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 ?gN gsoil^-1h^-1, the maximum net nitrification rate is respectively 40.75 and 22.77?gN 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 ?g g^-1 h^-1, separately. The maximum of net nitrification rates were 0.40,1.23,0.82 and 0.25 ?g 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 ?g g^-1 in 3 hours,56.67 ?g 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 15.69?g 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.