| Desert steppe is a kind of grassland with the strongest arid nature from steppe to desert.The habitat of the desert steppe ecosystem was fragmented due to long-term overgrazing and utilization,and the spatial distribution pattern of plants has changed,which often presents as patchy distribution.Studying the effect of grazing on the changing characteristics of spatial patterns of plants and soil,and their interaction rules is of great significance for understanding the succession mechanism of grassland communities.The experiment plots within the Stipa breviflora desert grassland were selected and fenced to establish a grazing trial which with four different stocking rates treatments(Control,CK;lightly grazed,LG;moderately grazed,MG and heavily grazed,HG).This research were mainly conducted on two aspects:(1)Exploring the response of species diversity(alpha diversity)to stocking rates at the community level and to discuss the spatial pattern characteristics of the four major species(constructive species:Stipa breviflora,dominants species:Artemisia frigida and Cleistogenes songorica,main companion species Convolvulus ammannii)and soil carbon and nitrogen,and the relationship between the plant characters and soil heterogeneity by using the method of spatial pattern analysis,niche width and overlap,spatial autocorrelation,semivariance analysis and kriging map;(2)The effects of stocking rates on species composition,species turnover and diversity from patches which were mainly composed of the selected four main species were studied by using alpha and beta diversity index.The seasonal dynamics and coupling relationship of plant-root-soil carbon and nitrogen contents were also studied for the main species from each patch.It aims to reveal the response of plant and soil spatial patterns and the nutrient coupling relationship response to grazing disturbance at the level of community and population(patch),and to clarify the differences and causes of the change of grassland ecosystem spatial pattern caused by long-term grazing,which would provide a scientific basis for formulating a reasonable grazing system and sustainable development of natural grasslands.The main results are as follows:1.Species diversity,spatial heterogeneity of the main species and soil,and the plant-soil relationship under different stocking rates(1)Species richness(Margelef),diversity(Shannon-Wiener)and evenness(Pielou)indices showed a decreasing trend as the stocking rate increased,and they were significantly lower in HG than other treatments(P<0.05).The four main species were all in an aggregated pattern under the four stocking rates.With the increasing of stocking rates,the aggregation intensity of Stipa breviflora and Cleistogenes songorica decreased,the niche width increased,and the aggregation intensity of Artemisia frigida and Convolvulus ammannii increased,the niche width decreased.The overlap between Stipa breviflora and Cleistogenes songorica increased with the increasing of stocking rates,and the interspecific competition between those two species was strengthened,which made them gradually became the dominant species in the community.However,the niche overlap between Artemisia frigida and Convolvulus ammannii was decrease,and the niche overlap between each of those two species and Stipa breviflora or Cleistogenes songorica decreased,indicating that they were at a disadvantage in the community.(2)The stocking rate had a significant effect on the density of the four species(P<0.05).With the increasing of stocking rate:the density of Stipa breviflora and Cleistogenes songorica gradually increased,and the density of Artemisia frigida showed a trend of increasing first and then decreasing,the density of Convolvulus ammannii decreased.The total density of the four species showed a trend of decreasing first and then increasing as the stocking rate increased.The four species showed different spatial patterns under different stocking rates:In CK,Stipa breviflora showed strong spatial autocorrelation,and was mainly affected by structure factors,but the spatial continuity of other species was poor,which were mainly affected by random factors,although they can form as aggregated patches.Grazing increased the total variation of Stipa breviflora,and the patch area became larger,the aggregation intensity reduced,which expands the populations of Artemisia frigida,Convolvulus ammannii,and Convolvulus ammannii in the community and made them forming a patchy pattern with gradient or circle in space layer structure,and the spatial autocorrelation were enhanced.(3)There were significant effects of stocking rate on soil total carbon(STC),Soil total nitrogen(STN),and carbon to nitrogen ratio(C/N)(P<0.05).As compared to CK,the spatial autocorrelation range increased and intensity of spatial autocorrelation decreased of STC in LG and HG;In MG,the spatial structure of STC was poor and was mainly affected by random factors.Soil total nitrogen showed a strong spatial autocorrelation,and the spatial autocorrelation range was lowest in MG,highest in HG,and they were similar in LG and CK.The soil C/N was affected by structural factors and random factors in HG,which showed a moderate degree of spatial autocorrelation,however,it showed a strong spatial autocorrelation in other treatments.Compared to CK,the spatial autocorrelation range of soil C/N increased in LG and HG,and it was relatively lower in MG as compared to other treatments.The spatial structure of STC was poor in MG,and were mainly affected by random factors,but it was good in other treatments,all of which were characterized by the patch structure of interconnected or gradient distribution.Soil total nitrogen was characterized by a gradient distribution which with good spatial continuity and was mainly affected by structure factors.In CK and MG,soil C/N showed a gradient distribution which combined with several small patches,while in the LG and HG,soil C/N showed patch distribution patterns combined with larger patch area.Species diversity and density characters were mainly affected by spatial heterogeneity of STN,but less affected by STC.The density of Stipa breviflora,Cleistogenes songorica and the total density of the four species were closely related to the spatial heterogeneity of STN,while the density of Artemisia frigida and Convolvulus ammannii were mainly affects the species richness and diversity.The spatial heterogeneity of STC and soil C/N was closely related to LG.2.Responses of species diversity at patch level,aboveground-root-soil carbon and nitrogen content and their coupling relationship of the major plants to stocking rates(1)With Stipa breviflora plaques as the background(substrate),the patch position and stocking rate had no significant effect on the alpha diversity of Artemisia frigida patch,while species similarity of Artemisia frigida was significantly affected by stocking rate(P<0.05),and the similarity between stocking rates were LG<MG<CK.The richness and diversity of Cleistogenes songorica patch were significantly affected by stocking rate,and the magnitude of them under different stocking rates were LG>CK>MG>HG.The evenness index of Cleistogenes songorica patch was affected by the stocking rate and patch position.There were significant effect of stocking rate and patch position on the richness and diversity index of Convolvulus ammannii patch,while the evenness index only differed significantly between patch positions(it was significantly lower in the center of a patch than that of the edge and outside of the patch).(2)Grazing reduced the above-ground biomass of different species patches,but the biomass of Stipa breviflora was higher in some months(August and September)under grazing treatments.As the stocking rates increased,the total C and C/N of Artemisia frigida,Cleistogenes songorica and Convolvulus ammannii decreased and total N increased,while the total C and total N of Stipa breviflora increased and the C/N decreased.At the beginning of the growing season,the total N of Artemisia frigida,Cleistogenes songorica,and Convolvulus ammannii was higher,and the total C and C/N was lower at the end of the growing season,while those of Stipa breviflora showed the opposite trend;The differences of STC,STN and C/N under different layer of roots were different between months or stocking rates,depending on patch species and depth;Except for the total C and total N of Artemisia frigida root,the corresponding nitrogen and carbon contents of the four plants'aboveground,roots and soil showed a very significant positive or negative correlation(P<0.01).Across all the stocking rate data,Stipa breviflora,Artemisia frigida and Cleistogenes songorica were closely related to the total C of aboveground,STC and STN,and those metrics were higher from the species above.While there were relatively higher total N from the aboveground and root of Convolvulus ammannii. |
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