A robust method confirming organic nitrogen uptake by mycorrhizal roots in a temperate forest

Background/Question/Methods

The uptake of nitrogen in organic form (for example, as amino acids rather than as ammonium or nitrate) by plants is important in tundra and boreal forest ecosystems, but the process is not as well quantified in temperate forests.  One criticism of previous experiments demonstrating organic N uptake is that concentrations of isotopically labeled amino acids added to soils may not be representative of concentrations available to roots and mycorrhizal hyphae in situ.  We developed an experimental approach that complements previous experimental work on this question, by adding ¹³C- and ¹⁵N-labeled whole organic matter (cyanobacteria) to root-ingrowth cores, and allowing soil proteolytic enzymes to release labeled organic N at a natural rate over a period of six weeks.

Results/Conclusions

We employed this method in eight forest stands on Inceptisols in southern New Hampshire, spanning a gradient of soil pH, N mineralization rate, mycorrhizal type, and root production.  Preliminary analyses show intact uptake of organic N compounds by roots of both ectomycorrhizal and arbuscular mycorrhizal tree species, in addition to the apparently predominant uptake of inorganic N.  Our findings thus far imply that previous results demonstrating uptake of organic N are not simply an experimental artifact of the choice of tracer compound or its added concentration.  Additional work remains to be done to quantify the importance of this process to overall N cycling across forest ecosystems under natural, N-enriched, and otherwise disturbed conditions.