Responses Of Soils And Plants To Water Addition In A Degraded Karst Ecosystem In Southern Yunnan Province Of China

Massive degraded karst plant communities have existed in southwestern China due to typical geological conditions and strong human disturbances like agricultural extension,grazing,firewood production,and urban intensification.Karst topography is mainly characterized by an extremely slow soil formation from underlying limestone,which leads to decrease the water retention capacity in the soils.Such extensive land degradation and deterioration of vegetation causes severe drought and soil erosion,and creates a desert-like landscape in the karst regions.Drought stress is one of the most common factors in limiting the growth,survival and distribution of plants in karst areas,as is the case with microbe communities.Thus,most of the growing plant species and microbial communities in the karst areas have adapted for the arid climate conditions in the long-term evolution.Furthermore,the chemical compositions of soils were defined with those plants and microbes under such climatic conditions.There is an increasing trend of precipitation according to the relevant predictions,but it is mainly uncertain how such increased moisture affects soils,microbial populations and plants.Climate change is one of the critical factors,posing serious threats to karst hydrogeology and affecting the growth and distribution of plants due to increasing trend of precipitation in the karst areas,southwestern China.In this study,we conducted an manipulated-precipitation experiment on a mixed grass and shrub community in the karst area in Jianshui County of Yunnan Province of southwestern China,with four levels of water additions(CK,T1,T2 and T3 indicating 0%,+20%,+40% and +60% relative to the current monthly precipitation,respectively).The experiment of water addition started at the beginning of April 2017,and soils and plants were sampled at the end of May and August.The chemical compositions of soils,the community of soil microbes,and the elemental stoichiometry and metabolites of dominant grasses and shrubs were examined under the water addition regime.Our study aims to understand how increasing precipitation impacts the karst ecosystems in the future.Main findings are as follows:1.Water addition altered elemental concentrations and stoichiometry of soilsSoil degradation,including geo-ecological destruction,of the karst regions in China is severe,and the elemental stoichiometry of degraded soil is much sensitive to variation in moisture due to the limiting roles of water on soil processes in karst ecosystems.Soil sampling was conducted at the end of May(dry season)and August(wet season),respectively.The concentrations of soil macroelements(C,H,N,P,K,Ca,Mg,and S),micro elements(Mn,Fe,Zn,and Cu),and non-essential elements(Na,Al,and Si)were measured.Soil water content(SWC)and soil p H were also measured.The results showed that SWC increased with increase in water addition in both the seasons(p < 0.01).Compared to CK,soil p H in the wet season increased significantly in water-treated plots(p < 0.01),whereas no significant changes were observed in the dry season.With water addition in both the dry and wet seasons,the concentrations of non-metal soil elements(C,H,N,S,and P)except for Si were relatively stable,and the metal elements(K,Na,Mg,and Fe)along with Si increased significantly,whereas Zn and Ca decreased.Soil Ca/Mg and Mg/K ratios showed a decrease with water addition in both the seasons(p < 0.01).With increasing moisture from CK to T3,Ca/Mg ratio decreased significantly and approached ideal condition for above-and below-ground communities.These results suggested that increasing precipitation in the future might differentially impact the stoichiometry and dynamics of multiple elements in soils in the degraded karst areas of southwestern China.2.Bacterial communities are sensitive to water addition compared to fungal communitiesVariation in the amount and patterns of precipitation may alter microbial communities and ecological processes,due to variations in soil moisture content.However,the mechanisms and patterns of soil elemental composition and microbial community responses to precipitation have not been well documented in karst soils.Sampling was conducted in May(dry season)and at the end of August(wet season).Total DNA of soil samples was analyzed through temperature gradient gel electrophoresis fingerprinting and gene libraries(16S r RNA).Our results showed that in the dry season,water addition did not alter the community compositions of both bacteria and fungi.However,with water addition in the wet season,the relative abundance for Actinobacteria increased and that for Acidobacteria,Planctomycetes and Verrucomicrobia decreased among the bacterial phyla,but no changes were observed for any fungal phyla.Furthermore,the alteration of bacterial community structure and diversity was mainly accounted for by an increase in soil K and Na concentrations due to water addition.These results suggest that with climate change in the future,increasing rainfall might alter the elemental composition of the karst soils,and bacterial communities may become more sensitive to variation in soil moisture as compared to the fungal communities.3.Differential stoichiometric responses of shrubs and grasses to water additionThe elemental concentration of both the soils and plants are sensitive to variation in precipitation due to the limiting roles of water on soil processes and plant growth in karst ecosystems.Plants grow under severe drought conditions in karst areas of southwestern China,but precipitation is predicted to increase in this region.However,it is unclear how elemental composition of soils and plants would respond to such increase in moisture,in particular,how K behaves as a key mediator in regulating water potential of plants.In this study,the responses of elemental composition and K stoichiometry in both soils and dominant plant species to water addition are investigated.Two perennial grasses(Cymbopogon distans and Arundinella setosa)and two shrubs(Carissa spinarum and Bauhinia brachycarpa)are investigated under four levels of watering treatments(CK,T1,T2 and T3).Macro-elements(C,H,N,P,K,Ca,Mg,and S),micro-elements(Mn,Fe,Zn,and Cu),and non-essential elements(Na,Al,and Si)are measured for soil and leaf samples.With water addition,leaf N and P kept unchanged in all the four species,while leaf K,Mg,and S decreased in both shrubs(higher C/K,N/K,and P/K),and leaf Fe,Si,and K increased in both grasses(lower C/K,N/K,and P/K),which suggests distinct K roles for water regulation potential in shrubs and grasses.These findings imply that elemental composition of both soils and plants would be altered with increasing precipitation in future,and different plant types would adopt distinct K-regulation strategies to cope with variable moisture in soil systems.4.Differential metabolic responses of shrubs and grasses to water additionWe investigated the impacts of water addition on leaf metabolites of two grasses and two shrubs in this area.For all four species,sphingolipids substantially increased in the leaveswith increasing water supply(p < 0.001),suggesting that these plants adopted biochemical strategy to tolerate the wet stress.However,both shrubs showed decreases in valine and threonine(amino acids),threonate,succinate and ascorbic acid(organic acids),galactose and rhamnose(sugars)and epicatchin and oleamides(secondary metabolites)with increasing water supply.Both grasses increased in the total metabolites at T1,but the total metabolites in A.setosa significantly decreased at T2 and T3 while unchanged in C.distans.Tri-carboxylic acid cycle and amino acid metabolism in shrubs and Shikimate pathways in grasses were strongly affected with water supply.Overall,shrubs and grasses respond differentially to variation in water addition in terms of metabolomics,which is helpful in understanding how plants respond to climate change.Soil degradation of the karst regions in southwestern China is extremely severe,and the plants and soil microbes showed sensitivity to variation in moisture due to the limiting roles of water on soil processes in karst ecosystems.Our findings imply that elemental concentrations of soils,microbial populations of soils,and stoichiometry and metabolomics of plants would be altered with increasing precipitation in future,which would help to better understand the impact of climate change on the ecological processes of karst regions in southwestern China.