Background: The accumulated residues of dead microbial cells (microbial necromass) are being recognised as a key component of the stable soil organic carbon (SOC) pools found in agricultural ecosystems. As such, the processes that control the accumulation and stabilization of microbial necromass are fundamental to develop improved strategies for carbon sequestration and sustainable soil management.
Objectives: (i) The purpose of this study was to analyze the amount and biochemical makeup of microbial necromass through amino sugar biomarkers from several relevant treatment methods (glucosamine, muramic acid, and galactosamine) used for composting and biochar. (ii) This project also seeks to establish the mechanisms by which microbial necromass stabilizes through composting or through biochar due to changing the structure and function of the microbial community. (iii) This study will determine the relative contributions of mineral associated organic matter formation, aggregate encapsulation, and surface adsorption of biochar to the long-term persistence of carbon.
Methods: A study, lasting five years, assessed the potential to build soil microbial biomass in Vigna radiata (mung bean) by applying compost (10 tonnes per hectare per year) and biochar (20 tonnes per hectare) separately , and also combined. We measured the buildup of microbial necromass as determined by amino sugar biomarkers, and assessed the impacts of treatments (compost, biochar and both) on organic carbon stock (soil organic carbon/soils), soil microbial community responses (furans, short duration, etc.), as well as soil enzyme activities associated with the carbon cycle (i.e., enzymes that break down plant matter).
Results: Combined application of compost and biochar resulted in significantly higher soil organic carbon (SOC) content than controls without amendment (18.9±0.8 vs. 8.2±0.4 g kg−1, respectively). Of the total SOC increase, about half was derived from microbial-derived necromass carbon (48%), demonstrating a large contribution to carbon sequestration. Fungal (317% increase in glucosamine) and bacterial (375% increase in muramic acid) community biomarker concentrations demonstrated synergistic stimulation of both types of microbial communities. Compost improved microbial carbon use efficiency (CUE) through increased substrate stoichiometry, whereas biochar provided stable microhabitat surfaces for necromass, which protected necromass from degradation by enzyme activity. The rhizosphere of Vigna radiata was a necromass accumulation hotspot through the rhizodeposition of simple sugars and amino acids, which stimulated microbial guilds such as Rhizobium, arbuscular mycorrhizal fungi (AMF), and free-living decomposers. Co-amendment treatments resulted in significantly higher microbial metabolic activity associated with elevated activities of carbon cycling enzymes (β-glucosidase, dehydrogenase, cellulase, and urease).
Conclusion: The use of compost and biochar together in cropping systems that use pulses provides a means of enhancing microbial necromass pathways and leading to the stable formation of organic carbon (SOC). The findings of this research indicate that combined amendments will provide a “climate smart” agricultural strategy with an important potential benefit for improving carbon sequestration as well as making positive contributions toward achieving both regional and global carbon-neutrality objectives.