| Azadirachta indica is planted extensively in Dry-hot valleys (DHV) of southwesternChina for its drought-enduring and multifunctionality, and Acacia auriculiformis is a kind offast growing species that can strongly improve soil conditions. It is important for soil and waterconservation, soil melioration, ecological prevention and regional economy to constructplantations with A. indica and A. auriculiformis that are suitable planting species in DHV.In order to ascertain the growing ability, nutrient utilization mechanism and distributionand characteristics of nutrient cycling of A. indica and A. auriculiformis in different forestationpatterns for further evaluation on effects of the mixed plantations, the biomass and itsallocation, litter fall and its decomposition, nutrient (N, P, K, Ca, Mg) resorption, allocation,and nutrient cycling characteristics of10year-old A. indica and A. auriculiformis in the pureand mixed plantations were studied with the method of sample plot from September2010toApril2012in Yuanmou DHV, Yunnan Province, China. The major conclusions were asfollows:(1) The biomass in the mixed plantation increased47.9%compared with the pure A.indica plantation and decreased30.8%compared with the pure A. auriculiformis plantation.Root biomass per plant increased for A. auriculiformis and decreased for A. indica in the mixedplantation compared with their pure plantation, respectively, but the biomass of medium, small,fine root had the opposite result. Proportion of the total lateral root biomass in the mixedplantation was larger than that in the pure plantation. Most of root (from65.8%to74.8%) ofspecies in the plantations distributed in the topsoil (from0.0m to0.2m), which is importantfor plantations to improve benefits in ecological protection and is the critical part formanagement of water and fertilization to artificial vegetation.(2) Standing crop of littler layer and annual amount of littler fall were largest in the pure A.auriculiformis plantation, and lowest in the pure A. indica plantation. The litter fall was largerin the dry season (from November to next May) than rainy season (from June to October). Decomposition rate of leaf litter of A. indica was promoted in the mixed plantation, and wereno significance for A. auriculiformis between in the pure and mixed plantation, and there wasmutual promotion while the leaf litter mixture of A. indica and A. auriculiformis decomposedin the mixed plantation. There was obvious nutrient resorption of N, P and K in leaves of A.indica and A. auriculiformis in DHV. The resorption efficiency of N in leaves was lower andthe content of N was highest in senesced leaves of A. auriculiformis. Resorption efficiency ofN and P of A. indica in the mixed plantation was higher than that in the pure plantation, whichindicated the intense requirement for N and P to A. indica in the mixed plantation. Ca and Mgwere accumulated constantly in senesced leaves along with the leaves senescing, whichreturned to the plantations in the form of litter fall.(3) The content of soil total or extractable N, P and K in the mixed species plantation werehigher significantly than that in the pure A. indica plantation, and there was no significantdifference for soil nutrient elements between A. auriculiformis in the pure and mixedplantations, in addition to K. The soil nutrient content was improved in the mixed plantationafter A. indica was planted with A. auriculiformis. Both contents of N and P in each organ (inaddition to the stem) of A. indica in the mixed plantation were higher than that in the pureplantation. There were different nutrient distribution structures in organs of A. indica butsmaller difference for A. auriculiformis between in the pure and mixed plantations, especiallythe N, of which distribution structure in organs had no difference. The lower N/P ratio (8.0) inleaves of A. indica in the pure plantation showed the scarcity of N compared with the N/P ratio(11.1) in the mixed plantation, and the higher N/P ratio (20.5) in leaves of A. auriculiformis inthe pure plantation showed a relative lack of P compared with the N/P ratio (16.1) in the mixedplantation. There were much high nutrient accumulation coefficients (from14.05to18.34) inplantations in DHV, which showed A. indica or A. auriculiformis could enhance there nutrientcontent by luxurious absorption.(4) The total nutrient (N, P, K, Ca, Mg) storage, retention, nutrient return, and nutrientabsorption in the pure A. indica plantation, the pure A. auriculiformis plantation and the mixed plantation ranged from333.045kg·hm-2to725.610kg·hm-2,62.719kg·hm-2·a-1to162.185kg·hm-2·a-1,48.818kg·hm-2·a-1to88.862kg·hm-2·a-1,251.047kg·hm-2·a-1to111.537kg·hm-2·a-1,respectively, which were highest in the pure A. auriculiformis plantation and lowest in the pureA. indica plantation. The nutrient utilization coefficient, cycling coefficient and recyclingperiod were estimated to be from0.335to0.389,0.393to0.438and6.540a to8.166a,respectively.The lower circulation rate and longer recycling period of N or P in the pure A.indica plantation showed the poor ability of maintaining soil fertility,which was disadvantageto keeping the stand productivity. The highest nutrient circulation rate and nutrient return andshortest recycling period of N or P in the pure A. auriculiformis plantation showed theadvantage to maintaining soil nutrient and stand productivity. The nutrient return and nutrientabsorption in mixed plantation were167.2%and186.2%of those in he pure A. indicaplantation, and the recycling period of Ca in mixed plantation was50%shorter than that in thepure A. auriculiformis plantation, which showed that the soil fertility was improved and theability of nutrient supplying was enhanced in the mixed plantation after A. indica was plantedwith A. auriculiformis.This study was to affirm ecological protection function of A. indica and A. auriculiformisplantaions in DHV, and suggested that some strategies of artificial vegetation restorationshould be taken according to the characteristics of biomass distribution and nutrient cycling ofplantations in different restoration patterns in DHV. |
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