Short-Term Effects of Three Tree Species on Soil Physicochemical Properties and Microbial Communities During Land-Use Change from Farmland to Forests

In recent decades, much of China’s farmland has been transformed into forests due to the Conversion of Farmland to Forests and Grasses Project. While past research has mainly examined soil nutrients and water conservation, less attention has been given to soil microbial communities. This study examined the effects of converting farmland to forests of Pleioblastus amarus (PA), Populus deltoides (PD), or Zanthoxylum bungeanum (ZB) on the soil physiochemical properties, enzymes, and microbial communities, using abandoned land (AL) as the control, over a period of five years. The results showed that PA increased the soil organic carbon (SOC) content, although not significantly, while significantly boosting the C:N and C:P ratios and urease activity compared to the AL. PD notably reduced the amylase and cellulase activities, as well as the fungal Shannon index. Additionally, the beta diversity of both the bacterial and fungal communities in the PA stand was clearly distinct from that of the AL and the other tree species. The SOC content, total potassium content, and cellulase activity showed significant correlations with bacterial communities. Moreover, the bacterial community changes in the PD and ZB stands were mainly driven by the genera Steroidobacter, Roseisolibacter, and Serendipita, and were negatively correlated with the SOC content, C:N and C:P ratios, and cellulase activity. In contrast, the fungal community changes in the PA stand were primarily influenced by the order Capnodiales, family Capnodiaceae, genus Chaetocapnodium, and species Chaetocapnodium philippinense, which were positively correlated with the soil pH, C:N and C:P ratios, and cellulase activity. Furthermore, “Metabolism” was identified as the primary bacterial function, and converting farmland to forest altered the fungal nutritional type from Saprotroph to Pathotroph–Saprotroph–Symbiotroph, particularly in the PA stand. These findings indicate that converting farmland to forest, particularly with bamboo P. amarus, significantly impacts the bacterial and fungal communities in the soil and changes the fungal trophic type due to the carbon source and cellulase activity of this tree species.