Soil Net N Mineralization In The Typical Temperate Grassland In Inner Mongolia

As a main limiting factor for plant growth and net primary productivity, soil nitrogen (N) availability and its responses to global environmental changes are critical for the projection of ecosystem and global C budgets. Net N mineralization, the transformation process from organic N to inorganic N, mainly determines soil N availability. A better understanding of the effects of land use history, temperature, soil water availability, and their interactions on net N mineralization in soils will facilitate our predictions of soil N dynamics and net primary production in terrestrial ecosystems. Undisturbed soil cores from the Inner Mongolia grassland in northern China were incubated in the laboratory to examine the effects of land use history, temperature, soil moisture, and their interactions on net N mineralization and nitrification rates. Land use history significantly affected ammonification, but not nitrification and net N mineralization. Ammonification, nitrification, and net N mineralization rates showed significant exponential relationships with temperature. Net N mineralization rates were not different at low incubation temperatures (?10, 0, and 5 ℃ irrespective of the significant main effects of temperature on N mineralization. Soil moisture content profoundly impacted nitrification and net N mineralization rates, but not ammonification rate, suggesting that nitrification was more sensitive to increasing soil moisture than ammonification. Our data showed significant interactions among land use, temperature, and moisture content. Land use history and soil moisture exerted great controls over the temperature sensitivity of net N mineralization. Similar net N mineralization and nitrification rates between the grazed site and the fenced sites may suggest that high N loss potential by leaching in degraded grassland ecosystems is inevitable, leading to a gradual depletion of N reserves and N limitations on plant productivity. The effect of land use history on soil inorganic-N pool and mineralization were also studied in situ in three Leymus chinensis grasslands with different use history (grazed grassland, grassland fenced for 25-years and grassland fenced for 5-years, which represent three intensities of disturbance). Soil net nitrogen mineralization and nitrification were determined using the PVC tube closed-top in situ incubation method in the three grasslands. Seasonal variation in the inorganic nitrogen pool and the net amminification, nitrification and mineralization rates and the annual net nitrogen mineralization were investigated. The results showed that: (1)soil inorganic-N, net nitrogen ammonification, nitrification and mineralization showed similar seasonal dynamics in the three grassland ecosystems except for some quite large differences in certain months; (2)there were significant differences in NO3--N concentration among three grasslands in June, July and September, but not in August. In October, NO3--N in the grazed grassland was significantly lower than those in the other two grasslands. NH4+-N concentration showed significance differences among the three grasslands in May, July and August. In June, NH4+-N in the grassland fenced for 5 years was significantly different from those in the grazed grassland and the grasslands fenced for 25 years. In September, NH4+-N in the grazed grassland was significantly lower than those in the grassland fenced for 25 years and the grassland fenced for 5 year. In October, NH4+-N in the grassland fenced for 25 years was significant higher than those in the other two grasslands. The effects of land use change on soil inorganic-N pool and mineralization observed in our study might have been attributable to the changes in soil temperature, soil moisture, species composition, and soil organic matter.