Evaluation Of Water Conservation And Fertilizer Conservation Function Of Dalbergia Odorifera And Acrocarpus Fraxinifolius Plantations In Karst Area

Introducing and cultivating precious species such as Dalbergia odorifera and Acrocarpus fraxinifolius in rocky desertification area can not only improve the ecological environment,but also greatly boost the economic benefit of woodland and increase the income of local farmers in the future.Mashan County of Guangxi province is one of the areas with the most serious rocky desertification in Guangxi.In this study,the 8-years old D.odorifera and A.fraxinifolius plantations planted in karst area were choosen as the research objects to the problem of vegetation restoration in karst area.The differences between D.odorifera and A.fraxinifolius plantations were compared and analyzed by field measurement and indoor analysis,including the forest productivity,water conservation,soil fertility improvement,enrichment and redistribution of plant nutrients,soil bacterial diversity and enrichment of nitrogen-fixing bacteria,which will provid the scientific basis for tree species selection,forest tending,and management in karst area.The results were as follows:（1）In karst area,DBH and tree height increase of D.odorifera plantation（3.2 cm,2.18 m）were slightly higher than those of A.fraxinifolius plantation（1.36 cm,1.67 m）.However,the biomass of branches,leaves and roots（3.01t/hm²,2.19 t/hm²and 1.49 t/hm²）in D.odorifera plantation was significantly higher than that in A.fraxinifolius plantation（0.62 t/hm²,0.45 t/hm²,0.44t/hm²）.（2）The interception ability of tree layer in D.odorifera plantation to precipitation was significantly higher than that of A.fraxinifolius plantation.The water conservation capacity of D.odorifera plantation ecosystem was about859.11 t/hm²,of which the soil water retention capacity accounted for 98.66%.The water content of the A.fraxinifolius plantation ecosystem was 836.24 t/hm²,and the soil layer accounted for 99.40%.（3）The contents of organic matter,C,N,P and K in soil of D.odorifera plantation were 39.84,23.16,3.71,1.48 and 4.62 g·kg^-1,and the N contents in leaves,branches,lateral roots and undergrowth vegetation were 25.28,11.64,12.47 and 18.48 g·kg^-1,C and P contents in lateral roots and K contents in undergrowth vegetation were 468.2,0.8 and 20.79 g·kg^-1,respectively,which were significantly higher than those in A.fraxinifolius plantation.But the content of C and P in leaves and branches,and the content of K in branches and litter were significantly lower than that of A.fraxinifolius.In general,D.odorifera had a higher enrichment capacity for soil N elements,while A.fraxinifolius had a higher enrichment capacity for soil P and K.In addition,the C reserves of leaves,branches,lateral roots and litter,the N reserves of each component,the P reserves of leaves,branches,lateral roots and litter,and the K reserves of other components except litter in D.odorifera plantation were significantly higher than those in A.fraxinifolius plantation.The stoichiometric ratio analysis showed that the C:N,C:P and C:K ratios of soil in D.odorifera plantation were 6.25,15.63 and 5.04,and the C:N ratios of leaves,branches,lateral roots and undergrowth were 15.84,38.73,38.00 and21.47,respectively,which were significantly or very significantly lower than those of A.fraxinifolius plantation.Except for the litters,the N:P ratio of each part in D.odorifera plantation was significantly higher or significantly higher than that of A.fraxinifolius.Finally,principal component analysis showed that the nutrient absorption and retention capacity of D.odorifera plantation was stronger than that of A.fraxinifolius plantation.（4）The fourαmicrobial diversity indices（Chao1 index,Faith pd index,Shannon index and Simpson index）of the two species showed that the rhizosphere soil of D.odorifera was significantly higher than that of A.fraxinifoliuss,indicating that the bacterial microbial diversity in the rhizosphere region of D.odorifera was better than that of A.fraxinifoliuss.At the phylum level,the dominant phylum of rhizosphere soil bacteria of D.odorifera and A.fraxinifoliuss was Actiobacteria,Proteobacteria,Acidobacteria,Chloroflexi,Nitrospirae and so on.The relative abundance of these five phyla in the rhizosphere of A.fraxinifoliuss accounted for 33.74%,22.42%,16.04%,11.58%and 6.83%,respectively,accounting for 90.61%in total.In the rhizosphere of D.odorifera,they were 29.98%,23.66%,18.56%,12.70%,and 5.83%,respectively,totaling 90.74%.At the same time,there were abundant nitrogen-fixing bacteria in the rhizosphere soil of the two species,and the main species were Rhizobium,Mesorhizobium,Bradyrhizobium,etc.,with obvious enrichment characteristics of root nitrogen-fixing bacteria.（5）The four indices of bacterial and microbial diversity in rhizosphere soil of D.odorifera and A.fraxinifoliuss were positively correlated with soil bulk density and soil p H,and negatively correlated with soil carbon content but the correlations were not significant.In addition,the correlation between different soil microbial diversity indices and soil physical and chemical properties in D.odorifera plantation was more consistent,and the soil microbial community and nutrient distribution were more uniform than those in A.fraxinifoliuss plantation,which was more beneficial to the diversity and development of soil microbial community and species.Generally,the forest productivity,water conservation,soil nutrient enrichment capacity and soil microbial diversity of D.odorifera in karst area are better than those of A.fraxinifoliuss plantation.