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

Authors: RIVAS, FRITZIE - Iowa State University; TABATABAI, MOHAMMED - Iowa State University; Olk, Daniel - Dan; THOMPSON, MICHAEL - Iowa State University

Submitted to: Biology and Fertility of Soils
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/22/2013
Publication Date: 3/15/2014
Publication URL: http://handle.nal.usda.gov/10113/58614
Citation: Rivas, F.A., Tabatabai, M.A., Olk, D.C., Thompson, M.L. 2014. Kinetics of short-term root-carbon mineralization in roots of biofuel crops in soils. Biology and Fertility of Soils. 50(3):527-535.

Interpretive Summary: Carbon contributes to several favorable soil processes, including nutrient availability, water infiltration, and biological activity. The amount of carbon in agricultural soils might decrease when large amounts of crop stems and leaves are removed from the field following harvest, leaving mostly only the roots in the soil. This situation can arise when crop materials are used for energy production. To better predict the effect that removing plant parts for energy production has on soil carbon, more information is needed on the stability of roots of different crops in soil. During laboratory incubations we found differences in the stability of corn roots and prairie grass roots, and these differences were related to the types of compounds found in the roots. These results will help us better predict the stability of soil carbon in fields where these crops are grown and the resulting effects on soil processes. This work will benefit scientists who are studying the long-term effects of energy production on soil behavior and farmers who are removing crop stems and leaves for energy production.

Technical Abstract: 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. Root samples were collected from experimental plots for four cropping systems: a multispecies reconstructed prairie, grown with and without nitrogen 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 carbon and total nitrogen) were assessed. The root materials were incubated for 30 days at room temperature using an incubation apparatus with continuous airflow. The decomposition rates and half-lives of rapidly and slowly decomposable carbon fractions were calculated by fitting a two-component first-order kinetic model to the data. Mineralizable carbon ranged from 7 to 13% of the added carbon. When the data for the two soils were combined, the evolved CO2-C per added carbon was positively correlated with both C/N ratio and lignin/N ratio 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 the range was 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, the rapidly decomposable fraction in roots of the unfertilized prairie treatment was mineralized most quickly. 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