Effects of sorption on biodegradation of low-molecular-weight organic acids in highly-weathered tropical soils

Abstract

Low molecular weight organic acids (LMWOAs) are key pools regulating carbon (C) stabilization and destabilization in tropical forest soils. The variation in clay contents and mineralogy between sites or within a profile might regulate magnitude of LMWOA sorption and biodegradation in tropical soils poor in short-range-order (SRO) Al/Fe oxides. We analyzed soil solution concentration, sorption isotherms, and mineralization kinetics of 14C-radiolabeled acetate, oxalate, citrate, and glucose. The sorption capacities of LMWOAs increased with clay contents, but not with the abundance of SRO clay minerals. Sorption can reduce mineralization rates of multivalent LMWOAs (oxalate and citrate) and its effects could increase with increasing clay contents, especially in the clay-rich Bt horizon or in the clayey soil profile. The microbial respiration rates from LMWOAs and monosaccharides are primarily regulated by substrate availability and microbial biomass in the tropical soils, while mineralization of multivalent LMWOAs can be limited by sorption especially in the clayey soil horizon or profile. The smaller sorption capacities in the organic and sandier soil horizons could contribute to fast turnover of organic matters through LMWOA pools in tropical forest soils.