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Fungi represent a rapidly cycling pool of carbon (C) and nitrogen (N) in soils. Understanding of how this pool impacts soil nutrient availability and organic matter fluxes is hindered by uncertainty regarding the dynamics and drivers of fungal necromass decomposition. Here we assessed the generality of common models for predicting mass loss during fungal necromass decomposition and linked the resulting parameters to necromass substrate chemistry. We decomposed 28 different types of fungal necromass in laboratory microcosms over a 90‐day period, measuring mass loss on all types, and N release on a subset of types. We characterised the initial chemistry of each necromass type using: (a) fibre analysis methods commonly used for plant tissues, (b) initial melanin and nitrogen (N) concentrations and (c) Fourier transform infrared (FTIR) spectroscopy to assess the presence of bonds associated with common biomolecules. We found universal support for an asymptotic model of decomposition, which assumes that fungal necromass consists of an exponentially decomposing ‘fast’ pool, and a ‘slow’ pool that decomposes at a rate approaching zero. The strongest predictor of the fast pool decay rate (k) was the proportion of cell soluble components, though initial N concentration also predicted k, albeit more weakly. The size of the slow pool was best predicted by the acid non‐hydrolysable fraction, which was positively correlated with melanin‐associated aromatics. Nitrogen dynamics varied by necromass type, ranging from net N release to net immobilisation. The maximum quantity of N immobilised was inversely related to cell soluble contents and k, as positively related to FTIR spectra associated with cell wall polysaccharides. Collectively, our results indicate that the decomposition of fungal necromass in soils can be described as having two distinct stages that are driven by different components of substrate C chemistry, with implications for rates of N availability and organic matter accumulation in soils. A free Plain Language Summary can be found within the Supporting Information of this article.

Increased nutrient inputs due to anthropogenic activity are expected to increase primary productivity across terrestrial ecosystems, but changes in allocation aboveground versus belowground with nutrient addition have different implications for soil carbon (C) storage. Thus, given that roots are major contributors to soil C storage, understanding belowground net primary productivity (BNPP) and biomass responses to changes in nutrient availability is essential to predicting carbon-climate feedbacks in the context of interacting global environmental changes. To address this knowledge gap, we tested whether a decade of nitrogen (N) and phosphorus (P) fertilization consistently influenced aboveground and belowground biomass and productivity at nine grassland sites spanning a wide range of climatic and edaphic conditions in the continental United States. Fertilization effects were strong aboveground, with both N and P addition stimulating aboveground biomass at nearly all sites (by 30% and 36%, respectively, on average). P addition consistently increased root

Soils store 2.5 and plants store 1.5 times as much C compared to the atmosphere, respectively (Schlesinger & Bernhardt, 2013). Thus, even small changes to the size of these C sinks, or to rates of photosynthesis or respiration, could have large consequences for atmospheric CO 2 levels. Importantly, other global change drivers, such as nitrogen (N) deposition (Stevens, 2019;Vitousek et al., 1997), may be

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