Effects And Regulatory Mechanisms Of Different Herbivore Assemblages On Soil N Mineralization Rate

Nitrogen as a major limiting nutrient in grassland,its circulation rate directly determines plant productivity and normal ecological function in grassland.Livestock grazing,as main management and use of grassland,exerts important influence on the quantity and quality of soil nutrient input in grassland,and the physical and chemical environment of the soil through defoliation,trampling and the return of excrement,thus affecting the soil microbial decomposition process and soil nitrogen mineralization rate,regulating grassland nitrogen cycle.Numerous studies have examined the effects of grazing intensity on the nitrogen cycle in grassland,however,fewer studies considered the effects of different grazing livestocks and assemblages on grassland nitrogen cycling.In this study,we examined the effect of different grazing livestocks assemblages(cattle grazing,sheep grazing,mixed grazing)at moderate grazing intensities on soil nitrogen mineralization rate(an important indicator of nitrogen cycling rate),in the meadow steppe,the typical steppe and the desert steppe,respectively,in the north of China.We obtained the following important research results.(1)Each of livestocks assemblages under moderate grazing intensity significantly increased soil nitrogen mineralization rate in grassland,except sheep grazing did not reach a statistically significant level in the desert steppe.This positive effect consistently occurs in the three main grassland types in the north of China: Meadow grassland,typical grassland and desert grassland.(2)The effect of grazing on soil nitrogen mineralization differs among grazing livestock assemblages,and this difference depends on different grassland types.In meadow steppe,cattle and sheep single grazing has higher soil nitrogen mineralization rates than mixed grazing of cattle and sheep.In typical steppe,the soil nitrogen mineralization rate under sheep grazing is significantly higher than the other herbivore assemblages.In desert steppe,there was no significant difference in soil nitrogen mineralization rates among three livestock assemblages.(3)Livestock grazing mainly affects soil nitrogen mineralization rate through two ways: Changes in grass to forb cover ratio caused by selectively forage(characterized by changes in plant litter input quality),and the rapid cycling pathway of grassland nutrition brought about by the return of animal excrement;The positive effect of animal excrement played a leading role in the improvement of soil nitrogen mineralization rate by grazing.(4)All grazing treatments had no significant effects on the soil moisture in the three grassland types.Therefore,the trampling behavior of livestock under the moderate grazing intensity may did not affect soil nitrogen mineralization rates by changing the soil physical environment(ie,soil moisture).In summary,moderate grazing in grassland has significantly promoting effect on soil nutrient cycling rate.During livestock grazing,return of nutrients such as faeces and other excrement are the main mechanisms for grazing to increase soil nitrogen cycling rate.Our research suggests thatmoderate grazing and utilization in grassland management can promote the nutrient cycling rate of grassland,and then improve the production performance of grassland.In addition,except grazing intensity,the species and assemblages of grazing livestock should also be further considered in grassland grazing management,because different grazing livestocks,even under the same grazing pressure,it may have a distinctly different effect on the grassland.