- Carbon (C) inputs and nutrient availability are known to affect soil organic carbon (SOC) stocks. However, general rules regarding the operation of these factors across a range of soil nutrient availabilities and substrate qualities are unidentified. "Priming" (stimulated decomposition by labile C inputs) and 'preferential substrate utilization' (retarded decomposition due to shifts in community composition towards microbes that do not mineralize SOC) are two hypotheses to explain effects of labile C additions on SOC dynamics. For effects of nutrient additions (nitrogen and phosphorus) on SOC dynamics, the stoichiometric (faster decomposition of materials of low carbon-to-nutrient ratios) and 'microbial mining' (that is, reduced breakdown of recalcitrant C forms for nutrients under fertile conditions) hypotheses have been proposed. Using the natural gradient of soil nutrient availability and substrate quality of a chronosequence, combined with labile C and nutrient amendments, we explored the support for these contrasting hypotheses.Carbon (C) inputs and nutrient availability are known to affect soil organic carbon (SOC) stocks. However, general rules regarding the operation of these factors across a range of soil nutrient availabilities and substrate qualities are unidentified. "Priming" (stimulated decomposition by labile C inputs) and 'preferential substrate utilization' (retarded decomposition due to shifts in community composition towards microbes that do not mineralize SOC) are two hypotheses to explain effects of labile C additions on SOC dynamics. For effects of nutrient additions (nitrogen and phosphorus) on SOC dynamics, the stoichiometric (faster decomposition of materials of low carbon-to-nutrient ratios) and 'microbial mining' (that is, reduced breakdown of recalcitrant C forms for nutrients under fertile conditions) hypotheses have been proposed. Using the natural gradient of soil nutrient availability and substrate quality of a chronosequence, combined with labile C and nutrient amendments, we explored the support for these contrasting hypotheses. Additions of labile C, nitrogen (N), phosphorus (P), and combinations of C and N and C and P were applied to three sites: 2-year fallow grassland, mature grassland and forest, and the effects of site and nutrient additions on litter decomposition and soil C dynamics were assessed. The response to C addition supported the preferential substrate hypothesis for easily degradable litter C and the priming hypothesis for SOC, but only in nitrogen-enriched soils of the forest site. Responses to N addition supported the microbial mining hypothesis irrespective of C substrate (litter or SOC), but only in the forest site. Further, P addition effects on SOC support the stoichiometric hypothesis; P availability appeared key to soil C release (priming) in the forest site if labile C and N is available. These results clearly link previously contrasting hypotheses of the factors controlling SOC with the natural gradient in litter quality and nutrient availability that exists in ecosystems at different successional stages. A holistic theory that incorporates this variability of responses, due to different mechanisms, depending on nutrient availability and substrate quality is essential for devising management strategies to safeguard soil C stocks.…
