inside trenching,while had no effect on these in outside trenching.In addition,correlation coefficient suggested that both soil aggregate mean mass diameter and the proportion of >2mm aggregate exhibited significantly positive correlation with total microbial PLFAs,bacterial PLFAs,fungal PLFAs,G+ bacterial PLFAs,G-bacterial PLFAs,AMF PLFAs and actinomycetic PLFAs.(3)Structural equation model showed that trenching soil,soil microbes are the major factors in affecting soil aggregate stability and >2mm aggregate,and soil moisture were the most important factor for 0.25-2mm and <0.25 mm aggregate.As for soil outside trenching,fine root exhibited the most significant effects on soil aggregate stability and >2mm aggregate than other parameters.In conclusion,the formation and development of soil aggregates is a complex process,which was co-regulated by various biotic and abiotic factors.The regulating mechanism were differ among different particle size.Meanwhile,there were still many uncertainty in the responses of different factors to drought,which probably complicate the formation of soil aggregate.Therefore,our findings highlight the importance of the effects of plant roots,microbes,soil C,soil N and other nonbiological and biological factors on soil aggregate,which may need to be incorporated into regional and global models for predicting effects of climate change on global forest and assessing the climate-biosphere feedbacks.