Experimental nitrogen fertilisation globally accelerates, then slows decomposition of leaf litter

Plant litter decomposition is a central process in the carbon (C) cycle and sensitive to ongoing anthropogenic nitrogen (N) fertilisation. Previous syntheses evaluating the effect of N fertilisation on litter decomposition relied largely on models that define a constant rate of mass loss throughout decomposition, which may mask hypothesised shifts in the effect of N fertilisation on litter decomposition dynamics. In this meta-analysis, we compared the performance of four empirical decomposition models and showed that N fertilisation consistently accelerates early-stage but slows late-stage decomposition when the model structure allows for flexibility in decomposition rates through time. Within a particular substrate, early-stage N-stimulation of decomposition was associated with reduced rates of late-stage decay. Because the products of early- vs. late-stage decomposition are stabilised in soils through distinct chemical and physical mechanisms, N-induced changes in the litter decomposition process may influence the formation and cycling of soil C, the largest terrestrial C pool.