Soil organic nitrogen transformations in typical karst and non-karst forests, southwest China

Proteins represent the major chemical forms of soil organic nitrogen (N) with their depolymerization being considered as the bottleneck step of soil N cycling. However, soil organic N transformations including protein depolymerization and their controls have not been well explored so far especially for the tropical or subtropical regions. In the current study, a 15N pool dilution assay was used to quantify the gross rates of protein depolymerization and microbial amino acid uptake in three soil depths, i.e., 0-10 cm, 10-20 cm, and 20-40 cm in a karst forest and a neighboring non-karst subtropical forest underlain by carbonate rocks and clasolite, respectively. The mean rates of soil protein depolymerization were comparable between the karst forest (29.31 ± 7.39 μg N g-1 d-1) and non-karst forest (40.58 ± 8.41 μg N g-1 d-1), but soil microbial amino acid uptake in the karst forest (104.82 ± 14.17 μg N g-1 d-1) was significantly lower relative to the non-karst forest (200.56 ± 18.65 μg N g-1 d-1). Soil protein depolymerization rates significantly decreased along with soil depth in both forests. Amino acid uptake rate was higher in the 10-20 cm soil layer than the other two layers in the karst forest, but showed no significant difference among soil layers in the non-karst forest. Particulate organic N and soil pH were identified as the strongest factors controlling the variation of protein depolymerization and microbial amino acid uptake, respectively. Our findings highlight the crucial role of lithology in regulating soil organic N transformations.