The role of plant functional traits and diversity in soil carbon dynamics within riparian agroforests

Restoration of agricultural riparian buffers with trees (agroforestry) provides an elegant solution to enhance carbon storage while also augmenting local biodiversity. Yet the scope and role of riparian plant community diversity in key soil dynamics remain unresolved. Operationalizing riparian age (young [<10 yr] and mature [>30 yr] since establishment] and forest stand type (coniferous and deciduous dominant) to capture the potential extent of plant diversity, we measured plant functional trait diversity and community weighted mean trait values, microbial composition, abiotic soil conditions, and rates of soil CO₂ efflux (mg CO₂ -C m^-2 h^-1 ). We used piecewise structural equation modeling (SEM) to further refine the role of biotic indices (leaf, root, and microbial characteristics), and abiotic factors (soil physio-chemical metrics) on soil C cycling processes in riparian systems. We found significantly lower rates of CO₂ efflux (F = 8.47; p < .01) over one growing season and higher total soil C (F = 3.46; p = .03) in mature buffers compared with young buffers. Using SEM, we describe influences on soil C content (marginal r²  = 61) and soil CO₂ efflux (marginal r²  = 53). Within young buffers, soil C content was significantly predicted by fungal/bacterial ratio and root length density, whereas in mature buffers, tree leaf characteristics were associated with soil C content. Soil CO₂ efflux was predicted by soil moisture, soil carbon content, and herbaceous root characteristics. Evidently, leaf and root functional traits in combination with broad soil parameters significantly describe soil C dynamics in the field; however, significant pathways are not the same throughout the life cycle of a riparian agroforest.