Analysis of Nitrogen Transformation Characteristics and Influencing Factors of Forestland Soil in the Qinghai-Tibet Plateau: A Case Study of the Qilian Mountains and Southeast Tibet

Abstract

Mineralization and nitrification, two important processes of the N cycle in forest ecosystems, play an important role in the maintenance of ecosystem function and soil environmental quality. The net mineralization rate and net nitrification rate are two common indexes for evaluation of the N supply and N loss risk in soil. In order to study the soil N transformation characteristics and their influencing factors in woodlands of the Qinghai-Tibet Plateau, the inorganic N concentrations, net N mineralization rates, and net nitrification rates of 327 soils sampled from the Qilian Mountains and Southeast Tibet were analyzed using indoor incubation experiments. The results showed that the inorganic N concentration, net N mineralization rate, and net nitrification rate of the woodland soil in Southeast Tibet ［109.70 mg·kg-1, 3.08 mg·(kg·d)-1, and 2.19 mg·(kg·d)-1］ were significantly higher than those in the Qilian Mountains ［49.47 mg·kg-1, 0.70 mg·(kg·d)-1, and 0.69 mg·(kg·d)-1］. The net mineralization rate of soil was positively correlated with the mean annual temperature (MAT), mean annual precipitation (MAP), moisture index, and soil organic matter concentrations (P<0.001). There were no significant positive relationships between the net nitrification rate and the MAT and MAP, but the net nitrification rate was positively correlated with the net N mineralization rate (P<0.001). These results indicated that the N supply ability of woodland soil in Southeast Tibet is significantly higher than that in the Qilian Mountains, and the hydrothermal conditions (MAT and MAP) are likely responsible for the difference in the net mineralization rate between the two studied regions. The net mineralization rate is the limiting factor for the net nitrification rate, which has an important influence on the spatial distribution of the forest soil nitrification process in the Qinghai-Tibet Plateau. These results emphasize the important roles of climatic and environmental factors, such as hydrothermal conditions, on the net N mineralization and nitrification rates of woodland soil in the Qinghai-Tibet Plateau, and will further deepen the understanding of the law of soil N transformation in woodlands in the Qinghai-Tibet Plateau.