Morphology and Decomposition of Roots Under Organic Management in the Southeastern U.S.

A better understanding of the belowground function of forage mixtures in organic systems can contribute to management recommendations for organic farmers in relation to soil fertility and C sequestration. The productive capacity of an organic system depends on building soil quality and enhancing biotic interactions between plants and soil, perhaps more than conventional forages. Roots are an important location for these interactions: roots scavenge nutrients and water and then release organic and mineral molecules into the soil through rhizodeposition, root sloughing, and after plant death. Factors of plant belowground biology included in this study are root morphology and root decomposition. Using data gathered from two replicated research farm sites in Tennessee and Kentucky, both conforming to USDA organic certification requirements, we found that forage species mixtures with varying functional groups (annual and perennial, warm-season and cool-season, legumes and grasses) differed in root biomass and root length density, but not in root fineness, which has implications for mixture function and soil quality. We evaluated decomposition of roots from three species used for forages which belong to three functional groups: a legume, a cool-season grass, and a warm-season annual grass alone and in mixtures. Although we thought that the addition of legume roots to grass roots would stimulate carbon mineralization, we found that there was no increase, perhaps because of root chemical composition. However, we expected that warm and cool season grasses would decompose differently due to the biochemical differences between C3 and C4 plants, and indeed we found that the warm-season annual grass caused a higher carbon mineralization. In summary, belowground characteristics of organic forages in the Southeast have some similarities with other low-input systems, and in other ways they are different.