Dry-wet Transition Effect On Soil Nitrogen Turnover In Forest And Semi-arid Grassland Ecosystem

Soil microbial nitrogen （N） transformation is one of the main processes for producing soil available N. Soil available N is the main nutrients for ecosystem net primary productivity which is limited by soil microbial N turnover. Environmental factors, such as soil moisture content and soil temperature directly affect soil microbial N transformations, especially in arid, semi-arid ecosystems. Dry and wet are two main processes in natural grassland and forest ecosystem before and after rain evidence, which has direct effect on N transformations to dry-wet alternate in natural forest, grassland ecosystems. Dry-wet alteration happened in dry season espially, can significantly improve N transformations. Nitrogen has two forms; the most of the soil N is organic N in natural ecosystem, which occupied more than 95%. However, organic N cann’t be obsorbed by plants except organic N turn to inorganic nitrogen under microbial decomposition. We adopted the method of laboratory culture, collected soil samples from arid and semi-arid forest ecosystem of Beijing and arid and semi-arid grassland ecosystems of Inner Mongolia. We used chloroform fumigation method, to simulate the response of soil N transformation to short-term rainfall events （i.e., dry-wet alternate）, and to clarify the role of microorganisms in the process of N transformation. We try to understand the rapid response to rainfall in arid areas. The conclusion as follows:1. Soil net N mineralization rate:The response of soil net N mineralization rate to short-term rainfall events is larger in Beijing forest ecosystem than that in Inner Mongolia grassland ecosystems. The maximum （RN-Max） net N mineralization rates were 40.87 and 22.86 μgN gsoil^-1h^-1, the maximum net nitrification rate is respectively 40.75 and 22.77μgN gsoil^-1h^-1 at the top and in the bottom of mountain in forest ecosystems, respectively. A period of time at the top of the net N mineralization and nitrification rates reached the maximum value of time （TRN-max） were for 6 h, and for 5 h at the bottom, the top half of the net N mineralization and nitrification rates maximum duration （D1/2max-RN） were 1.5 h, but at the bottom was 2.5 h.Culture experiment results from the east to the west of Inner Mongolia grassland ecosystem sample showed that:Nitrogen mineralization rate of maximum （TRN-max） of SAL,TGX,TGM,DGJ were 0.92,1.69, 1.10 and 0.61 μg g^-1 h^-1, separately. The maximum of net nitrification rates were 0.40,1.23,0.82 and 0.25 μg g^-1 h^-1.Net N mineralization and net nitrification rates reached the maximum time （TRN-Max） at 8 h, D1/2 Max-RN at 7 h, respectively; Net N mineralization rate was significantly lower in grassland ecosystem than that in forest ecosystem.2.Soil microbial biomass nitrogen （MBN）:Microbes is sensitive to rainfall pules effect. The maximum soil MBN were 67.16 μg g^-1 in 3 hours,56.67 μg g^-1 in 2 h at the top and foot of the mountain of forest ecosystem in our sampling sites, respectively. The peak of soil MBN of SAL, TGX, TGM, DGJ grassland ecosystem were all in 8 h, and the maximum value were 31.59,28.79,15.22 and 15.69μg g^-1, respectively. Soil MBN and the maximum value of time in grassland ecosystem was far lower than that in forest ecosystem. Soil MBN was 2.13～4.41 times in grassland ecosystem than that in top of forest ecological system,1.79～3.72 times of grassland ecosystem than that at the bottom of the soil microbial biomass nitrogen of forest ecosystems.3. Dry and wet alternation affected soil microbial N transformation and MBN in forest and grassland ecosystem.