Changes in microbial biomass and the metabolic quotient with biochar addition to agricultural soils: A Meta-analysis

- Microbial responses to biochar in agricultural soils assessed by meta-analysis.
- Biochar addition overall increased SMBC but decreased qCO2.
- Changes in qCO2 impacted greatly by soil rather than by biochar conditions.
- Long term field experiments needed to explore microbial responses to biochar.

Biochar has been increasingly recommended for world agriculture, but the effects on microbial activities in agricultural soils has not yet thoroughly assessed. In this study, using a meta-analysis of experiment data retrieved from literature published up to March 1, 2015, changes were examined in microbial biomass and soil respiration in agricultural soils with biochar addition. Microbial responses to biochar addition were quantified in soil respiration quotient (RQ), microbial quotient (MQ) and metabolic quotient (qCO2) and their differences were evaluated between with and without biochar addition, and among groups of biochar production conditions and experiment conditions. There was an overall increase by 25% in soil microbial biomass carbon (SMBC) and nitrogen (SMBN) but a decrease by 13% in qCO2, under biochar compared to the control. Whereas, microbial biomass carbon was increased by 26% but total soil CO2 production unchanged, across all short term experiments up to 6 months following a single biochar addition. A significant reduction (by <20%) in qCO2 was found under crop residue and manure biochars in term of feedstock, and biochars pyrolyzed at high temperature over 500 °C in term of pyrolysis temperature. Whereas, the reduction was great (by over 30%) both in clay soils and in neutral soils but moderate (by 15%) in soil organic carbon (SOC) depleted soils, respectively in terms of soil texture, reaction and SOC level. Thus, soil conditions exerted great impacts on microbial metabolic quotient changes compared to biochar conditions. Nevertheless, microbial responses to biochar addition to agricultural soils were much uncertain with respect to both biochar and experiment conditions. Long term field experiments are still deserved to monitor soil microbial processes as long as sustainable soil managements are concerned with biochar technology in agriculture.