| The nitrogen?N?cycle is one of the three major geochemical cycles.The industrialization and the rapid development of the global economy have continuously led to rapid nitrogen deposition increase worldwide,which has made a strong impact on terrestrial and aquatic ecosystems.As a result,it has become an environmental issue that humans have to face,which is one of the hot issues in field of global change research.At the same time,increasing human activities causes highly intensified degradation and has significantly affected community productivity,species composition and ecosystem processes.Overgrazing as an important human disturbance is considered to be one of the main causes of vegetation and soil degradation in semi-arid and arid regions of northern China.Therefore,to restore vegetation and soil,enclosure has been widely used in the area.Studies on the effects of enclosures on plants,communities,and ecosystems have mainly focused on grassland ecosystems.There is still insufficient understanding of the species composition of desert ecosystems and their responses to global changes in arid regions.Due to the fragility of the ecological environment in the arid regions and its sensitivity to human activities and global changes,it is necessary to study the impact of nitrogen deposition and enclosure on the structure and function of the arid ecosystem.In order to ascertain the effects of nitrogen addition and enclosure on desert plants and soil in arid regions,4-year field experiments of gradient nitrogen addition(0,10,30,50 kg N hm-22 year-1)and continuous exclosure treatment(carrying capacity:0 Vs 2.43 sheep hm-2)had been conducted in a typical arid desert dominated by Alhagi sparsifolia?A.sparsifolia?and Lycium ruthenicum?L.ruthenicum?in northwest of China,and the response of characteristics of soil,plant nutrition and plant community to the treatments and possible mechanisms were analyzed.The research results help to further understand the response of plants to nitrogen deposition and enclosure management and the succession trends of plant communities in arid ecosystems,and provide a theoretical basis for desert ecosystem management in arid areas.The main results are shown below:?1?After the enclosure treatment,the total nitrogen content of the soil increased significantly from 0.76 mg g-1to 1.51 mg g-1,the nitrate nitrogen content increased significantly from 1.10 mg kg-11 to 4.72 mg kg-1,however,the soil pH decreased significantly from 8.57 to 7.47.Except that the soil ammonium nitrogen content increased significantly with increasing nitrogen concentration,the soil pH and other physical and chemical properties were not significantly different between different concentrations of nitrogen addition.?2?The exclosure significantly increased the productivity of plant communities.After the enclosure treatment,vegetation height?48.33 cm?,coverage?51.01%?and above-ground biomass(76.30 g m-2)in the exclosure?EX?area were significantly higher than those in the free grazing?FG?area(13.63 cm?32.61%?35.85 g m-2),but there was no significant difference in vegetation density;both Shannon-Wiener Index and Simpson Index increased significantly.?3?The exclosure significantly increased the relative coverage?from 24.97%to56.52%?and above-ground biomass(from 21.80 g m-22 to 56.71 g m-2)of A.sparsifolia,reduced the relative coverage?from 55.45%to 23.21%?and relative density?from 16.24%to 6.66%?of L.ruthenicum,and had no significant effect on the above-ground biomass of L.ruthenicum.In EX,the relative coverage and density of A.sparsifolia are significantly higher than those of L.ruthenicum.?4?Nitrogen addition has different effects on the leaf element content of the different plants.Overall,there was no significant difference in leaf element content between different gradient nitrogen addition.The concentrations of N,P,and K gradually decreased with the change of the season,while the concentrations of Na gradually increased and tended to accumulate in the senescent leaves.Exclosure presented divergent effects on different leaf elements of the two plant.In September,the leaf nitrogen concentration of A.sparsifolia in EX-0 was significantly lower than in FG-0;in May,the P concentration of both L.ruthenicum and A.sparsifolia in EX-0 were significantly lower than that in FG-0;the K concentration of both L.ruthenicum and A.sparsifolia in EX-0 was significantly higher than that in FG-0 in the whole growing season.?5?Nitrogen addition presented divergent effects on leaf element resorption efficiency of the two plants.Nitrogen addition increased N,P,K resorption efficiency of A.sparsifolia;K resorption efficiency of L.ruthenicum increased significantly with the increase of nitrogen addition concentration;nitrogen addition has no significant effect on Na resorption efficiency of both L.ruthenicum and A.sparsifolia.A.sparsifolia had significantly higher NRE and PRE than L.ruthenicum with the exception of the PRE in FG-0 treatment.However,A.sparsifolia had significantly lower KRE than L.ruthenicum in exclosure area.In summary,4-year exclosure treatment promoted the more rapid growth of A.sparsifolia in the A.sparsifolia-L.ruthenicum community in an arid desert ecosystem,significantly affecting the structural composition of the plant community,and increasing the productivity of the plant community;nitrogen addition did not significantly affect the composition of the plant community during the growing season.It is the exclosure rather than the nitrogen addition that has a greater impact on the soil physical and chemical properties and plant communities in the arid area. |
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