Mechanisms of soil C storage in bioenergy cropping systems

Soils often accrue carbon (C) when land is converted from grain based row crops such as corn to perennial bioenergy crops. This is likely a function of the increase in root biomass production that accompanies the change from grain crops to perennial grasses, however very little is known about the stability of this additional soil C or the priming effect increased root inputs may have on the decomposition of older soil C. Increased root growth associated with perennial grasses used as bioenergy crops may also expose older, physically protected soil C to further decomposition or mineralization by physically altering aggregate structure and soil pore sizes. In order to better understand how soil C storage in bioenergy cropping systems differs from more traditional cropping systems, we assess the stability of soil C pools under switchgrass, giant miscanthus, a native perennial grass mix and continuous corn treatments at the two Biofuel Cropping Systems Experiments located at W. K. Kellogg Biological Station, Michigan and Arlington Agricultural Research Station, Wisconsin, which are part of the Great Lakes Bioenergy Research Center (GLBRC). We collected three 10 cm deep soil cores in November 2011 from five replicate plots of each cropping system. The soils were dry sieved into three size fractions, > 2000 µm, 500-2000 µm and < 500 µm. We then determined total soil C and permanganate oxidizable C (POXC) and microbial activity (i.e. extracellular enzyme activity (EEA) and respiration rates during short term soil incubations) associated with these fractions. We also use preliminary data of belowground net primary productivity (BNPP) from root ingrowth cores collected in Fall 2011 at both sites.

Differences by site in soil type - sandy loam Alfisols in Michigan versus silt loam Molisols in Wisconsin - greatly influenced mean aggregate size, aggregate stability, soil C and microbial activity. At the Michigan site, we found higher total soil C and POXC in the >2000 and 250-2000 µm compared to the < 500 µm fraction across all cropping systems. In the Wisconsin soils, the opposite was true with higher C concentrations in the < 500 µm compared to the >2000 and 250-2000 µm size fractions. Michigan soil microbial activity (EEA and respiration rate) was highest in the > 2000 µm aggregates overall, higher under the perennial grasses compared to corn, and was related to increasing BNPP. In contrast, microbial activity in the Michigan soils small aggregate size class was highest in soils under corn compared to perennial grasses and was related to total soil C and POXC. In Wisconsin soils, microbial activity was highest across treatments in the < 500 µm aggregates, with microbial activity in the larger aggregates related to soil C and POXC and not BNPP. Microbial activity, assessed by EEA, was higher in soils under corn compared to perennial grasses, but respiration rates were lowest in soils under corn compared to perennial grasses. We suggest that these results demonstrate that interactions between soil physical properties and microbial communities are as important as belowground productivity in determining the influence cropping systems have on soil C stocks.