pH and substrate regulation of nitrogen and carbon dynamics in forest soils in a karst region of the upper Yangtze River basin, China

Nitrogen (N) is the most limiting nutrient for forest development. In this study, using a natural pH gradient, N and carbon (C) mineralization was investigated and the effects of soil pH as well as the total C and N contents and the soil C/N ratio were evaluated in forest soils after mountain closure in a karst region. The N availability was poor based on the low N mineralization potential (N (0)) and the low active fraction of soil total N (N (0)/total N), while high microbial decomposition activity was indicated by a high mineralization rate constant (k (N)). N (0) was positively correlated with soil pH as well as the total C and N contents. Additionally, multiple regression analysis revealed that total CN (the product of the soil total C and total N contents) and the C/N ratio had more significant effects on N (0) than soil pH. In contrast, the mineralization rate constants k (N) and k (C) were positively affected by soil pH. The results indicated that N availability was regulated by soil organic matter (SOM), while microbial activity was restricted by soil pH. Also, the lack of nitrification and the high C (0)/N (0) ratio observed at soil pH < 5.5 may be a strong indicator of alterations to the microbial composition prompted by severe soil acidification. Further research is required to determine the changes in soil microbial composition with the drop in soil pH and their effects on SOM decomposition and nutrient availabilities.