关键词: 马尾松针阔混交林, 石栎-青冈常绿阔叶林, 温度敏感性, 氨化速率, 硝化速率

Abstract: Global warming profoundly affects the nitrogen (N) cycle in forest soils, but the direction of its impact (promotion or inhibition) on soil N mineralization processes (ammonification and nitrification) in different forest types and its temperature sensitivity (Q₁₀) remain unclear. This study selected Pinus massoniana mixed conifer-broadleaf forest (PLL) and Lithocarpus glaber-Cyclobalanopsis glauca evergreen broad-leaved forest (LCC) in the hilly areas of central-southern Hunan Province. Using a constant-temperature incubation method at 10, 25, 30, and 40 ℃, the net ammonification rate (N_AR), net nitrification rate (N_NR), and net N mineralization rate (N_MR) in the 0—10 cm and 10—20 cm soil layers were measured. The relationship between the mineralization rate and temperature was fitted by an exponential model to calculate Q₁₀, and the relationships with biological and abiotic factors were analyzed. The results showed that: (1) The interaction between temperature and forest type and soil depth was significant, manifested as: the N_AR, N_NR, and N_MR of LCC were generally significantly higher than those of PLL, and the differences varied with soil depth and temperature (especially for N_NR); the N_AR and N_MR of both forest soils increased significantly with increasing temperature, while NNR reached a peak at 30 ℃ and then decreased; all rates decreased with increasing soil depth, and the advantage of soil mineralization rate in the surface layer (0-10 cm) was more significant at high temperatures; the activity of soil ammonia-oxidizing microorganisms was the strongest at 30～40 ℃, while the activity of nitrifying microorganisms was the strongest at 10～25 ℃, resulting in a decrease in N_NR at high temperatures (40 ℃). (2) The N_AR-Q₁₀ (2.00—2.01) and N_MR-Q₁₀ (2.01—2.02) of all soil layers in PLL were significantly higher than those in LCC (1.89—1.95 and 1.93—1.94), indicating that the N transformation in PLL soil was more sensitive to warming. Only in the 10—20 cm soil layer of LCC was the N_AR-Q₁₀ significantly higher than that in the surface layer (0—10 cm) soil. (3) N_AR was mainly regulated by the carbon-nitrogen ratio of the litter layer and soil total nitrogen; N_NR and N_MR were mainly driven by microbial biomass phosphorus (MBP) and total nitrogen in the litter layer; N_AR-Q₁₀ was mainly affected by MBP and urease activity; while N_MR-Q₁₀ was mainly regulated by soil inorganic nitrogen. In conclusion, warming significantly promoted the N mineralization process in subtropical forest soils (except for NNR at >30 ℃), and the Q₁₀ of N mineralization in PLL soil was higher than that in LCC. LCC showed higher N mineralization rates and lower temperature sensitivity, highlighting the importance of protecting existing natural forests for maintaining the stability of the soil N cycle and responding to climate change. This study provides a scientific basis for understanding the response mechanism of the soil N cycle in subtropical forests to climate change and optimizing forest management measures.

Key words: Pinus massoniana mixed conifer-broadleaf forest, Lithocarpus glaber-Cyclobalanopsis glauca evergreen broad-leaved forest, temperature sensitivity, net ammonification rate, net nitrification rate