Underexplored microbial metabolisms in agricultural soils: can they contribute to ecological nutrient management?

Mary Ann Bruns
Department of Ecosystem Science and Management, The Pennsylvania State University

Presented at the All Scientist Meeting (2015-04-15 to 2015-04-16 )

Agricultural producers are increasingly seeking management methods that minimize soil physical disturbance and return organic matter to soils. More of today’s agricultural soils are being managed with cover cropping, green manures, and reduced tillage. Such soils are intended to have greater proportions of rhizosphere to bulk soil, longer periods of root activity, and more micro-site heterogeneity. This presentation addresses the question, “How might integrated soil management methods achieve nutrient cycling efficiency to approach that observed for native, undisturbed soils?” The integration of ‘low-disturbance-higher-carbon’ agricultural practices could facilitate more diverse and efficient microbial metabolisms by altering soil physical and biogeochemical properties. Examples of potentially desirable microbial processes include greater H2 cycling, heterotrophic fixation of CO2, free-living and associative N2 fixation, and dissimilatory nitrate reduction to ammonium, all of which are well- documented but underexplored processes in soils. Under conditions of improved soil biophysical integrity, microbial uptake or syntrophic exchange of gases could be enhanced by greater supplies of specialized carbon inputs and sustained biogeochemical gradients. Increased heterogeneity of soil micro-sites also could result in more diversified and complex N transformation pathways to improve N use efficiency. While the major end products of nitrate reduction are commonly assumed to be either N2 or the potent greenhouse gas N2O, reduced physical disturbance in the presence of carbon substrates could facilitate dissimilatory nitrate reduction to ammonium, the form of inorganic N which is much less susceptible than nitrate to losses from soil. Conditions created through integrated soil management could therefore help lengthen N residence time in soils and potentially lower N2O emissions. Effective measurement tools are needed to determine nutrient-conserving processes and to characterize the soil conditions that facilitate them.

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