Integrated Management, Underexplored Microbial Metabolisms, and Nutrient Flow in Soils.

Agricultural management practices that both protect and build soil organic matter (SOM) are important strategies for achieving soil conditions more similar to those found in less disturbed, native plant-soil systems, where more efficient, less leaky nutrient cycling is thought to occur. This presentation reviews possible microbial metabolisms that could be stimulated when soils are no longer subjected to tillage and efforts are made to building SOM pools (i.e., no-till; diversified rotations; cover cropping; greater use of organic amendments). Under reduced disturbance, restoration of biophysical integrity is expected to create more heterogeneous biochemical gradients, lower overall oxidation-reduction potentials, and greater niche differentiation. Microbial processes known to occur, but which are relatively unexplored for their potential to affect nutrient flows in agricultural soils, include 1) re-use of metabolic wastes such as H2, CO2, and organic compounds; 2) nitrate respiration pathways other than denitrification; and 3) heterotrophic CO₂ consumption. In undisturbed soils, more metabolic byproducts such as H2, CO₂ and volatile compounds are retained within soil pores and aggregates. Such intermediates can serve as substrates to be recycled in situ, rather than lost to the atmosphere. Lower redox potentials, combined with increases in available carbon are known to promote dissimilatory nitrate reduction to ammonium, which has implications for extending N residence times in soils. Moreover, it is known that CO₂ stimulates microbial growth when it is assimilated in anaplerotic reactions of the Krebs cycle. Taken together, soil conditions resulting from SOM-building management should promote the in situ recycling of metabolic intermediates, expand microbes’ energy supply beyond organic carbon. The study of such underexplored metabolic transfers therefore has implications for carbon sequestration and can yield a better understanding of how SOM-building practices improve nutrient use efficiency.