Activity and anoxia: Methanogenesis follows high rates of decomposition in an unsaturated litter patch experiment

Background/Question/Methods

There is a fundamental need to understand how anoxic microsites, and other types of small-scale redox heterogeneity in unsaturated soils, form and function because of the strong effects they can have on nutrient availability, greenhouse gas flux, soil contaminant mobility, and microbial community structure. Anoxic microsites are created due to low oxygen diffusion, high oxygen consumption, or a combination of the two. However, the relative importance of primary factors influencing oxygen diffusion and consumption has not been determined, nor the precise impact these factors have on biogeochemical processes. To these ends, we carried out an incubation experiment of plant litter patches in soil mesocosms under a full factorial design across soil moisture (15%, 35%, 65%, or 85% WHC), litter patch dispersion (1, 3, 9 patches or uniform dispersal), and litter type (alfalfa or wheat). We determined the effects these factors had on greenhouse gas fluxes, both of aerobic (CO₂) and anaerobic (CH₄) origin, as well as a variety of soil properties. We measured net gas fluxes frequently and calculated integrated budgets of total gas flux during the 10 week incubation. 

Results/Conclusions

Moisture had a strong, positive effect on CO₂ (p<0.0001) and CH₄ (p<0.001) fluxes, with the greatest increases in CO₂ and CH₄ production occurring between 15% and 35% WHC and 65% and 85% WHC, respectively. Incubations with alfalfa litter had much greater CH₄ budgets (640%, p<0.0001), CO₂ budgets (30%, p<0.01), and initial CO₂ fluxes than those with wheat litter. Further, differences in CO₂ flux between litter types dissipated more quickly in incubations where the litters were more dispersed, implying that the spatial dilution of litter, even at sub-centimeter scales, can moderate biogeochemical effects. These results show that soil moisture and litter type can cause strong variations in CH₄ and CO₂ flux, proxies for redox potential and decomposition rates, respectively. Most interestingly, high early CO₂ fluxes were correlated with high CH₄ fluxes later (R=0.40, p<0.0001), indicating that elevated rates of aerobic microbial activity can create anoxic regions that contain redox potentials low enough to host methanogenesis.

Our findings show that soil moisture, litter type, and litter dispersion can affect CO₂ and CH₄ fluxes in a variety of ways and that there is strong promise for measurements of anaerobically produced gases to be a simple, inexpensive, non-invasive way to infer redox heterogeneity.