Rhizosphere influence on soil microbial biomass and enzyme activity in banj oak, chir pine and banj oak regeneration forests in the central Himalaya

The present study aims to investigate the rhizosphere effect on soil organic carbon, nutrients and soil microbial biomass in three forests viz, banj oak (Quercus leucotrichophora), chir pine (Pinus roxburghii) and banj oak regeneration forest (banj oak regeneration in chir pine) in the central Himalaya. Soil samples were collected seasonally from three depths (0-10 cm, 10-20 cm, 20-30 cm) at two soil positions (i.e., rhizosphere and bulk soil). The soil physicochemical and biological properties were compared between rhizosphere soil (RS) and bulk soil (BS), among forest types using two-way ANOVA and comparison of means was assessed by Tukey's HSD post hoc test. Correlation analysis was performed to observe the association among physical, chemical and biological properties of the soil. Additionally, a Linear Mixed Effect Model (LMM) was applied to investigate the effect of forest types, soil positions, seasons, depths and their interactions on soil biological properties. In the rainy season, soil organic carbon (SOC) and total nitrogen (TN) in RS of banj oak were significantly higher (p < 0.05) than the BS in the upper soil layer (0-10 cm). However, the difference was less prominent in the middle (10-20 cm) and lower soil layer (20-30 cm). Soil microbial biomass (Cmic, Nimc and Pmic) in RS of banj oak and chir pine were significantly higher than the BS in upper and middle soil layers, while both RS and BS of banj oak regeneration did not vary significantly. Forests differed significantly in the rhizosphere effect. Banj oak showed a strong rhizosphere effect through the combined impact of favourable microclimatic conditions and high soil microbial biomass. Chir pine grows in nutrient-depleted soil, showed a low rhizosphere effect compared to the rhizosphere effect of banj oak. The SOC, Cmic and Pmic were generally higher in the RS of banj oak regeneration forest than the RS of chir pine, reflecting that oak regeneration enhances microbial diversity through an increased supply of root induced (root exudates) microbially mineralised nutrients. The rhizosphere effect changed significantly with soil depths and was found higher in the upper soil layer (due to high soil nutrients and microbial activity) and decreased with increasing soil depths. Across the forests, the rhizosphere effect was more prominent in the rainy season than the winter and summer seasons, mainly driven by high soil moisture content. Principal Component Analysis (PCA) was performed to obtain rhizosphere soil microbial index (RSMI). Our study suggests that Cmic, Cmic: SOC and Pmic: AP were the most important parameters to evaluate rhizosphere soil quality of this temperate forest of the central Himalaya.

Automated summary

This study investigates the rhizosphere effect on soil organic carbon, nutrients, and soil microbial biomass in different forests in the central Himalaya. The results show that the rhizosphere significantly affects soil properties, with banj oak exhibiting a strong rhizosphere effect and chir pine showing a low rhizosphere effect. The rhizosphere effect decreases with increasing soil depth and is more prominent in the rainy season. The study suggests that the rhizosphere soil microbial index can be used to evaluate soil quality in temperate forests.