Interactive effect of root exudation and texture on anoxic microsite dynamics in the rhizosphere

Roots are presumed to be key controls on anoxic microsites, which partially regulate nutrient availability and the fate of carbon and contaminants in soils. However, how root activity interacts with edaphic factors to regulate anoxic microsite formation is poorly understood. Here, we sought to define how root exudation interacts with soil texture to alter soil oxygen dynamics around a model plant root. We used reverse microdialysis to deliver 13C-labelled model exudates to soil mesocosms of two distinct textures. Over the course of three diurnal cycles, we mapped the 2D-distribution of oxygen, defined the contribution of exudates to total soil respiration, and measured the production of Fe(II) and fermentation products around our model plant root. We show that root exudation spurs the formation of anoxic microsites by intensifying microbial respiration around the plant root. These effects are lessened during periods of no exudation (“nighttime”) and enhanced in finer textured soils. Additionally, we show that anaerobic Fe-reducing bacteria partially contribute to Fe-oxide dissolution. The transient nature of anoxic microsites in the rhizosphere and the potential for Fe-redox cycling within them raise important questions about prevailing concepts of rhizosphere contaminant availability, nutrient acquisition, and carbon dynamics.