Effects Of Rubber-based Agroforestry Patterns On Soil Carbon And Nitrogen Stocks And Microbial Communities

Rubber（Hevea brasiliensis）monoculture plantations uauslly generate environmental problems.It is very important to improve the ecosystem function and service of rubber plantations through transformation of forest stand and other methods.Agroforestry systems are considered as the alternatives strategy to promote rubber tree monocultures,but information is limited on the effects of rubber-based agroforestry systems on soil organic carbon storage（SOCs）and total nitrogen storage（TNs）.The study was conducted in Xishuangbanna of Southwest China,and the aim was to reveal the effects of rubber-based agroforestry systems on SOCs and TNs.The four intercropping models include rubber monoculture（RM）and three agroforestry treatments,i.e.intercropping with Camellia sinensis（HCS）,Coffea liberica（HCL）,and Theobroma cacao（HTC）.Moreover,effects of exogenous carbon addition on soil organic carbon mineralization,soil physicochemical properties and enzyme activities were studied using ^{1 3}C labeled glucose.Soil microbial community composition,diversity and functional gene copies（cbb L and nos Z genes that related to the C and N cycling）in different rubber-based agroforestry systems were analyzed by high-throughput sequencing and quantitative PCR,and their relationships with soil physicochemical properties were explored.The main results are as follows:（1）In the study area,SOC storage was between 56.14–70.95 Mg hm^-2,and TN storage was between 5.7–7.0 Mg hm^-2.The HCL treatment facilitated SOC and TN contents and enhanced SOC storage（SOCs）and TN storage（TNs）（0–40 cm）in both dry and rainy seasons.Compared with rubber monoculutre,the SOCs and TNs in the 0–10cm soil layer of the two modes of HCS and HCL increased significantly.Structural equation modeling revealed that the copy numbers of cbb L and nos Z were mainly affected by soil p H and bulk density（BD）（R²=0.37 and 0.35,respectively）,and the abundance of cbb L and SOC content directly affected SOCs（R²=0.46）,and the abundance of nos Z and TN content directly affected TNs（R²=0.22）.Our results indicate that the use of C.liberica as intercrop should contribute to retard soil degradation in rubber monoculture.Combining the abiotic and microbial pathways,we suggest that the enhancement of C and N sequestration by intercropping C.liberica with rubber can be attributed to the increased inputs of root and litter,and to the stimulation of the microorganisms that regulate C and N cycles,therby promoting soil carbon and nitrogen retention.（2）Incubation experiments showed that the addition of glucose increased the content of microbial biomass carbon（MBC）in three intercropping treatments,and the ^{1 3}C-glucose of three intercropping treatments was more integrated into the MBC instead of the SOC in the soil,indicating that the utilization efficiency of soil microorganisms in the intrcropping treatemnts were higher than that of rubber.Meanwhile,the MBC content of HCS and HCL was significantly higher than that of the treatment without glucose additionduring the 30-day incubation,indicating that glucose addition could effectively promote the microbial activities in HCS and HCL.After glucose addition,the content of NO₃^--N and NH₄⁺-N in three intercropping treatments decreased,which might be that soil microorganisms obtained the carbon source for growth from the newly added glucose,and the cell proliferation also increased the requirement of nitrogen and thus led to a decarease in the content of the soluble nitrogen in the soil.The results of the changes of enzyme activity may also prove this:in the three intercropping modes,theβ-1,4-glucosidase activity was significantly reduced in the early period of culture(the 5^{t h} day)and the N-acetylglucosaminidase（NAG）enzyme activity was significantly increased,confirming that the addition of glucose alleviated the the microbial carbon limitation while increased the nitrogen limitation.（3）The addition of glucose generally induced the positive priming effect（PE）at the surface soil（0–10 cm）in the three intercropping treatments,and accelerated the SOC mineralization.While in the rubber monoculture,we found the negative PE and the highest net carbon increase,indicating that increasing the input of exogenous carbon through intercropping or other agricultural measures would promote the accumulation of SOC.For deep soil（20–40 cm）,the carbon loss caused by PE exceeds or is marginaly equivalent to the residual amount of glucose,resulting in a decrease or marginaly change of net carbon,indicating that deep soil is more sensitive to the addition of exogenous carbon compared with surface soil.Increasing the carbon input to deep soil will stimulate the decomposition of SOC,which may lead to the loss of SOC.（4）In general,rubber intercropping models have significantly changed the structure of soil microbial communities,but no significant impact on the diversity of microbial comunitites.The dominant phyla of soil bacteria in this study area are Acidobacteriota,Proteobacteria,Actinobacteriota and Chloroflexi,among which the relative abundance of Acidobacteriota is the highest（24%–36%）.The two dominant phyla of Chloroflexi and Proteobacteria have significant differences between treatments,and there are also significant differences in the rare phyla,such as Verrucomicrobiota,Nitrospirota,Planctomycetota,Myxococcota,GAL15,WPS＿2 and RCP2＿54.The dominant phylum of soil fungi are Basidiomycota and Ascomycota.The soil p H,bulk density,ratio of C:N,SOCs,TNs and other indicators of different soil layers have correlations with the stucture of soil bacteria and fungi,indicating that these soil physical and chemical properties are important in driving changes in mirobial communities.Overall,the study indicated that the proper intercropping systems will benefit the storage of SOC and TN storage and prevent soil degradation in rubber monoculture.The soil respiration process and the microbial-related mechanisms need more studies in agroforestry systems in the future.