Assessment of soil carbon stabilisation and soil microbial diversity following stoichiometric inputs of carbon, nitrogen, phosphorus and sulphur to the soil

The soil microbial community requires N, P and S in proportion to C to effectively metabolise C-inputs. Meeting microbial CNPS requirement presents as a strategy to increase the rate of soil carbon (C) sequestration. The aim was to test C:N:P:S stoichiometric inputs for the stabilization of soil carbon in the soil and assess the role of the microbial community ‘microbial diversity and function’ as a driver of soil organic matter stabilization. A laboratory incubation study with straw as C-input was incorporated in soil with or without stoichiometrically balanced nutrients to convert a target 30% fresh C-input to fine-fraction C (FF-C; total C in the mineral associated organic matter). Treatments included C-input, stoichiometrically balanced C-input, C-input with surplus nutrients, nutrient limiting treatments (-N, -P and -S limiting) and five soils with increasing clay content. A significant effect of treatment and soil type was reported for total CO2-C produced (P < 0.001). The same treatment effect was observed for FF-C in heavy textured soils but was not large enough to be significant. Findings suggest balanced inputs of CNPS is a strategy to stabilise soil C in heavy textured soils but may increase C turnover in light textured soils with limits to C storage. A soil incubation experiment with four consecutive 12-week incubation cycles based on the same treatments and soil types found a response to C-inputs in the stabilised soil fraction was observed in cycle 2, 3 and 4 for heavier textured soils, clay 20% and clay 24%. In the final cycle 4, a treatment effect began to emerge in soil clay 8% with a significant treatment effect for the Fert-150% (surplus nutrients) treatment. It is concluded that at the end of the experiment the FF-C response was not strong enough to determine treatment effect. The effect of soil type was highly significant in all incubation cycles and indirectly highlights that the effect of soil type might have a role to play in the rate of soil C stabilisation following stoichiometric inputs of CNPS to soil. Metagenomics analysis was performed to establish the taxonomic profile of the microbial community in response to stoichiometrically balanced CNPS input. The bacterial community structure was found to be altered in a significant way by treatment, with reductions in species richness and species diversity when supplementary inorganic nutrients were added to C-input. In all soils Proteobacteria, Actinobacteria, Acidobacteria and Verrucomicrobia emerged as the dominant taxa based on total abundance yet at phyla level, a different range of bacteria Verrucomicrobia, Gemmatimonadetes and Chloroflexi were found to positively respond to treatment. A strong effect of soil type was evident in this study persisting across phylum, class and genera levels of classification. A treatment-soil interaction was the driver of reduced Gemmatimonadetes abundance with increased soil clay content and this was possibly also the case for Chloroflexi. The functional response of soil microbial communities to variations in C:N:P:S stoichiometry is important in understanding the constraints to soil C sequestration. To link microbial taxa to soil C processes metatranscriptomic analysis was conducted. Different approaches to RNA extraction were trialled with automated and manual extraction procedures executed. The quality control and sequencing library results converged, underscoring an RNA integrity issue. These findings demonstrate that the soil may inherently possess a limited quantity of intact RNA. Metatranscriptomics for the intended objectives has been limited due to the difficulties associated with obtaining RNA yield and quality to generate high-quality cDNA from the soil.