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

HUYGENS, D; URCELAY, C.; BOECKX P. ; DÍAZ, S

SOIL BIOLOGY AND BIOCHEMISTRY

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2017 vol. 101

0038-0717

Plant uptake of dissolved organic nitrogen (DON) has been proposed to explain inconsistency in theecosystem N balance of semi-arid systems. Nevertheless, direct evidence for an ecologically importantrole of DON in plant nutrition in these systems remains elusive under field conditions. Here, naturalabundance 15N signatures of NO3, NH4þ, DON and whole plants from a semi-arid model forest wereanalyzed to provide robust estimates of plant N source partitioning and relative N cycling rates under insituconditions. Concurrently, architectural and symbiotic root traits were determined to assess theirrelationship to plant N acquisition strategies. Bayesian isotope mixing models indicated an insignificantcontribution of DON to ecosystem plant N nutrition. Nitrate was the dominant plant N source in thisecosystem, while the contribution of NH4þ to plant nutrition varied between herbaceous (26%) andwoody plants (8%). The low C:N ratio of the dissolved organic matter pool - ranging from 12.7 to 4.9within the soil profile e indicated microbial C-limitation in this ecosystem. Dissolved organic N wassignificantly enriched in 15N relative to NH4þ and NO3, corroborating the importance of dissolved organicmatter recycling as a cost-effective pathway that simultaneously supplies C and nutrients for microbialmetabolism. Plants exclusively depend on inorganic N forms made available through microbial Nmineralization and free-living atmospheric N2 fixation, followed by autotrophic nitrification