Mycorrhizal and nutrient controls of carbon sequestration in tropical rainforest soil

Tree mycorrhizal associations have substantial consequences for soil organic carbon (SOC), but it remains unclear how nutrient availability will regulate the performance of arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) trees, and then consequently affect SOC sequestration in tropical forest soils. This study characterized the performances of AM and ECM trees, SOC content, and soil microbial functions under variable soil nitrogen (N) and phosphorus (P) content across an intact tropical rainforest based on the spatial dataset from a 60-ha dynamic plot and fitted statistical models to examine the mycorrhizal and nutrient controls on SOC stocks. ECM trees showed a better performance in soils containing higher N in total or in NH4+ forms and enhanced SOC content via increases in both species richness and basal area, which led to an increase in SOC as soil N content evaluated. AM trees had a greater basal area at N-richer (i.e., available N and NH4+) niches while a higher species richness under higher soil P levels (i.e., total and available P). The AM tree community patterns had inconsistent regulations on SOC, with basal area showing a positive while species richness exerting a negative effect on SOC content. Such counteracting effects from AM trees might attenuate SOC accumulation along the P gradient, resulting in a positive trend in SOC with soil total N:P ratios. As soil available P increased, species richness of AM trees increased, which was accompanied by a higher abundance of pathogens while a lower abundance of AM fungi. This indicated a decreased dependence of AM trees on mycorrhizal P acquisition, accompanied by the high susceptibility of roots to pathogen attacks, which may promote AM tree diversity. The performances of ECM trees were positively related to ECM fungi abundance, offering ECM trees a competitive P strategy and pathogen resistance. Summarily, our results suggest that both the basal area and species richness of mycorrhizal trees act as significant regulators for SOC sequestration along soil N or P gradient in tropical forests. Such findings provide a mechanistic understanding of soil C dynamics during vegetation changes under the rising global stoichiometric imbalance between N and P.