The multi-pathway mechanism of soil organic carbon accumulation in Pinus massoniana plantations under nitrogen and phosphorus addition: from soil biota to carbon stability

Abstract Background The decline in soil organic carbon accumulation caused by intensified nitrogen deposition is concerning. Although phosphorus input may alleviate the negative impacts, there is still a research gap regarding the mechanisms, particularly those involving the soil biota, that drive the stability of soil organic carbon. Methods We conducted a 2-year nitrogen (0, 30 and 90 kg N ha – 1 yr – 1 ) and phosphorus (0, 30 kg P ha – 1 yr – 1 ) addition experiment with six treatments in a 25-year-old Pinus massoniana plantation in subtropical China. Results The addition of external nutrients improved soil nutrient availability but led to a decrease in pH. Low nitrogen input promoted the particulate organic carbon (POC) and total organic carbon, whereas high nitrogen input had the opposite effect. Phosphorus addition alleviated these negative impacts to some extent. Nitrogen and phosphorus addition significantly affected the dissimilarity of soil biological communities. Nitrogen treatments generally reduced the alpha diversity index of soil bacteria, while the trend for fungi was the opposite. Arthropods showed a rise followed by a decline, with phosphorus addition weakening these effects. Soil respiration decreased with increasing nitrogen addition, and phosphorus addition didn’t alter this trend. The POC was primarily influenced by the soil environment-microorganism-respiration and environment-microorganism pathways, whereas the mineral-associated organic carbon (MAOC) was mainly influenced by the soil environment-arthropod pathway. POC (Path coefficient, pc = 0.524) and MAOC (pc = 0.237) directly determine the accumulation of organic carbon. This conceptual model explained 59.4% of the variation in total organic carbon (Goodness-of-fit, GOF = 0.594), thereby delineating the integrated mechanisms underlying SOC accumulation. Conclusions Excessive nitrogen input was unfavorable for organic carbon accumulation, while phosphorus addition partially mitigated the negative effects of nitrogen excess. Under this context, active organic carbon was significantly influenced by soil microorganisms and soil respiration, whereas stable organic carbon was primarily affected by soil arthropods. Graphical Abstract