Net Mineralization of Carbon and Nitrogen Under Aerobic and Anaerobic Conditions

Abstract

Abstract A study was conducted to compare the net C and net N mineralized during the decomposition of alfalfa ( Medicago sativa L.) under aerobic and anaerobic conditions. Dried and ground alfalfa was mixed with soil at a rate of 5 g kg −1 and incubated either aerobically at optimum water in a CO 2 —free atmosphere or anaerobically (flooded) in an N 2 atmosphere. The decomposition process was followed by measuring the CO 2 and CH 4 evolved, water soluble organic C, and inorganic N. Aerobic C mineralization was best described using CO 2 evolution as a first‐order kinetic process. Three distinct phases of decomposition were observed (rapid, intermediate, and slow) with rate constants ( k ) of 0.123, 0.059, and 0.0095 d −1 , respectively. Under anaerobic conditions, CO 2 evolution remained fairly constant throughout the experiment. The decomposition rate constant for anaerobic CO 2 evolution was 0.006 d −1 . Carbon mineralization under anaerobic conditions was best described as the sum of CO 2 and CH 4 evolved plus the water soluble organic C produced. This treatment resulted in three distinct phases of anaerobic decomposition. The C mineralization rate constants for the rapid and intermediate phases were 0.118 and 0.024 d −1 , respectively. No rate constant was determined for the slow phase. A computer simulation model was used to estimate the C and N mineralized under both aerobic and anaerobic conditions. The linear relationship between the predicted and observed mineralization was evaluated. For C mineralization the slope was 1.1 and that of N mineralization was 1.0, with R 2 values of 0.961 and 0.946, respectively.