Nitrogen (N), together with water in dryland areas, is the most limiting resource for plant growth. Because precipitation patterns are expected to change dramatically in response to climate change, an understanding of how soil N cycling changes in response to rainfall events is essential. In forests, dominant tree species and their mycorrhizal types are determinants of soil N cycling, but the response of N cycling to rainfall as a function of forest type has not been investigated. We asked whether the response of soil N dynamics to a rainfall event differed between different mycorrhizal forests, and if so, how. We intercepted rainfall and partly sprinkled water on a test area of both an arbuscular mycorrhizal black locust forest and an ectomycorrhizal oak forest in a dryland area of China. Soil samples were collected between 0 and 9 days thereafter. The levels of ammonium N in soil, and nitrate N as well as the net transformation rate were measured. We also analyzed the abundance and structure of the prokaryotic and ammonia-oxidizing communities to elucidate the mechanism underlying changes in N cycling.
Results/Conclusions
In both forests, prokaryotic abundance and community structure did not change in response to changes in soil moisture, which reflected the number of days after treatment. However, the activities of the community increased in response to artificial rainfall, evidenced in both forests by the positive correlations between ammonium N levels and net N mineralization rate, and soil moisture. By contrast, nitrification clearly differed between the two forests. In the black locust forest, rainfall apparently stimulated nitrification, increasing both the net nitrification rate and nitrate N levels. In the oak forest, rainfall did not increase nitrate N levels but did increase the net nitrification rate. In addition, it reduced the abundance of ammonia-oxidizing bacteria in the oak forest. In the oak forest, the increase in ectomycorrhizal absorption of N due to the rainfall may limit ammonia-oxidizing bacterial growth, and ectomycorrhizal absorption or some immobilization of N may prevent an increase in nitrate N levels after a rainfall event, in spite of the increase in the net nitrification rate measured in the laboratory. Our results suggest that the response of nitrification to a rainfall event differs depending on the forest type, especially on mycorrhizal type.