Root exudates mediate N transformations and plant N availability in marginal land soils

Abstract:

Nitrogen (N) availability is a primary constraint on the yields in bioenergy cropping systems, especially in marginal lands with inherently low fertility. N availability is determined by the balance of net N mineralization from soil organic matter and N fixation from atmospheric N₂. Root exudate chemistry changes with plant nutrient status, however, how changes of root exudates affect N transformations in marginal land soils is not well characterized. To explore this question, we sampled soils from two sites in Michigan, part of the Great Lakes Bioenergy Research Center. Each site contains replicated monoculture split-plots of switchgrass (var. Cave-In-Rock); the split-plot treatment is with or without N addition. In a 14 d lab incubation, we added simulated root exudates (carbohydrates (Ch), organic acids (OA), and combination of Ch and OA) at a rate of 100 μg C g^–1 day^–1. We measured soil respiration every day and at the end of incubation, we measured gross N mineralization and immobilization using ¹⁵N pool dilution and N fixation with the ¹⁵N₂-incorpration technique. We found that both Ch and OA treatments significantly increased soil respiration rates, while this increase was relatively higher in Ch treatment than in OA treatment. In contrast, OA treatment caused significantly greater increases than Ch treatment in dissolved organic carbon (DOC) and nitrogen (DON) in soils from both sites. In addition, across sites Ch significantly increased N fixation, while OA did not change or slightly reduced N fixation. Interestingly, we found OA addition significantly increased the soil pH by 2–3 units after 2 weeks. Furthermore, the increases in N fixation rate was negatively correlated with the increased DON. These changes suggest that the N fixation in marginal lands is regulated by the change of available N in soil, which is mediated by root exudates. Overall, we found addition of simulated Ch or OA root exudates differentially mediated N transformations in marginal land soils, which may lead to a better understanding of the plant-microbe interactions in the rhizosphere.