The extent to which agricultural management practices might influence belowground soil properties, such as plant-available-water-holding capacity or the residence times of soil organic carbon, remain poorly understood. Here we report initial findings from a long-term experiment in southwestern Montana that takes advantage of a wheat/cover-crop rotation to define the effects of four functional groups of cover crops on belowground patterns and processes. The four functional groups included representatives of Brassicaceae (e.g., Raphanus sativus/Camelina sativa), legumes (e.g., Pisum sativum/Vicia sativa), fibrous-rooted plants (e.g., Avena sativa/Lolium perenne), and taprooted (e.g., Carthamus tinctorius/Brassica rapa). We collected and analyzed biomass samples from each of 7 experimental plots (including additional pea-only, fallow, and a full combination of the four functional groups; n=4 replicates), in addition to chacterizing a broad range of soil properties at co-located sites. Soil measures included total elemental analyses, soil inorganic carbon measured by loss-on-ignition at 800C, and soil organic carbon as well as soil organic matter, indexed by loss-on-ignition at 450C. We also measured soil respiration rates in order to define soil carbon residence times assuming a single-pool model.
Most of the soil carbon pool in these soils was dominated by calcium carbonate; SIC was approximately three times more abundant than soil organic carbon (SOC). In the upper 10 cm, SIC levels were ~1.7%, while SOC levels ranged from 0.7 (fallow) to 1.1 (cover-crop)%. These modest differences in SOC between the fallow and cover crop cocktail treatments were restricted to the uppermost 10 cm of the soil; below 10 cm, SOC values converged at 0.4%, while SIC levels rose to ~2.2%. We estimated soil carbon residence times at ~15 years, with no appreciable differences between cover crop treatments. Overall, our results suggest that efforts to build SOC, particularly SOC with increased residence times, with cover crop cocktails in these types of calcareous and loess-dominated soils characteristic of southwestern Montana may require considerably longer investments than the three rotations that have occurred at this site.