Response Of Litter Decomposition To Karst Habitat Heterogeneity And Seasonal Variation

Litter is an important carrier connecting the above-ground and under-ground ecosystems,and is the main source of nutrients that flow into the soil through decomposition and nutrient release.It is closely related to the above-ground and underground ecological processes,and is crucial for maintaining the geochemical cycle and plant species diversity.Karst region is characterized by atrocious nature condition,nutrient deficiency and high habitat heterogeneity,which greatly limit the nutrient cycle of the ecosystem.Litter decomposition can continuously provide soil organic carbon(SOC)and nutrients for the karst ecosystem,thus promoting plant growth and affecting the productivity and stability of the entire forest community.The evergreen and deciduous broad-leaved mixed forest developed in the karst areas of southwestern China is a representative vegetation type,characterized by high biodiversity and wide habitat heterogeneity.Due to the fragility of the ecosystem,it is extremely easy to cause various ecological problems such as the loss of surface nutrient resources,land degradation and biodiversity reduction.Some studies have shown that litter decomposition can alleviate nutrient deficiencies in karst forest ecosystems to a certain extent,thereby improving soil quality and function.However,how litter decomposition affects soil properties and its response to seasonal variation under different karst heterogeneous habitats remains unclear.Therefore,this study focuses on typical karst forest communities and quantifies different survey plots as low,moderate and high heterogeneity habitats by calculating heterogeneity index.Then,two evergreen tree species(Ligustrum compactum and Itea yunnanensis)and two deciduous tree species(Carpinus pubescens and Quercus variabilis)were simulated decomposition in three types of heterogeneous habitats and four decomposition times(90 d,180 d,270 d and360 d)by decomposition bag method.Analyze the decomposition characteristics and explore the decomposition process of litter using decomposition model in different heterogeneous habitats.The content and release of nutrients and the degradation characteristics of refractory compounds in litter under different heterogeneous habitats were studied.Meanwhile,we studied the effects of litter decomposition on soil p H,C,N,P,K and enzyme activities,so as to clarify the interaction between litter decomposition and soil properties.Further,the response of litter decomposition traits and soil properties to seasonal climate change were revealed.The main research conclusions are as follows:(1)Decomposition time,heterogeneity and species treatment significantly affected the litter mass loss rate,it was showed as 0-90 d>90-180 d>180-270 d>270-360 d at different decomposition phases.In different heterogeneous habitats,the mass loss rate was as follows low>high>moderate;the mass loss rate was showed as L.compactum>I.yunnanensis>C.pubescens>Q.variabilis in different tree species.Also,the Olson model showed that the decomposition time of 95% on litter of L.compactum,I.yunnanensis,C.pubescens and Q.variabilis required 4.419-4.801 a,4.763-5.653 a,5.538-6.242 a and 7.219-7.416 a,respectively.(2)The nutrient release of litters under different decomposition time was significantly different.The organic carbon(OC)release from litters observed a trend of gradual decline in the early stage and fluctuated up in the middle and late stages of decomposition,showing a net release characteristics;N release was characterized by enrichment in the early stage and release in the middle and late stage of decomposition;P release was characterized by relative enrichment;K release presented a decreasing trend,which was characterized by net release during decomposition.Heterogeneity treatment significantly affected the release of C,N,P and K from litter,OC and P release were showed as low>moderate and high,and the release of N and P were showed as:high>low and moderate heterogeneous habitat.The different species treatment significantly affected the release of C,N,P and K,among which the OC release were presented as C.pubescens>L.compactum>I.yunnanensis>Q.variabilis and the release of N,P and K were showed as evergreen>deciduous tree species in different heterogeneous habitats.Besides,the mass loss of litter was positively correlated with the release of C,N,P and K.(3)Decomposition time,heterogeneity and their interaction significantly affected the content and release of lignin,cellulose,condensed tannin,and total phenol in the litter.The release of lignin,cellulose,condensed tannin,and total phenol in the middle and late stages was greater than that in the early stages of decomposition,showing the characteristics of net release.The release of lignin and total phenol were showed as moderate>low and high,the cellulose release was presented as low>moderate and high,and the release of condensed tannin was showed as high>low and moderate heterogeneous habitats.Heterogeneity and species treatments significantly affected the release of refractory compounds in litter,which was showed as deciduous>evergreen tree species,and the litter mass loss was significantly positively correlated with the release of refractory compounds.(4)Decomposition time and heterogeneity treatment significantly affected soil p H,organic carbon(SOC),total nitrogen(TN),total phosphorus(TP),total potassium(TK),ammonium nitrogen(AN),available phosphorus(AP),and available potassium(AK)content.Soil p H and SOC were greater in the middle and late stages than in the early stages of litter decomposition,showing an overall increase with the increase of time.Soil TN presented a trend of decreasing-increasing-decreasing-increasing,while TP and TK contents in soil presented the characteristics of increasing-decreasing.Meanwhile,the contents of AN,AP and AK in soil increased at the early stage and decrease at the middle and late stages of decomposition.Soil p H and TK were showed as low>moderate and high heterogeneity,SOC and TN were observed as moderate>low and high heterogeneity,and TP was observed as high>low and moderate heterogeneity.The C,P and K of litter were positively correlated with the availability of soil resources,while the N content and refractory compounds of litter were negatively correlated with the availability of soil resources.(5)The activities of urease,phosphatase,FDA hydrolase,β-glucosidase and cellulase in soil were significantly affected by the decomposition time of litter.Soil urease and cellulase activities showed a descending-ascending-descending trend with the decomposition of litter,soil phosphatase and β-glucosidase activities showed an ascending-descending-ascending trend,and FDA hydrolase activity presented an ascending-descending-ascending-descending trend.Meanwhile,the refractory compounds were negatively correlated with multiple soil enzyme activities.SOC,TN,TP and TK were positively correlated with soil enzyme activities,and increased with the increase of soil enzyme activities.(6)The decomposition response phase and heterogeneity interaction significantly affected the response ratio of litter C,N,P,K,lignin,cellulose,condensed tannin,and total phenol,in which litter C and nutrients were mostly negative in the early stage,positive in the middle and later stages of decomposition,and refractory compounds were mostly negative in different stages of decomposition.The phase response ratio of OC was presented as moderate>low and high,N and K phase response ratio was presented as low>moderate and high,and P phase response ratio was high>low and moderate heterogeneous habitat.The phase response ratios of soil p H,SOC,TN,TP,TK,AN,AP and AK were significantly affected by the interaction of decomposition response phase and heterogeneity.Among them,the phase response ratios of soil p H,SOC,TN,TP and TK were mostly negative in the early stage,but mostly positive in the late stage of decomposition;the phase response ratios of AN,AP and AK in different decomposition stages were mostly negative.Besides,the phase response ratios of soil p H,SOC and AN were moderate>low and high,and the phase response ratios of TN,TP,TK,AP and AK were high>low and moderate heterogeneous habitat.Mean annual temperature(MAT),mean annual precipitation(MAP),accumulated temperature(CT)and accumulated precipitation(CP)were significantly positively correlated with the response ratios of litter C,P,K,cellulose,total phenol,and soil C and N.(7)The interaction of decomposition time and heterogeneity had a significant effect on the decomposition response ratios of SOC,TN and TP,and the heterogeneity had a significant effect on the decomposition response ratios of soil AN,AP and AK.The decomposition response ratios of SOC and TN were presented a negative effect under different decomposition time and heterogeneous habitats.Soil TP response ratio showed positive effect in different decomposition time and in low and high heterogeneous habitats,but negative effect in moderate heterogeneous habitats;the response ratio of soil TP and TK decomposition showed a positive effect in different decomposition times and heterogeneous habitats;the response ratios of AN,AP and AK showed positive effects at different decomposition times and high heterogeneous habitats,but negative effects at low and moderate heterogeneous habitats.MAT,MAP,CT and CP were significantly positively correlated with the response ratios of soil p H,TP,TK and AN.In conclusion,there are significant seasonal differences in the decomposition characteristics of litter,the content and release of nutrient elements and refractory compounds under different heterogeneous habitats.Litter decomposition can accelerate the release of nutrient elements and refractory compounds,and promote the improvement of soil nutrients and enzyme activities,thus improving soil fertility.At the same time,habitat heterogeneity significantly improved litter decomposition,soil nutrients,and enzyme activities,but their changes were significantly different in low,medium,and high heterogeneous habitats.Further,litter decomposition traits decreased gradually with seasonal changes,showing a negative effect,while soil properties mostly show positive effects,indicating that litter decomposition can actively respond to seasonal climate changes,and its decomposition process is jointly regulated by decomposition time,habitat heterogeneity and climate factors.The results of this study contribute to the in-depth understanding of the biogeochemical cycle mechanism of karst forest ecosystems,and provide theoretical support for predicting the carbon sequestration and nutrient turnover potential of degraded ecosystems under the background of global climate change,which is beneficial to the maintenance of soil ecological functions and the protection and management of forest vegetation.