The Mechanisms By Grazing On Grassland Plant And Soil Spatial Heterogeneity And Their Correlation

Spatial heterogeneity is one of the key characteristics of grassland ecosystems, which drives many ecological processes in a wide array of grassland ecosystems. Human disturbance caused the simplification of ecosystems and the decrease of grassland spatial heterogeneity and plant diversity. In order to maintain grassland biodiversity and ecological processes, deep understanding of the causes of the formation of the grassland heterogeneity become increasingly important. Large herbivores are one of the major ecological agents that strongly influence plant and soil spatial heterogeneity of grassland ecosystems. The effect of herbivore grazing on grassland spatial heterogeneity is an important research topic in both grassland ecology and landscape ecology. We conducted the deep research on the study of the mechanisms by grazing on grassland plant and soil spatial heterogeneity and their correlation. This study is of great importance for maintaining and protecting grassland biodiversity and guiding the grassland grazing management.Cattle and sheep, as the two principal grazers in our meadow steppe of northeastern China, are the research objects in this study. Under the backgrounds of different plant diversity, we systematically study the effects of herbivore assemblage(no grazing; sheep grazing;cattle grazing and mixed grazing) on grassland plant and soil spatial heterogeneity. The study revealed the characteristics of plant and soil spatial heterogeneity and their responses to herbivore assemblage of meadow steppe in northeast, China. We also analysis the relationships of plant and soil spatial heterogeneity and discuss how herbivore assemblage and plant diversity interact to impact grassland plant and soil spatial heterogeneity and the mechanisms behind them. Based on a large number of experimental data and statistical analysis of the data. We obtained the main findings as follows.1) In songnen grassland ecosystems, soil variates(ammonium nitrogen, nitrate nitrogen, total inorganic nitrogen; total nitrogen; nitrogen availability, organic matters, total phosphorus, p H, electrical conductivity and water content) exhibited high degree of spatial heterogeneity and there were large variations both in two components of spatial heterogeneity(coefficient of variation, C.V.; spatial structural variation; C/C+C0). The magnitude of coefficient of variation were as follows: nitrogen availability > electrical conductivity > nitrate nitrogen > ammonium nitrogen > total inorganic nitrogen>total nitrogen>organic matters>total phosphorus>water content>p H. The magnitude of structural heterogeneity were as follows: organic matters>water content > total inorganic nitrogen > total nitrogen >total phosphorus > nitrate nitrogen > ammonium nitrogen > electrical conductivity > p H > nitrogen availability. Take soil nitrogen(N) as examples, the degrees of spatial heterogeneity of different forms of soil nitrogen were different. The coefficient of variation of soil N availability was highest, and the coefficient of variation of soil total N was the lowest. There was no necessary link between coefficient of variation and spatial structural variation. The value of coefficient of variation of soil N availability was highest, but the value of spatial structural variation was lowest. The value of coefficient of variation of EC was larger, but spatial structural variation was smaller. For soil p H, both the value of coefficient of variation and spatial structural variation were low.2) The results showed that there was a significant effect of herbivore assemblages on plant and soil spatial heterogeneity and the effects strongly depended on pre-grazing plant diversity. In high and low plant diversity sites, sheep grazing decreased plant community heterogeneity although the result was not significant in high plant diversity site. Cattle grazing increased plant community heterogeneity in high plant diversity site, but the result was not significant in low plant diversity site. The effects of mixed grazing on plant community heterogeneity depended on pre-grazing plant diversity that mixed grazing had no significant effect on plant community heterogeneity in high plant diversity site but decreased it in low plant diversity site. Herbivore assemblage had different effects on different soil variates. In high plant diversity grassland, all the grazing treatments improved the spatial heterogeneity of soil nutrient elements(soil available N, total N and organic matter). Besides, cattle single grazing consistently increased spatial heterogeneity of soil available N in two diversity sites. Cattle or sheep single grazing had consistent effects of soil p H and electrical conductivity that they both increased soil p H and electrical conductivity in two diversity sites. Mixed grazing had contrary effects on soil p H and electrical conductivity. The effects of sheep and cattle grazing on soil water content were different and the effects were opposite in two diversity sites. However, mixed grazing decreased spatial heterogeneity of soil water content in two plant diversity site.3) Herbivore assemblages had significantly different effects on soil N availability, but the effects were independence of pre-grazing plant diversity. Cattle grazing increased spatial heterogeneity of soil N availability but sheep grazing and mixed grazing had no effects on spatial heterogeneity of soil N availability. Although the effects were different among different grazing treatments, however, any grazing treatment did not decrease but maintained or increased spatial heterogeneity of soil N availability, which indicated herbivore did not degrade Songnen grassland ecosystems by changing soil spatial heterogeneity.4) The relationships of spatial heterogeneity of soil N availability and their response to herbivore assemblage strongly depended on herbivore assemblage and the components of spatial heterogeneity(C.V. and C/(C+C0)). In ungrazed treatment, there was no correlation between C.V. of soil N availability and plant diversity. In all the The correlation between C.V. of soil N availability and plant diversity was not changed by any grazing treatment. The relationship of C/(C+C0) of soil N availability and plant diversity depended on herbivore assemblage. There were significantly positive relationships between C/(C+C0) of soil N availability and plant diversity(plant richness and Shannon wiener index) at both plot and quadrat scales.Based on the above outcomes we obtained in this study, we acquired the further knowledge and deeper understanding. The effects of herbivore assemblage on plant and soil spatial heterogeneity and their relationships are complex due to the differences in selection of different herbivores and the interactions of different herbivore. Herbivore assemblage had different effects on plant and soil spatial heterogeneity and the effects strongly depended on pre-grazing plant diversity. However, the effects of herbivore assemblage on spatial heterogeneity of soil N availability were independence of pre-grazing plant diversity. We found that in high plant diversity grassland, herbivores and plant diversity interact to improve spatial heterogeneity of soil N availability when the grazing intensity is moderate. High soil spatial heterogeneity can provide better ecosystem service. In addition, heterogeneity –diversity hypothesis indicate higher soil spatial heterogeneity can provide more niches for plant species and generate positive animal-plant-soil feedback loop. We also highlighted the importance of protecting plant diversity in maintaining and improving grassland ecosystem structure and function. Further, herbivore grazing didn’t decrease spatial heterogeneity of soil N availability and the effects independent of herbivores assemblage and pre-grazing plant diversity. From a perspective of maintaining grassland structure and ecosystem function, cattle grazing was optimal grazing regime of our Songnen meadow steppe. This study provided important scientific basis of guiding the grazing management and making reasonable grazing policies.