Can larch- Aralia elata agroforestry systems improve the soil chemical and microbial properties of larch plantations?

The conversion of secondary forests to larch plantations has resulted in soil degradation in Northeast China. Previous studies have proven that introducing native broadleaved tree species into larch plantations could improve soil quality, but it could not provide economic benefits in the short term. To gain short-term economic benefits, Aralia elata (Miq.) Seem., a native broadleaved shrub or small tree with high economic value, has been introduced into larch plantations and thus formed a larch- A. elata agroforestry system. However, the effect of this practice on degraded soil of larch plantations remains unclear. Here, we compared the soil chemical and microbial properties at four soil depths (humus, 0–10 cm, 10–20 cm, 20–30 cm) in paired stands of larch plantations and adjacent larch- A. elata agroforestry systems with different years since inter-planting (1, 3, 5, and > 10 years). The results showed that compared with larch plantations, most chemical and microbial properties significantly changed with inter-planting years in larch- A. elata agroforestry systems, especially at the humus layer and 0–10 cm soil layer. Particularly in the larch- A. elata agroforestry system with inter-planting for over 5 years, the soil chemical (mineral nitrogen, available phosphorus, and pH) and microbial (microbial biomass of C, N, and P, β-glucosidase, β-cellobiohydrolase, N-acetyl-β-glucosaminidase, acid phosphatase, phenol oxidase, and peroxidase) properties significantly increased by 5–97% in the humus layer and by 3–110% in 0–10 cm soil layer. Most of the chemical and microbial properties were mainly affected by the number of years since inter-planting, basal area, litterfall, and C/N ratio of the forest floor. Conclusively, inter-planting with A. elata could improve the soil chemical and microbial properties of larch plantations, especially after > 5 years since inter-planting, while providing economic benefits simultaneously in the short term.