Oilseed Meals and Crop Residue: Distinct C and N Dynamics.

In addition to decreasing dependency on fossil fuels, biofuels offer the added benefit of being potentially carbon neutral or negative and not likely to increase atmospheric concentrations of greenhouse gases. Increased production of biofuel from oilseeds will result in large amounts of oilseed meals, the co-product after oil extraction from seeds. Land application of oilseed meals provides an important disposal method. However, oilseed meals are known to contain large amounts of C and N compounds that could be rapidly mineralized by soil microorganisms and therefore release large amounts of CO₂ and possibly other greenhouse gases into the atmosphere. Little is known about the cycling of C, N, and other nutrients in oilseed meals following land application. A laboratory study was conducted to determine the fractional C and N dynamics by adding different amount (0.5, 1, and 5% w/w) of jatropha, camelina, and flax seed meals and wheat biomass to Weswood soils (fine-silty, mixed, superactive, thermic, Udifluventic Haplustepts). Soil C and N were partitioned into microbial biomass, mineralizable, organic, and total fractions. All of these fractions were significantly affected by both the type of amendment and application rate. A dramatic increase in microbial biomass was observed during the first week after addition and then decreased over time. Seed meal addition significantly increased soil total organic C and N. This was true even after a prolonged incubation period through which the most labile fractions of C and N were mineralized. The soil carbon mineralization rate was highest within 2 weeks after addition of oilseed meals then declined to a relatively stable rate. For wheat, the highest C mineralization rate was observed within the first week of addition. Net N mineralization followed a linear pattern with application rate for oilseed meal treatments while adding wheat caused significant N immobilization. These distinct C and N dynamics between the oilseed meals and crop residue were likely due to major differences between their C:N ratios. C:N ratios of oilseed meals used in this experiment ranged from 7 to 11, while for wheat it was 32. In addition, our results indicated that the composition of C compounds was different in oilseed meals and wheat in that oilseed meals contain more easily degradable organic C source.