Different coupled carbon-nitrogen terrestrial biosphere models (TBMs) make different assumptions about the mechanisms that determine nitrogen uptake from soils into vegetation. The majority assume that uptake is proportional to fine root mass. The remainder assume that uptake is independent of fine root mass. Dybzinski et al. (2019, Global Change Biology) found that plant community nitrogen uptake was largely independent of fine root mass in greenhouse microcosms across a variety of species and across a variety of growing conditions. To determine if this result holds in established forests, we measured nitrogen uptake and fine root mass in 18 mono-dominant stands of different gymnosperm and angiosperm species established at the Morton Arboretum (Lisle, Illinois, USA), mostly in the early 1920s to 1930s. We measured nitrogen uptake as the nitrogen concentrations of leaves, wood, and fine roots multiplied by their respective growth increments, with all measurements normalized to gN m-2 y-1. In addition to fine root mass, we measured a variety of additional fine root traits, including root length density and mean diameter.
Results/Conclusions
Consistent with the earlier greenhouse results, nitrogen uptake rate was independent of fine root mass. Nitrogen uptake rates spanned 3 to 10 gN m-2 y-1, fine root mass spanned 75 to 400 g m-2, fine root length density spanned 700 to 3200 m m-2, and mean fine root diameter spanned 0.55mm to 0.8mm (for roots less than 1mm). These results suggest that fine root mass is “over-proliferated” for game-theoretic reasons relative to nitrogen availability and thus independent of uptake rate, analogous to the way a large sponge and a very large sponge are equally capable of soaking up a relatively small spill. Although it may be tempting to include fine root mass in the nitrogen uptake equations of TBMs, these results suggest that it may be a mistake.