Anoxic microsites in soils: regulating factors, impacts on greenhouse gas fluxes and role in nitrogen cycling

Background/Question/Methods

Anoxic microsites appear to host the majority of denitrification in well-drained soils and may also be important regions for carbon storage and phosphorous availability.  Their low redox potential also allows for qualitatively different profiles of greenhouse gases emission, thus changing the radiative forcing potential caused by soil processes. However, we have only weakly integrated anoxic microsites into biogeochemical theory because we have inadequate understanding of the soil properties that cause microsites to form and we have not yet defined the potential magnitude of their influence.

To address these knowledge gaps, we studied the effects of soil and litter properties on microsite formation , greenhouse gas fluxes and N-cycling.  We constructed soil microcosms (n=174) with different degrees of spatial aggregation of plant litter, intended to create a gradient in sizes of anoxic microsites.  To further vary rates of microbial metabolism we used two types of litter (wheat, alfalfa) and then maintained the soils over a gradient of soil moistures.  Over 12 weeks we quantified the degree of microsite anoxia using measures of gross CH₄ production as a proxy.  We also measured net N₂O and CO₂flux.   Upon sacrificing we measured soil properties, soil organic matter structure (FTIR), and microbial biomass.

Results/Conclusions

We found that anoxic microsites were created in incubations (n=92/174) across the soil moisture gradient by using gross CH₄ production as a non-destructive proxy for microsite presence.  Nitrous oxide dynamics were dominated by two pulses, the pulse timing and amplitude were dependent on soil moisture, litter type, and litter distribution.  Gross CH₄ production was controlled by litter type and soil moisture, while gross CH₄consumption was greatest in soils of moderate moisture and positively correlated with CH4 production.  Alfalfa litter caused soil pH to decrease (versus wheat litter) and shifts in microbial biomass.

We found that anoxic microsites can be caused by small amounts of litter (<1.0g) in relatively dry soils and produce biogeochemically significant amounts of CH₄ and N₂O.  Their effects on N-cycling include increases in gaseous loss and variations in the form of inorganic N available. Our results indicate that anoxic microsites may have a much greater impact on greenhouse gas fluxes than previously recognized.  The findings will help inform parameters needed for the full integration of anoxic microsites into N-cycling models, however, accounting for the non-linearities and multiple controls involved in simultaneous CH₄ production and consumption may not be easily integrated.