The Effects Of Rainfall Reduction And Phosphorus Addition On Leaf Litter Decomposition In Lowland Tropical Monsoon Forest

Litter refers to the general term of all organic matter produced and returned to the ground by the aboveground plant components in the ecosystem,which is the material and energy for the decomposer,so as to maintain the function of the ecosystem.Litter decomposition is one of the key processes of nutrient cycle in ecosystem.The study of decomposition process is helpful to understand the biogeochemical cycle of ecosystem and the formation of soil organic matter.Drought and biodiversity caused by global climate change will directly or indirectly affect the decomposition rate and nutrient loss dynamics of litter.At present,a lot of researches have been focused on the effects of drought on litter decomposition and the diversity effect in the decomposition of mixed litter.But the tropical area is generally limited by phosphorus,so it is very important to study the interaction between drought and phosphorus addition in tropical areas.Therefore,the paper chooses the Ganzha Ridge tropical lowland monsoon forest Ecological Reserve in Sanya City of Hainan Province as the research area,and selects four-tree species with high relative abundance,different chemical and morphological characteristics as the research objects,respectively,Hopea exalta,Koilotepas bainnense,Memecylon nigrescens,and Acronychia pedunculata.And the experiment is under the design of the two factors interactive control experiment of the reduction of rainfall（simulated drought）and phosphorus addition.There are three districts and blocks.Each district block is set with 4 treatments:simulated drought treatment D（-50%rainfall）,P-addition treatment(+50kg P hm^-2 yr^-1),simulated drought×P-addition treatment DP（-50%rainfall）×P(+50Kg P hm^-2 yr^-1),Control treatment C.Firstly,the dynamic changes of decomposition constant and C and N loss of four-tree species leaf litter under 4 treatments were studied to explore the effects of simulated drought and P-addition on the leaf litter decomposition and the interaction effect of simulated drought and P-addition.Secondly,the diversity effects of 11 groups of mixed leaf litter（6 groups of two tree species+4groups of three tree species and 1 group of four tree species leaf litter=11 groups）were studied,which were composed of four tree species leaf litter.The compensation effect,selection effect and net diversity effect were calculated to explore the diversity effect.And the relationship between C and N loss in the mixed leaf litter decomposition and the richness of mixed leaf litter was explored.The effects of simulated drought,P-addition and interaction effect on net diversity effect,C and N loss were studied.The main conclusions are as followed:（1）The decomposition constant,C and N loss of litter of different tree species were significantly different due to the different litter qualities.Among them,the litter with higher initial N content,and lower C/N decomposed faster;The N was released by the litter with higher initial N content,and N was enriched when the initial N content was lower;In the decomposition process,the C content of leaf litter of 4 species was always released,and the N content was different due to different initial litter qualities.（2）The simulated drought treatment inhibited C and N loss of litter decomposition,but it was related to the species;The P-addition treatment promoted C loss and inhibited N loss in litter decomposition;The interaction effect of simulated drought and P-addition treatment existed,and P-addition could regulated the effect of drought on litter decomposition.（3）Among the 11 mixed leaf litter,10 groups had significant diversity effect,and the net diversity effect and compensation effect were significantly greater than zero;The diversity effect,C and N loss rate of mixed litter decomposition were positively correlated with the richness of leaf litter mixtures.（4）The effects of simulated drought,P-addition and interaction of the two did not significantly affect the diversity effect.The main conclusions are as followed:Simulated drought treatment indirectly inhibit C and N loss of litter decomposition by reducing the physical effects of rainwater leaching and crushing,reducing the abundance and activity of soil animals and microbial decomposers,however for four-tree species leaf litter,the effect was different;In the litter decomposition,P-addition treatment could indirectly promoted C loss and inhibited N loss by improving the ability of soil microorganism to decompose high C content substances,and stimulated microbial decomposer to take use of soil P first,rather than to abttain P from litter;The effect of phosphorus addition treatment alleviated and even reversed the effect of simulated drought treatment;The diversity effect existed in the 10 groups mixed leaf litter,and the compensation effect was dominant,that is,the diversity effect may be from the complementary effect because of the mutual promotion of different niche species;The diversity effect,C and N loss rate increased with the increase of tree richness in mixed litter,and the mixed litter richness increased the abundance of material resources and the micro-environment conditions,improved the richness and activity of soil microbial community,therefore promoted litter decomposition,enhanced diversity effect and C and N loss;The effects of simulated drought,phosphorus addition and interaction effect had no significant influence on diversity effects.