Fine root inputs to soil reduce growth of a neighbouring plant via distinct mechanisms dependent on root carbon chemistry

P> Plant carbon (C) and nitrogen (N) inputs to soil interact with microbes and abiotic factors like climate and pH to influence soil fertility and plant productivity. Although root exudates and root litter are important factors affecting the cycling of nutrients critical to plant growth, many studies remain focused on effects of above-ground litter inputs. Using two species that co-dominate alpine moist meadows as a model system (the phenolic-rich forb Geum rossii, and the fast-growing grass Deschampsia caespitosa), we asked whether C from G. rossii fine roots could reduce D. caespitosa growth. We hypothesized that root C would indirectly reduce D. caespitosa growth by stimulating soil microbes, thus restricting plant N supplies. We predicted that this effect could be overcome by fertilization with N. We also hypothesized that G. rossii root phenolics could allelopathically inhibit D. caespitosa growth, and we predicted that direct, allelopathic inhibition would not be reversible with N additions. Both cellulose and phenolic-rich root C additions inhibited D. caespitosa growth in field microcosms, relative to controls. N additions reversed the negative effects of cellulose on plant growth but did not reverse the inhibitory effects of phenolic-rich C additions. A companion soil incubation experiment showed that both types of C addition reduced N availability relative to controls, and there was a strong negative correlation between N availability and substrate-induced soil respiration. Our results indicate that root C inputs to soil can have substantial, negative effects on plant growth. Reduced plant growth was likely to be caused by (i) indirect interactions between root C and soil microbes lowering plant N supply; and (ii) a mechanism independent of N supply, possibly involving allelopathic effects of G. rossii phenolic compounds on D. caespitosa root growth. Synthesis. Fine root inputs to soil (i.e. phenolic-rich C and cellulose C) reduced a neighbouring plant's growth and the chemical composition of the root C influenced whether growth inhibition could be overcome by increased plant N supply. Negative effects of phenolic-rich root inputs on plant growth could be an under-appreciated factor structuring plant communities, particularly in N-limited systems dominated by phenolic-rich species.