| The ecological function of the agro-pastoral ecotone is of vital importance because of its particularity of the geographical location and diversity of the utilization mode.Nitrogen cycle is one of the most important material cycle in soil,and the nitrogen availability is a key parameter to evaluate ecosystem.One of the main ways of grassland utilization is grazing,which can interfere with the nitrogen transformation process in soil.Researching the effect of grazing on soil nitrogen mineralization in agro-pastoral ecotone can provide the bases for long-term grazing test and establish the foundation for sustainable development of grassland.This experiment was carried out at the Field Observation and Research Station of Grassland Ecosystem in the agro-pastoral ecotone(Youyu)of Shanxi Agricultural University.It was used complete random blocking design and set up four grazing intensities,which were non-grazing(CK),light grazing(2.35 Sheet Unit(SU),LG),medium grazing(4.80 SU,MG)and heavy grazing(7.35 SU,HG).There were four replicates per treatment.Grazing activities were beginning in June and stopping at the end of September in 2017-2019.The contents of ammonium(NH4+-N),nitrate(NO3--N)and inorganic nitrogen in soil were determined during the growing season,and the cumulative mineralization contents and net nitrogen mineralization rates were calculated.The response of soil nitrogen mineralization to grazing intensity was analyzed.And the relationship between soil physical and chemical properties,above-ground biomass,microbial biomass and nitrogen mineralization rate was explored,in order to explore the effect factors of nitrogen mineralization.The results showed that:1.The soil inorganic nitrogen contents with different grazing intensities had obvious seasonal dynamics with the change of month.The monthly dynamics of ammonium nitrogen(NH4+-N),nitrate nitrogen(NO3--N)and inorganic nitrogen contents were different.The ammonium nitrogen(NH4+-N)content was higher in June,September and October,and the nitrate nitrogen(NO3--N)and inorganic nitrogen contents were higher in July and August.2.Nitrate nitrogen(NO3--N)content,which accounted for 89% of inorganic nitrogen content,was the main component of inorganic nitrogen.The monthly dynamics of net nitrification rate was basically consistent with the net mineralization rate,and nitrification was the main transformation process of nitrogen mineralization.3.The cumulative mineralization content increased significantly with the increase of grazing years(P<0.05),and moderate grazing increased more than 30 times in 2018 and 2019 than in 2017,reaching a maximum of 111.12 mg·kg-1 in 2019.4.The net nitrogen mineralization rate increased with the increasing soil temperature.But the relationship between soil moisture and net nitrogen mineralization rate was not significant(P>0.05).Both soil bulk density and soil p H were positively correlated with net nitrogen mineralization rate(P<0.05).There was a significantly negative correlation between net nitrogen mineralization rate and N2 O flux(P<0.05).5.Different grazing intensities reduced soil organic carbon,microbial biomass carbon and microbial biomass nitrogen.There was a very significant negative correlation between inorganic nitrogen and soil carbon-nitrogen ratio(r=-0.51,P<0.01),and a significant negative correlation between inorganic nitrogen and microbial biomass carbon-nitrogen ratio(r=-0.33,P<0.05).In summary,the higher the grazing intensity,the higher the net nitrogen mineralization rate,and the increasing grazing intensity could promote the process of soil nitrogen mineralization,so accumulated more effective nitrogen.The effect of soil temperature on soil nitrogen mineralization was greater than that of soil moisture.Animal trampling resulted in bulk density increasing,and decreased water content,then increased soil temperature.And increasing soil temperature promoted the net nitrogen mineralization rate.Soil temperature was the limiting factor for soil nitrogen mineralization in this experiment field. |
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