Microbial carbon use efficiency during plant residue decomposition: Integrating multi-enzyme stoichiometry and C balance approach

Accurate estimation of microbial carbon use efficiency (CUE) in soil is challenged by a high degree of genetic and environmental variability. Different methods vary in their estimates of soil microbial CUE giving the room to select the optimal method for a specific research task, while integrating different methods could improve our understanding of processes controlling CUE variability. Aiming to estimate CUE during plant residue decomposition in different soils, we applied the conventional C-balance method, single C-cycling enzymatic stoichiometry (SCE-STM) and newly proposed "multi"-C-cycling enzymatic stoichiometry (MCE-STM) methods. The STM approach derives CUE from elemental ratios of microbial biomass, substrate, and activities of C and nutrient (e.g. N) acquiring exoenzymes. The extended MCE-STM is a modification of the SCE-STM method, where we used the sum of three C-cycling enzymatic activities (beta-glucosidase (BGL), beta-D-cellobiohydrolase (BCL), beta-xylosidase (BXL)) as proxy for CUE calculation, instead of using a single C-acquiring enzyme (BGL). We hypothesized that MCE-STM provides a more reliable estimation of microbial CUE in soils amended with complex plant residues than the SCE-STM or the C balance approach. The comparison of methods was done in a laboratory incubation experiment, using two soils differing mainly in acidity level mixed with two specimen of plant residues differing in lignin (L) and polyphenol (PP) content. We anticipated a higher microbial CUE in less acidic (pH 5.1) soil amended with higher quality (lower (L + PP)/N) ratio)) plant residues than in more acidic (pH 4.3) soils amended with medium quality (higher (L + PP)/N ratio) plant residues, due to less energy investment in microbial metabolism in the former case. Microbial CUE estimations were completed at 7, 15, 30, 45 and 60 days. Lower CUE values (0.09-0.18) were recorded by MCE-STM as compare to those (0.24-0.47) obtained by C-balance and SCE-STM methods. Irrespective of applied CUE estimation methods, higher CUE was recorded in less acidic (pH 5.1) soil amended with residues of higher quality than the other three combinations. Microorganisms invested more energy to support growth in low pH soil in order to tolerate soil acidity, which, in turn, suppressed N-acquiring enzymatic activity and further decreased CUE. The modification of the MCE-STM method for CUE determination proposed in this work was capable to quantify the combined effect of soil pH and plant residue quality on efficiency of microbial metabolism. It, therefore, can be considered as viable alternative to the original stoichiometric modeling approach (SCE-STM).