Controls on mineral-associated organic matter formation in a degraded Oxisol

Oxisols are the dominant soil type in humid tropical and subtropical regions and are subjected to both drying rewetting (DRW) cycles and fluctuating oxygen (O-2) availability driven by warm temperatures and abundant rainfall in surface layers. Drying-rewetting cycles and O-2 fluctuations may critically affect the microbial transformation of plant litter and subsequent stabilization as mineral-associated organic carbon (MAOC), but experimental data are still limited. We examined the impacts of DRW cycles, and variable O-2 regimes with constant moisture, on carbon (C) and iron (Fe) dynamics in a degraded Oxisol (under long-term fallow) with added plant residues. In laboratory incubations (> 3 months), both DRW cycling and fluctuating O-2 availability induced a flush of respiration and a temporary increase in microbial biomass C (MBC) following soil rewetting or O-2 exposure, although MBC was consistently suppressed in these treatments relative to the control (60% water holding capacity under constantly aerobic condition). Consequently, DRW cycles significantly increased but O-2 fluctuations significantly decreased cumulative C mineralization relative to the control. Concentrations of short-range-ordered Fe oxides peaked immediately after litter addition and decreased five-fold during the remainder of the experiment. Mineral-associated C (defined as the chemically dispersed < 53 mu m soil fraction) increased 42-64% relative to initial values but was significantly lower under DRW cycling and fluctuating O-2 relative to the control. Correspondingly, these treatments had greater fine particulate organic C (53-250 mu m), despite increased CO2 production under DRW cycling. Our data indicate the potential for rapid and significant accrual of MAOC in a degraded Oxisol, but environmental factors such as DRW cycling and fluctuating O-2 can inhibit the conversion of plant litter to MAOC-possibly by suppressing microbial biomass formation and/or microbial transformations of organic matter.