Notes from literature reading:

“Empirical evidence that soil carbon formation from plant inputs is positively related to microbial growth”,

Bradford et al. 2013

Plant inputs include aboveground litter, root litter and root exudation. The chemical composition of all these forms of plant inputs contain low molecular weight C compounds (LMWC), i.e. dissolved sugars, organic acids and amino acids. LMWC inputs are not directly decomposed and transferred to CO₂ efflux, but could be used by microbes and then becomes the dominant precursors for formation of stable soil organic carbon. However, the different respiration rates and microbial use efficiencies of different LMWC forms, over flow metabolism theory and preferential assimilation all suggest the different contributions of various LMWC forms to SOC formation. The forms of LMWC ortheir proportions would vary with plant species shift under global environmental changes. Therefore, it is urgently needed to investigate how LMWC compound identity influences SOM formation under global environmental changes.

In this study, the authors added ¹³C- glucose and glycine to the soil collars in field experiments to quantify C partitioning to respiration,microbial biomass, SOM, soil solution and plants and, therefore, clarified which LMWC form with higher contributions to SOC formation. This experiment was designed with four treatments (water, N, P, and N+P) and replicated three times, which totally twelve plots. The twelve plots were part into three blocks.Three soil collars were put in each plot and were added with ¹³C-glucose and glycine for 28 weeks. The measured variables included: soilrespiration, ¹³C contents of CO₂ efflux, and the mass of plant shoots, roots and microbes, and their %C, %N, ¹³C- and ¹⁵N-contents.Linear mixed-effects models were used for the statistical analysis conducting in the freeware statistical package R. The results suggested microbial biomassis a dominant precursor for SOM, and glucose had greater contribution to SOM formation from three hypotheses relating to growth efficiencies, preferential assimilation and biomass turnover.

This study investigated controls on SOM formation from LMWC compounds, not on how addition of these compounds influenced bulk SOM stocks. ¹³C stable isotopes were used to completely clarify the fates of LWMC inputs and the C partition in different pools. The chronic amendments of dissolved compounds, not pulsed additions, are more representative of processes such as root exudation. The random effects from the spatial (i.e. blocks) and temporal (e.g.soil respiration measure time) were accounted for by using the linear mixed-effects models for statistical analysis. In addition, the results were closely related to three hypotheses relating to growth efficiencies, preferential assimilation and biomass turnover on SOM formation from glucose and glycine.