The characteristics of soil N transformations regulate the composition of hydrologic N export from terrestrial ecosystem

摘要

It is important to clarify the quantity and composition of hydrologic N export from terrestrial ecosystem and its primary controlling factors, because it affected N availability, productivity, and C storage in natural ecosystems. The most previous investigations were focused on the effects of N deposition and human disturbance on the composition of hydrologic N export. However, few studies were aware of whether there were significant differences in the concentrations and composition of hydrologic N export from natural ecosystems in different climate zones and what is the primary controlling factor. In the present study, three natural forest ecosystems and one natural grassland ecosystem that were located in different climate zones and with different soil pH range were selected. The concentrations of total dissolved N, dissolved organic nitrogen (DON), NH_4~+, NO_3~-in soil solution and stream water, soil properties, and soil gross N transformation rates were measured to answer above questions. Our results showed that NO_3~-concentrations and the composition pattern of hydrologic N export from natural ecosystems varied greatly in the different climate zones. The NO_3~-concentrations in stream water varied largely, ranging from 0.1mgNL~(-1) to 1.6mgNL~(-1), while DON concentration in streamwater, ranging from 0.1 to 0.9mgNL~(-1), did not differ significantly, and the concentrations of NH_4~+ were uniformly low (average 0.1mgNL~(-1)) in all studied sites. There was a trade-off relationship between the proportions of NO_3~-and DON to total dissolved N in stream water. In subtropical strongly acidic forests soil site, DON was the dominance in total dissolved N in stream water, while NO_3~--N became dominance in temperate acidic forests soil site, subtropical alkaline forests soil region, and the alpine meadow sites on the Tibetan Plateau. The proportions of NO_3~-to total dissolved N in both soil solution and stream water significantly increased with the increasing of the gross autotrophic nitrification rates (p<0.01). Our results indicated that the characteristics of soil N transformations were the most primary factor regulating the composition of hydrologic N losses fromecosystems. The nitrification was the central soil N transformation processes regulating N composition in soil solution and hydrologic N losses. These results provided important information on understanding easily the composition of hydrologic N export from terrestrial ecosystem.