Soil Enzyme Stoichiometry Indicates the Alleviation of Microbial Carbon and Nutrient Limitations After Forest Restoration in Rubber Monocultures, Southwest China

The rapid expansion of rubber monocultures over the past two decades has degraded extensive areas of tropical rainforest, raising concerns about their restoration. A key factor influencing the recovery of these forests remains their microbially mediated biogeochemical cycling processes. Here, we investigated changes in soil carbon and nutrient concentration, the carbon (C), nitrogen (N), and phosphorus (P)-acquiring soil extracellular enzyme activities and their stoichiometric ratios (reflecting microbial nutrient limitations) following forest restoration in rubber monocultures. Furthermore, we evaluated the effects of restoration strategies (natural regeneration and restoration plantings) and soil abiotic properties on enzyme activities and examined correlations between soil nutrient concentration and enzyme activities stoichiometric ratios. Our findings revealed that the enzyme activities in restored forests differed significantly from those in rubber monocultures, with higher or lower activities depending on the enzyme types and the restoration strategies. As restoration advanced, the enzyme C:N:P became relatively balanced, indicating an alleviation of microbial C- and N-limitation. Both restoration strategies alleviated microbial C-limitation to a similar extent, but restoration plantings showed a higher alleviation of microbial N-limitation than natural regeneration. Soil pH emerged as the main factor influencing enzyme activities. The increase in soil total P concentration significantly decreased microbial C-limitation but increased N-limitation. Furthermore, the increase in soil C:P and N:P ratios significantly alleviated the microbial N-limitation. Our findings highlight that converting monoculture rubber plantations back into tropical forests through natural regeneration and restoration plantings promotes positive changes in soil microbial activity, alleviates microbial nutrient limitations, and fosters a more balanced nutrient acquisition strategy. These results provide critical scientific support for ecological restoration efforts in tropical regions.