Incorporation of fresh-C, as straw, with supplementary inputs of N, P and S increased the rate of mineralisation in heavier textured soils (S3 and S5) demonstrated at peak concentrations of CO2-C. Subsequently, no effect of treatment was found in the stable fraction of OM (FF-C) at twelve weeks.
No statistical significance between C-input and stoiochiometrically balanced CNPS input was found in lighter textured soils. This result aligns somewhat to that of (Creamer et al. 2014) where CO2 production increased without either detectable changes in glucose-derived microbial biomass or SOC in an 8-week incubation in which stoichiometric nutrient addition (CNPS) using a low molecular weight substrate (glucose) and eleven increasing levels of N, P and S based on carbon use efficiency (CUE) were added to a nutrient poor sandy soil. The difference between studies is the type of C-input, crop residues (complex substrate in the current study) v’s glucose (simple soluble substrate in Creamer et al. 2014), and it is acknowledged that unlike the glucose which should be utilised entirely by the soil microbial community under all nutrient treatments, other factors are at play such as microbial extracellular enzyme production. Indeed, peak CO2 evolution rates declined at week 2 to 3 in the current study due to the slower decomposition of straw, unlike other studies Khan et al. (2019) and Creamer et al. (2014) in which peak concentrations occurred in week 1. Creamer et al. (2014) hypothesised that as nutrient addition increased, an increasing proportion of the glucose-C would yield lower CO2 concentrations and be incorporated into microbial biomass and stabilised as SOC. Yet nutrient addition altered the magnitude of glucose-C mineralization by increasing the rate of glucose-C turnover. In terms of SOC, that study suggested that a sandy soil with low capacity for physical protection of SOM, nutrient addition does not immediately promote C sequestration in the soil microbial community.
In microbially processed SOC, firstly microbial-derived compounds are made that subsequently require further stabilisation in soils. In an effort to make a link between microbial anabolism and soil organic carbon stocks in the field, Poeplau et al. (2019) conducted a short-term (24 h) incubation experiment with 18O labelling simulating abundant organic matter breakdown using two long-term experiments of grassland soils and reported a significantly higher microbial growth (+ 102 ± 6%), lower respiration (-16 ± 7%) and higher microbial C utilization efficiency (+ 53 ± 21%) of C-input in mineral fertilized (NPK) compared to control treatments. Linking this to the change in SOC in field experiments (modelled with the introductory carbon balance model (Andrén 1997) found that on average only 77% of the change in SOC could be explained by C-inputs, and a strong relationship between C:N root and NPK treatments confirmed a link between microbial anabolism and substrate C:N ratio. In addition to ex vivo modification, in vivo turnover, explains the process of microbial catabolism (energy-yielding pathways) and anabolism (regeneration pathways) to produce organic compounds of further SOC stabilisation (Liang et al. 2017).
In the current study, the sandy loam soils (S2 & S4) indicate slightly lower rates of mineralisation, as CO2-C. Interestingly, sandy loam soils had the highest SOC concentrations at the start of the experiment (3.56 and 4.86% based on the < 2 mm fraction) and comparative plant available P compared to the loam soils, indicating these soils were well managed for crop production despite their lighter texture that might otherwise impact on crop production in a less well managed scenario. Both soils had received imported organic matter inputs.
Despite no statistical difference detected for the treatment effect on soil FF-C, the N1 treatment had higher FF-C concentrations than in C2 in the loam soils (S1, S3 & S5), following the same trend as CO2-C production. Nutrient limiting treatments in the loam soils showed lower FF-C concentrations, particularly for P- and S-limiting treatments. This strengthens the hypothesis set, that regardless of a large labile-C supply, limited N, P or S can limit the size of the FF-C pool. In contrast, the sandy loam soils show a different trend, suggesting a limited capacity to stabilize C, as discussed by Creamer et al. (2014), with higher clay soils providing more binding sites to the new C pool based on chemical or physicochemical binding between SOM and soil minerals (Six et al. 2002). In summary, whilst the treatment effect on CO2-C production and FF-C is not statistically significant across all soils, there is an indication that stoichiometrically balanced CNPS input is indicative of soil C sequestration in the loam soils and small changes are detectable in soils with a higher content of clay (based on CO2-C). This is expected to be further evident in a follow-up multi-cycle laboratory incubation. Whether CNPS stoichiometry as a management strategy to increase SOC in well-managed sandy soils is possible, remains to be determined.
Interestingly, all five soils collected for this study had a high P fertility status, yet lower CO2-C was produced in the P-limiting treatment indicating that the plant-available soil P was not immediately available during straw decomposition in loam soils. Furthermore, the S-limiting treatment produced the least CO2 and FF-C among all the nutrient treatments in loam soils demonstrating that microbial growth was more limited by S in soils and a direct relationship between microbial-C and S (Banerjee and Chapman 1996). Few studies have considered sulphur as a major building block of SOC. Microbial S metabolism has been demonstrated to be particularly important under situations of P-limitation and rapidly growing soil microorganisms, and a possible explanation is that sufficient S supply can support microbial growth (i.e sulfur-oxidizing bacteria) to form ATP when P is limited (Khan and Joergensen 2019). This serves to fuel carbon dioxide assimilation into organic compounds via the Calvin cycle or reverses the tricarboxylic acid cycle, ultimately providing the host with metabolites required for survival (Abouna et al. 2015). This might be because adding sulphur to the soil produces H2SO4, which slightly lowers soil pH whilst increasing P solubility in soil due to a shift in the microbial community to increase P solubilizing microorganisms in P-deficient conditions (i.e. Bacterial genus Paenibacillus) (Kaya et al. 2020).