Thinning Promotes Soil Phosphorus Bioavailability in Short-Rotation and High-Density Eucalyptus grandis × E. urophylla Coppice Plantation in Guangxi, Southern China

Abstract

Thinning can improve soil nutrient supply, but the effects of thinning on soil phosphorus (P) contents and bioavailable mechanisms in high-density and short-rotation Eucalyptus coppice forests are not well reported. Therefore, we conducted five intensities of thinning treatments, which were 83% (283 tree ha−1, T1), 66% (566 tree ha−1, T2), 50% (833 tree ha−1, T3), 33% (1116 tree ha−1, T4), and 0% (1665 tree ha−1) in a 2nd 6-year-old E. grandis × E. urophylla coppice plantation with 8 years as a rotation, investigated soil nutrient contents, microbial biomass, and extracellular enzyme activities of 0–20 and 20–40 cm soil layers after two years of thinning, and analyzed the relationship between available phosphorus (AP) and other indicators. The results showed that soil total phosphorus (TP) contents in 2nd Eucalyptus coppice plantations were lower than in native forest ecosystems, but T1 significantly increased (p < 0.05) TP by 81.42% compared to CK of 0–20 cm, whereas T2 and T3 improved available phosphorus (AP) by 86.87%–212.86% compared to CK. However, soil organic carbon (SOC), dissolved organic carbon (DOC), total nitrogen (TN), and alkaline hydrolysable nitrogen (AN) were not significantly different (p < 0.05) among all treatments. According to the analysis, soil TP contents were significantly positively related (p < 0.001) to SOC; soil total nutrients and DOC contents had the highest standardized total effect on AP; meanwhile, the quotient of microbial biomass directly conducted soil AP contents. These results highlighted that thinning can be used to alleviate soil P shortages by promoting multinutrient and biological cycles in Eucalyptus coppice forests.