Nitrogen-fixing tree species modulate species richness effects on soil aggregate-associated organic carbon fractions

Soil aggregation is an important mechanism shaping soil organic carbon (SOC) fractions, but the effects of tree species richness on soil aggregate-associated organic carbon fractions are poorly understood. In this study, a manipulative experiment was conducted to examine whether and how tree species richness (1, 2, 4 and 6) together with or without the presence of nitrogen (N)-fixing tree species affect soil aggregate-associated organic carbon. Soil aggregates were classified into four fractions: large macroaggregates (>2 mm), small macroaggregates (0.25–2 mm), microaggregates (0.053–0.25 mm), and silt and clay (<0.053 mm). And then large and small macroaggregates were further separated into coarse (c-iPOC), fine intra-aggregate particulate organic carbon (f-iPOC) and mineral associated organic carbon (MAOC), and microaggregates were divided into f-iPOC and MAOC. We observed that the effects of tree species richness on soil aggregate-associated organic carbon fractions were depended on the presence or absence of N-fixing tree species. In the presence of N-fixing tree species, organic carbon contents of small macroaggregate, microaggregate and silt and clay fractions were increased across the gradient of tree species richness (+10.4%, +31.3% and + 26.7% from monocultures to 6-species mixture), and the similar responses occurred for c-iPOC within small macroaggregates (+42.9%) and f-iPOC within microaggregates (+50.0%). In the absence of N-fixing tree species, however, only f-iPOC within large macroaggregates (+60.0%) had a positive linear relationship with tree species richness. Structural equation modeling was then used to dissect the mechanism underlying the positive effects of tree species richness on the aggregate-associated organic carbon fractions, indicating either direct or indirect effects by increasing root length density. Our study highlights the important roles of N-fixing tree species in shaping tree species richness effects on soil aggregate-associated organic carbon fractions, and therefore, N-fixing tree species should be preferentially considered in mixed-species plantations to enhance long-term SOC stabilization.