Responses Of Plant And Soil Communities To Nitrogen Input And Liming In A Fallowing Red Soil Cropland

Soil degradation caused by agricultural intensification seriously threatens the agricultural ecosystem functioning in southern China.How to effectively manage soil degradation to restore the soil ecological function sustainability has attracted much attention.At present,the red soil region is facing the dual impacts of nitrogen deposition and anthropogenic managements.Although there have been many studies on the restoration of ecosystem functioning of terrestrial ecosystems in red soil region,the mechanism of the restoration of degraded red soil by fallowing is still unclear.Here,a field experiment was conducted on degraded red soil upland in southern China,which stimulated N deposition with lime application,to study the plant and soil communities’chronological trajectories after fallow and to better understand potential effects of environmental changes on restoration process.Variations and connections among plant,soil microbial as well as soil nematode diversity,community dynamics and stability were analyzed to investigate the effects and potential mechanisms of nitrogen input and lime application as well as their interactions on the changes aboveground-belowground community after fallow.This research provides new experimental evidence illustrating the aboveground-belowground processes of terrestrial ecosystem and provides a theoretical reference for sustainable agriculture.The main results are as follows:In general,the concentration of soil organic carbon,total nitrogen,and dissolved organic carbon（DOC）increased fallow while soil mineral N and available P decreased after 5 years fallow with or without N input and liming,and soil microbial biomass carbon（MBC）decreased at first but increased later,microbial biomass nitrogen（MBN）and phosphorus（MBP）.Soil p H decreased by 0.2 and 0.3 under lower(45 kg N ha^-1 y^-1)and higher N(90 kg N ha^-1 y^-1)input respectively while increased by 1.2-1.8 under liming in 5 years.High level N input and liming significantly increased soil organic carbon,Nitrogen input could increase MBC but decrease MBN and MBP while liming showed opposite effects on MBC,MBN and MBP compared to N input.Nitrogen input showed no significant effects on soil enzyme activities and liming significantly decreased hydrolase but increased oxidase activities.Overall,changes in soil stoichiometry and enzymatic stoichiometry implied increased N and P limitation after fallow,additionally,nitrogen input exacerbated P limitation and liming exacerbated both N and P limitation.In general,plant aboveground biomass increased first and then decreased while plant richness and abundance showed multiple-polynomial relationship with time since fallow with or without N input and liming.Plant Shannon-Weiner index increased with increasing time since fallow.Both N and lime input could increase plant biomass.N input could decrease forbs richness and abundance,while could increase graminoids richness and decrease the abundance.Liming decreased graminoids abundance but increased forbs abundance.Nitrogen input and liming increased plant Shannon-Weiner index while their interaction decreased plant Shannon-Weiner index.Nitrogen input could decrease the stability of graminoids or forbs and liming showed no effects on plant community stability.Thus,N input and liming could drive plant community changes by altering different plant functional groups after fallow.In general,the gram-positive to gram-negative ratio（G+:G-）as well as the fungi to bacteria ratio（F:B）characterized by phospholipid fatty acid and the microbial community Shannon-Wiener index increased with increasing time after fallow.Soil microbial diversity increased with increasing time since fallow.N input showed negative effects on microbial Shannon-Wiener index and decreased the community stability of bacteria and fungi.Liming could increase fungi PLFAs content,F:B ratio as well as microbial Shannon-Wiener index but decreased the G+:G-ratio.The relative abundance of microbivores（bacterivores and fungivores）increased,and relative abundance of herbivores decreased with increasing time since fallow with or without N input and liming.The relative abundance of omnivores-predators fluctuated with time after fallow.The richness and Shannon-Weiner index of soil nematode increased after fallow.Nitrogen input significantly increased bacterivore and herbivore abundance.Liming could increase nematode abundance except for herbivores.Furthermore,interactions of liming and N input increased herbivore abundance.Nitrogen input decreased nematode richness and liming increased nematode Shannon-Weiner index,however,liming decreased nematode Shannon-Weiner index under N input.Both nitrogen input and liming could increase nematode species turnover rate.Liming could increase bacterivore community stability but decrease their synchrony and N input could increase fungivore community stability but decrease their synchrony.The results showed that N input and liming could change the development of nematode community,however,the interactions of N and lime had an adverse effect on nematode diversity.The partial least squares path model results demonstrated that for fallowing agricultural ecosystem of red soil,N input could regulate plant-soil community,thereby ecosystem functioning by enhancing soil resources（soil organic carbon and nutrients）and decreasing soil p H.In contrast,liming affected ecosystem by significantly increasing soil p H but exacerbating N and P limitation.In detail,soil resource promoted graminoids but stress forbs,and soil p H only promoted graminoids and soil microbial community.Graminoids negatively influenced microbial community,nematodes,and eco-functions.Forbs could promote microbial community and ecosystem functioning.Changes in soil resources and p H induced by N input and liming could affect soil community and functions by altering the competition of graminoids and forbs.In conclusion,fallow could be effective management to restore the biodiversity of degraded red soil,however,natural N deposition or anthropogenic N input may have potential adverse impact on the restoration process by exacerbating soil acidification and phosphorus limitation.Lime application could be an effective approach to alleviate and offset soil acidification caused by N input.Nevertheless,the potential threats to nutrient limitation induced by liming and the negative impact on nematode diversity of liming and N input could not be ignored.This study will help to understand the aboveground-belowground interactions in soil biodiversity and function restoration,and will also expand understanding of the structural and functional regulation mechanism of soil ecosystems.