The Effect of Soil Quality and Saturation Deficit On Carbon Mineralization Response to Wheat Residue and Inorganic Nitrogen Fertilizer Additions.

The dynamics of soil carbon(C) mineralization and sequestration play an important role in soil fertility maintenance and global carbon cycle. In order to separate the effects of dynamic soil properties (eg: particulate organic matter, mineralizable N) from the effects exerted by inherent soil properties (eg: clay content, soil organic carbon(SOC) concentration), soils were taken (0-20cm) in spring after soybean or grains from the Marsden Plots (Iowa) and the Farming Systems Project (Maryland), USA. Both studies compare intensively managed 2yr conventional rotations amended with inorganic fertilizers with longer (3yr and 4yr) rotations that receive animal wastes and lower amounts of external inputs. Carbon mineralization and its response to a wheat-residue(R) or inorganic nitrogen fertilizer(N) applied alone or in combination were compared to controls in a 230-day aerobic incubation. Carbon mineralization was divided into three segments representing to estimate active (0-23 days), slow/protected (24-120 days), and slow/stabilized C (121-230 days). For the same R or N treatments, carbon mineralization rate and cumulative amount of CO₂-C released from soils ranked 3yr > 4yr > 2yr rotations, and were positively correlated with initial SOC concentrations. When the cumulative CO₂-C curves were fitted to a simple first order kinetic model: Ct = Co(1-e^-kt), the Co values for different treatments ranked: R>RN>>control >N. The results suggest that residue addition increased soil carbon mineralization potential, and N fertilization suppressed CO₂ emission regardless of soils’ initial condition. The C mineralization with respect to stabilization of sequestrated C in soils of this study needs to be further explored.