Tracing Study On The Fate Of Labeled Nitrogen Applied To The Temperate Desertificated Grassland Under Different Grazing Intensities

The study was carried out in temperate desertificated steppe of Jingtai county of Gansu province.The influences of different grazing intensities on the vegetation and soil of temperate desert steppe were researched.The ¹⁵N tracer technique was selected to research the absorption,distribution,loss and utilization ratio of labeled nitrogen under different nitrogen fertilizer applications.The results are showed as follows:(1) The values of height,density,coverage,above-ground biomass of vegetation decreased with the increasing of grazing intensities.The species composition and structure were also affected by grazing stress.With grazing intensities increasing,the ratio of high quality forage reduced,and the contents of CP decreased.The contents of CF changed from 2.09%to 3.33%.The contents of crude ash were the highest in light grazing grassland while the minimum in high grazing grassland.In the temperate desertificated steppe,the contents of Ca were all small in different grazing grassland,non-grazing and high grazing were both not conductive to the absorption of phosphorus to forage.(2) With the increasing of grazing intensities,the soil moisture contents decreased while the values of soil bulk density increased.The values of soil pH in different grazing grassland showed weak alkaline,and they changed from 8.4 to 8.9.The contents of soil organic matter and soil total nitrogen showed a decreasing trend with the grazing intensities increasing.Soil organic matter decreased from 8.01%to 1.93%,and the average of total nitrogen decreased by 41.7%.With the increasing of grazing intensities, the influence to the contents of available nitrogen,available phosphorus and available potassium were significant,especially to the surface layer soil.The results showed that the urease activities of different grazing grasslands have significant differences,and the urease activities are more intense in the surface layer than in the under layers.The influences of grazing intensities to the contents of soil catalase activity were significant in the surface layer.(3) The influences of nitrogen applications to the aboveground biomass and the belowground biomass in surface layer were significant.The aboveground biomass and the belowground biomass in surface layer of middle grazing intensities grassland were higher than high grazing intensities grassland.For the aboveground plant pool,the total nitrogen content in fertilizer application rate of 24.5gN·m² in middle grazing intensities grassland was the maximum while in fertilizer application rate of 12.25gN·m² in high grazing intensities grassland.The total nitrogen contents of two soil layers were both increasing with the increasing of nitrogen rates in middle grazing intensities grassland and high grazing intensities grassland.(4) The recovery rates of the applied ¹⁵N were range from 3.84%to 11.67%in the plant pool.With the increasing of nitrogen rates,the absorption of fertilizer nitrogen by plants showed an increasing trend,suggesting that nitrogen fertilizer were significantly affected.The recovery amounts by aboveground and belowground organs significantly increased while the recovery rates decreased.According to the recovery amounts of labeled nitrogen,the aboveground organs of high grazing grassland were less than middle grazing grassland.In one growing season about 18.19%～48%of the labeled nitrogen derived from ferlizer while about 52%～81.81%derived from soil.Soil nitrogen was still the main nitrogen source which was absorpted and utilized by plants.(5) After one growing season about 50.43%～84.41%of labeled nitrogen was retained in the soil pool,and the labeled nitrogen retained in the soil pool was distributed equally to the 0～10cm soil layer and 10～20cm soil layer.The ¹⁵N retained by the soil increased significantly with increasing of the nitrogen application rates.The labeled nitrogen amounts retained in the surface soil pool were more than in the under soil pool.(6) The loss amounts and loss rates of fertilizer nitrogen were all significant among different nitrogen rates in two grazing intensities grasslands.The loss amounts of fertilizer nitrogen were range from 0.48gN·m^-2 to 22.41gN·m^-2 while the loss rates of fertilizer nitrogen were range from 3.92%to 45.73%.The loss amounts and loss rates of fertilizer nitrogen showed an increasing trend with the increasing of nitrogen rates in two grazing intensities grasslands.(7) Loss/recovery analysis showed that under the climatic and soil conditions prevailing during this experiment,fertilizer application rate of 12.25 gN·m^-2 was associated with the lowest risk/benefit ratio,so it was the most economical nitrogen application rate.These results provide a reference for the future management strategies of grassland ecosystem.