Microbial recycling of dissolved organic matter confines plant nitrogen uptake to inorganic forms in a semi-arid ecosystem

Plant uptake of dissolved organic nitrogen (DON) has been proposed to explain inconsistency in the ecosystem N balance of semi-arid systems. Nevertheless, direct evidence for an ecologically important role of DON in plant nutrition in these systems remains elusive under field conditions. Here, natural abundance 15N signatures of NO3−, NH4+, DON and whole plants from a semi-arid model forest were analyzed to provide robust estimates of plant N source partitioning and relative N cycling rates under in-situ conditions. Concurrently, architectural and symbiotic root traits were determined to assess their relationship to plant N acquisition strategies. Bayesian isotope mixing models indicated an insignificant contribution of DON to ecosystem plant N nutrition. Nitrate was the dominant plant N source in this ecosystem, while the contribution of NH4+ to plant nutrition varied between herbaceous (26%) and woody plants (8%). The low C:N ratio of the dissolved organic matter pool - ranging from 12.7 to 4.9 within the soil profile - indicated microbial C-limitation in this ecosystem. Dissolved organic N was significantly enriched in 15N relative to NH4+ and NO3−, corroborating the importance of dissolved organic matter recycling as a cost-effective pathway that simultaneously supplies C and nutrients for microbial metabolism. Plants exclusively depend on inorganic N forms made available through microbial N mineralization and free-living atmospheric N2 fixation, followed by autotrophic nitrification.