Divergent Effects of Understory Vegetation Manipulation on Soil Carbon, Nitrogen, and Phosphorus in a Temperate–Subtropical Transition Zone Mixed Forest

Understory vegetation (shrubs and herbs) mediates belowground biogeochemical processes in forests through litter inputs, root exudation, and microenvironmental regulation; however, the magnitude of these regulatory effects remains poorly quantified. Here, we conducted a 10-year small-scale understory vegetation manipulation experiment in a coniferous–broadleaf mixed forest in central China, aiming to systematically assess the impacts of understory vegetation on soil carbon (C), nitrogen (N), and phosphorus (P) dynamics. Two experimental treatments were established: (1) the “None” treatment (removal of both understory vegetation and litter) and (2) the “Understory” treatment (litter removal while retaining understory vegetation). Results indicated that compared with the “None” treatment, the “Understory” treatment did not significantly alter the concentrations or stocks of soil organic C (SOC) and total N (p > 0.05), suggesting a weak responsiveness of SOC and total N to understory vegetation presence. In contrast, understory vegetation exerted a significant positive effect on soil P fractions: total P concentration and stock increased by 3.97% and 2.68%, organic P by 6.65% and 5.32%, and available P by 46.38% and 43.96%, respectively (p < 0.05). These results demonstrate that understory vegetation exerts a more pronounced regulatory effect on soil P dynamics than on C and N dynamics. In conclusion, understory vegetation plays a pivotal role in promoting soil P sequestration and improving P availability in coniferous–broadleaf mixed forest ecosystems. We recommend retaining understory vegetation in forest management practices to sustain soil P availability and mitigate widespread P limitation in such ecosystems.