Soil carbon, nitrogen, phosphorus contents and enzyme activities in different mixed fir and broad-leaved plantations

Mixed planting of fir and broadleaf trees is an effective management method for improving the productivity. We investigated soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) contents, along with their stoichiometric ratios across 10 different combinations of Chinese fir and broadleaf species (Mytilaria laosensis, Michelia macclurei, Michelia maudiae, Cinnamomum porrectum, Castanopsis hystrix, Cinnamomum burmanni, Liquidambar formosana, Michelia chapensis, Schima superba, and Michelia odora, respectively) with the stand age of 20 yr. Additionally, we examined the activities and stoichiometric characteristics of various enzymes: β-1,4-glucosidase, β-xylosidase, cellobiohydrolase, β-1,4-N-acetylaminoglucosidase, leucine aminopeptidase, and acid phosphatase to analyze nutrient limitation status in different mixed stands. The results showed that: 1) In the 0-10 cm and 10-30 cm soil layers, the highest SOC and TN contents were found in M. macclurei, whereas in the 30-50 cm soil layer, the highest contents were observed in S. superba. The highest TP content across all three soil layers was recorded in C. burmanni. Soil SOC and TN contents of the 10 forest stands decreased with the soil depth, while TP was not affected by the soil depth. 2) The measured ranges across the 10 stand types were 4.82-32.69 g·kg^-1 for SOC, 0.50-2.16 g·kg^-1 for TN, and 0.24-0.55 g·kg^-1 for TP, indicating nutrient deficiency. Only the stands of M. macclurei and S. superba reached national average levels for SOC and TN, while the TP content of all forest stands was below the national average. 3) The results of the enzyme quantification vector model indicated that soil microbial metabolism was jointly limited by nitrogen and phosphorus. 4) Redundancy analysis revealed that TN, available phosphorus (AP), ammonium (NH₄⁺-N), soil water content (SWC), pH, and TP were the primary factors influencing enzyme activities. Additionally, pH, AP, TP, SWC, and the carbon-to-phosphorus (C:P) ratio emerged as key determinants of enzymatic stoichiometry. In summary, M. macclurei and S. superba were effective in promoting soil nutrient levels in mixed forests of fir and broadleaf trees, being the optimal tree species for the mixed forest renovation in the South subtropical region. Given the prevalent limitations of nitrogen and phosphorus, external nitrogen and phosphorus inputs should be considered in future management practices.