Kinetics of short-term carbon mineralization in roots of biofuel crops in soils

Various crops have been used for energy production while striving to maintain C storage derived from roots in soils. To better understand and document the rates of root decomposition in biofuel cropping systems, we compared the evolution of CO2 from roots incubated with samples of two Iowa Mollisols (Clarion and Nicollet). Root samples were collected from experimental plots for four cropping systems: a multispecies reconstructed prairie, grown with and without N fertilization, and continuous corn, grown with and without a ryegrass cover crop. Major structural components of the root samples (lignin, cellulose, and hemicellulose) as well as chemical composition (total C and N) were assessed. The root materials were incubated for 30 days at room temperature (22 °C) by using an incubation apparatus with continuous airflow. The decomposition rates and half-lives of rapidly and slowly decomposable C fractions were calculated by fitting a two-component first-order kinetic model to the data. Mineralizable C ranged from 7 to 13 % of the added C. When the data for the two soils were combined, the CO2-C evolved was positively correlated with both the C/N ratio (r = 0.88**, p < 0.01) and the lignin/N ratio (r = 0.89**, p < 0.01) of the roots. For the Clarion soil, first-order decomposition rate constants for the rapid fraction ranged from 0.07 to 0.69 day−1, whereas for the Nicollet soil they ranged from 0.09 to 0.66 day−1. For both soils, the half-lives of the rapidly decomposable fraction ranged from 1 to 10 days, and half-lives of 204 to 770 days were observed for the slowly decomposable fractions. Among the cropping systems studied, the rapidly decomposable fraction of roots derived from the unfertilized prairie treatment was the largest. Those root residues also had the highest hemicellulose index and higher concentrations of arabinose, galactose, glucose, and xylose sugars than did roots of the other crops. Other decomposition parameters, such as the decomposition rates and half-lives obtained from the two-component model, were not correlated with the root composition parameters studied. Our results suggest that lignin did not inhibit the early rate of root C mineralization in mixed perennial crops. That information could be useful in refining models of root C dynamics in daily time steps or at the scale of a single growing season.