Planting density is one of the most important factors affecting soil properties and thereby directly impacting plantation yields. However, the effect of planting density on soil quality and productivity in plantation ecosystem mostly remain unknown. 

In a study published in Applied Soil Ecology, using a soil quality index (SQI) derived from soil quality indicators which are selected by principal component analysis, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences evaluated the effect of planting density of pine (Pinus kesiya) plantations on soil quality up to one meter soil depth. 

The researchers assed soil quality in four mature pine plantations with increasing planting densities (low, moderate, high, and very high) respectively, up to one meter soil depth. The study was conducted in the Ailaoshan mountains (24°32′N, 101°01′E), situated in the central Yunnan province, southwest China. 

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 contributed more towards SQI.  

They found that the soil quality index significantly decreased with increasing planting density, showing that high planting densities in subtropical plantations have led to soil degradation, especially in the deep soils. Stem biomass and SQI were positively related, indicating that soil quality increased yields.

With low planting density, the soil quality was higher, characterized by a higher understory vegetation cover, demonstrating that restoration of understory vegetation can recover soil quality in high planting density and very high planting density. Therefore, soil quality of pine plantations with high planting densities can be improved by thinning and maintaining a moderate number of pine trees through selective logging. 

"To our knowledge, this is the first study to assess soil quality along a soil profile (one meter) and to identify the relationship between SQI and stem biomass of pine plantations," said Prof. YANG Xiaodong of XTBG.