Decoupling of nitrogen, phosphorus, and carbon release from fine and coarse roots during 7 years of decomposition

Below‐ground litter decomposition represents an important source of the limiting nutrients nitrogen (N) and phosphorus (P) to forest soils, but roots also immobilize these nutrients during the decomposition process.

Despite clear implications for soil fertility, the rates and drivers of nutrient immobilization and release (as the percent of increase and decrease of the initial pool) from root litter remain poorly understood, especially in coarse roots (>2 mm diameter).

To address this gap, we conducted a 7‐year field decomposition experiment using roots from three species, across five diameter classes (<1, 1–2, 2–5, 5–10 and 10–20 mm) in a temperate forest.

Nitrogen dynamics were largely decoupled with P and carbon (C) over the course of the experiment, and both varied by species and root diameter. Roots released P to the surrounding soil within the first year of decomposition. In contrast, roots immobilized N for much longer, with the coarsest roots remaining a net N sink after 7 years. Long‐term N release was jointly controlled by initial nutrient and C quality, whereas P release and decomposition rate were better predicted by initial C quality. Initial root nutrients well predicted the difference between long‐term N versus P release.

Synthesis. Our results highlight the fact that N and P dynamics should be considered separately when modelling nutrient release during root decomposition, and suggest that the functional diversity of below‐ground biomass may have considerable afterlife effects on the relative availability of N and P in soil. We conclude that root litter, especially coarse root litter, represents an underappreciated N sink in forest soils.