| As the main nutrient limiting factors in desert steppe ecosystem,nitrogen(N)and phosphorus(P)play an important role in nutrient cycle and utilization.At present,it is limited to carry out different nutrient addition experiments at the same time in the same desert ecosystem.Therefore,in order to study the nutrient utilization strategies and the changes in carbon(C)distribution pattern of desert herbaceous plants after nutrients addition.In this paper,a long-term(2007-2018)field N,P and N,P co-addition experiment was carried out in a typical desert steppe on the southeastern fringe of the Tengger Desert.We analyzed the changes of soil physical and chemical properties,soil extracellular enzyme activities and the stoichiometric characteristics in roots and shoots of different functional plant groups in desert steppe.The main results are as follows:(1)Long-term application of fertilizer significantly changed the soil physical and chemical properties and stoichiometric ratios,and led to the accumulation of SOC in desert steppe.The addition of N significantly decreased the soil p H and restricted the surface soil P,while the addition of P increased the content of soil total N,but the soil N was also limited with the increase of fertilization concentration.On the contrary,the N,P co-addition increased the contents of total N,total P and available nutrients in soil,but the limitation of N was greater than that of P.Therefore,in the N-limited desert steppe,the N,P co-addition is more beneficial to the supply of soil nutrients.(2)Long-term application of fertilizer changed the soil extracellular enzyme activity and its stoichiometric ratios.The addition of high N significantly decreased the soil extracellular enzyme activity and C/N,increased the soil enzyme C/P and N/P,while the effects of P application and N,P co-application on soil enzyme activities were not consistent.In addition,the average ratios of soil C,N and P related enzymes all deviating from the global ratio of 1:1:1,and showed that the demand of microorganisms for N was greater than that of C and P,which demonstrated that soil microorganisms were more likely to be limited by N in desert grassland.(3)Fertilization,plant types,plant organs and their interaction had significant effects on the contents of C,N and P in plants.The addition of N promoted the uptake of P by perennials,especially forbs and legumes,but significantly decreased the content of P in roots of annuals.The addition of P also promoted the uptake of N by plants,but the high P addition decreased the contents of N and P in legume roots.After N,P coaddition,the contents of N and P in annuals and perennials were increased,but the N content in shoots of legumes was significantly decreased,at the same time,it significantly decreased the P contents in roots of annual plants and in shoots of perennial forbs.Moreover,fertilization can reduce the contents of C in plants,while the C content of perennial grasses showed an obvious shoot-root difference,in which the C content of shoot decreased significantly,while that of the roots increased significantly.Perennial grasses have large root surface area,and the Mn content of its shoot is higher than that of other plants.(4)Long-term application of fertilizer changed the plant stoichiometric ratios of desert steppe.Fertilization can decreased the C/N and C/P in shoots of different functional plant groups,but the N,P co-addition significantly increased the C/P in shoots of perennial perennial forbs.However,the effects of fertilization on the C/N and C/P in roots of different functional plant groups were not consistent.In addition,the N/P ratios of plants(except for legumes)significantly increased with the addition of N and the N,P co-addition,especially the changes trend of annuals was more significant with the improvement of fertilization level,but the N/P of legumes decreased significantly.On the contrary,the addition of P decreased the N/P in shoots of perennial forbs and legumes,but did not reduced the N/P of shoots in annuals and perennial grasses.The results showed that long-term fertilization changed the nutrient utilization rates and the C assimilation ability of herbaceous plants,and enhanced the distribution of C to plant roots.In summary,different functional plant groups have different nutrient utilization strategies,the desert steppe more likely to be limited by N,while the legumes have the effect of fixing N and are more likely to be restricted by P.The improvement of soil N availability resulting from exogenous N input will highlight the restrictive effect of P on plant growth,while P input limited the N uptake in legumes.The co-application of N and P is more beneficial to the supply of soil nutrients in desert steppe,but it also limited the absorption of P in the roots of annual plants and the shoot parts of perennial forbs.In addition,soil physical and chemical properties play an important role in the changes of soil extracellular enzyme activity and plant stoichiometric ratios. |
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