Planting density affects soil quality in the deep soils of pine plantations

Planting density is one of the important factors affecting soil properties, thereby directly impacting plantation yields. However, the effect of planting density on soil quality and productivity in plantation ecosystem mostly remain unknown. The present study aims to assess soil quality in four mature pine (Pinus kesiya) plantations with increasing planting densities i.e., 700 trees ha−1 (low planting density, LPD), 1080 trees ha−1 (moderate planting density, MPD), 1260 trees ha−1 (high planting density, HPD), and 2340 trees ha−1 (very high planting density, VHPD) respectively, up to 1 m soil depth. The soil samples collected at 0–20, 20–60, and 60–100 cm depth intervals were analyzed for a total of thirty-nine physical, chemical, and biological properties. The soil quality index (SQI) was calculated using linear scoring and weighted additive method. Total carbon, arbuscular mycorrhizal fungi (AMF) biomass, gram-positive bacteria biomass, soil moisture content, and β-glucosidase enzyme were selected as the important drivers of SQI. Total carbon, AMF, and moisture content had higher contribution to SQI. The results showed that increasing planting density adversely affected soil quality. SQI was higher in the surface (0–20 cm) soil layer, and it decreased with depth. Planting densities, soil depths, and their interaction had a significant effect on SQI. Increasing SQI promoted the stem biomass with a positive linear response. Therefore, maintaining a moderate to low planting density by thinning practices and selective logging could recover natural understory vegetation, acquire high timber yield, and sustain soil quality in pine plantations.