Distribution and utilization of nitrogen on moderately and heavily grazed temperate desert steppe using the 15N tracing technique
Abstract Due to overgrazing, grasslands in northern China have been severely degraded since 1980s. Grassland biomass and plant populations decreased dramatically. Application of fertilizer is a common technique for pasture management in many countries, however, it has not been widely used on China grasslands. Nitrogen (N) is an important productivity driver in grassland ecosystems, but its transportation, allocation and utilization in temperate desert steppe remained unclear. In this study, a sequential nitrogen addition experiment was conducted on a farm in southwestern Jingtai County, Gansu Province, in the transition zone between Loess Plateau and Tengger Desert. Two different grassland plots with moderate grazing (MG) and heavy grazing (HG) were fenced in 2008. N (NH 4 NO 3 ) was applied at the rate of: 12.25gN·m −2 , 24.5gN·m −2 , 36.75gN·m −2 and 49gN·m −2 , respectively. To determine the amount and distribution of nitrogen applied to the grassland, the 15 N tracer technique was adopted. The results demonstrated that the recovery rates of 15 N ranged from 3.8% to 11.7% in the plants. The nitrogen recovery amount from both shoots and roots increased significantly ( P 0.05), while the recovery rate decreased gradually with increasing nitrogen application. The 15 N recovery from shoots under HG was much less than from MG. In one growing season, only 18.2%–8.0% of labeled 15 N was derived from fertilizer, so soil nitrogen is still the main source absorbed and utilized by plants. As much as 50.4%–84.4% labeled 15 N remained in soil, which was evenly distributed throughout 0–20cm soil layer, and significantly increased with nitrogen application rates. The nitrogen remained in the top soil (0–10cm) pool was much less than that of 10–20cm layer ( P 0.05). The nitrogen loss amounts and rates were significantly different among N treatments, which indicated that there was an increasing trend in N loss with increasing N application rates. Loss/recovery analysis showed that 12.25gN·m −2 was the recommended application rate. This work is expected to provide the lowest risk/return ratio and produce both economic and environmental benefits for the degraded grassland in Gansu and analogus regions in China. Highlights Grazing intensity negatively affected the nitrogen (N) recovery by plants. Soil N was still the main N source absorbed and utilized by plants. With increasing N application, the N loss increased in temperate desert steppe. 12.25gN·m −2 was the recommended for the degraded grassland in Gansu.
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