Fungal-to-bacterial dominance of soil food-webs in a changing world: Consequences for the priming effect and resource-use efficiency
Food-web models have established a widely held belief that the bacterial decomposer pathway in soil supports high turnover rates of easily available substrates, while the slower fungal pathway supports the decomposition of more complex organic material, thus characterising the ecosystem biogeochemistry. Three field-experiments simulating how SOM-quality is modulated by a changing climate were assessed. (1) the Detritus Input, Removal, and Trenching – DIRT – experiment in a temperate forest in Harvard Forest LTER, US. There, experimentally adjusted litter input and root input had affected the SOM quality during 23 years; (2) a litter addition and warming experiment in subarctic tundra soils, in Abisko, Sweden. There, leaf litter additions (90 g m-2 y-1), or warming using open-top chambers, had modulated SOM quality; (3) field-application of 14C-glucose to UK grassland soils, sampled over 13 months to generate an age-gradient of SOM. A combination of stable isotope (13C) and radio-isotope (14C) studies, fungal and bacterial growth measurements, and C and N mineralisation (15N pool dilution) assays were used to investigate how SOM-quality influenced fungal and bacterial food-web pathways and the implications this had for C and nutrient turnover and microbial use as well as for the ability of labile C supplements to ‘prime’ SOM mineralisation.
In the DIRT experiment at Harvard Forest, we found no evidence to substantiate that detrital food-webs dominated by bacteria support high turnover rates of easily available substrates, while slower fungal-dominated decomposition pathways support the decomposition of more complex organic material. Rather the opposite, an association between high-quality SOC and fungi, emerged from the results. We found that simulated climate change both via increased plant litter input and as a result of warming could influence the susceptibility of subarctic tundra soils to priming by labile C. However, while C mineralisation was inhibited by labile C additions by <60%, N mineralisation was powerfully simulated (<300 %), suggesting selective microbial N-mining. Both C and N mineralisation responses were surprisingly rapid, faster than responses in growth, suggesting that catabolic rather than anabolic changes were at least partially responsible for the dichotomy. In the UK grassland soils, we could detect pronounced (<350%) priming of SOM decomposition by labile C supplements, but found that these were not systematically connected to fungal and bacterial growth dynamics. Taken together, our results suggest that we need to revise our basic understanding of soil microbial communities and the processes they regulate in soil.