Soil microbial responses to labile carbon input differ in adjacent sugarcane and forest soils

Abstract

Soil microbial activity can be constrained by availability of energy because soil carbon (C) occurs mostly as complex soil organic matter (SOM), with relatively small quantities of high-energy, labile C. Decomposition of SOM is mediated by energy-requiring processes that need extracellular enzymes produced by soil microbial communities. We examined how an increase in energy status via sucrose supplementation affects the production of SOM-degrading enzymes, comparing matched soils under forest and sugarcane agriculture with histories of contrasting inputs of complex and labile C. Activities of SOM-degrading enzymes increased in both soils after sucrose addition, but CO2 production increased more rapidly in the sugarcane soil. The forest soil had greater increases in phosphatase and glucosidase activities, whereas the sugarcane soil had greater increases in protease and urease activity. The contrasting microbial community-level physiological profiles of the soils further diverged at 30 and 61 days after sucrose amendment, before returning to near pre-treatment profiles by 150 days. We interpreted the increasing soil enzyme production as indicative that enzyme production was limited by energy availability in both soils, despite contrasting histories of labile v. recalcitrant C supply. Quicker responses in sugarcane soil suggest pre-selection towards populations that exploit labile inputs.