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Net Mineralization of Carbon and Nitrogen Under Aerobic and Anaerobic Conditions

Dep. of Agronomy, 115 Plant Sciences, Univ. of Arkansas, Fayetteville, AR 72701

* Corresponding author.

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 CO₂—free atmosphere or anaerobically (flooded) in an N₂ atmosphere. The decomposition process was followed by measuring the CO₂ and CH₄ evolved, water soluble organic C, and inorganic N. Aerobic C mineralization was best described using CO₂ 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, CO₂ evolution remained fairly constant throughout the experiment. The decomposition rate constant for anaerobic CO₂ evolution was 0.006 d^–1. Carbon mineralization under anaerobic conditions was best described as the sum of CO₂ and CH₄ 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 R² values of 0.961 and 0.946, respectively.

Published with the approval of the director of the Univ. of Arkansas Agric. Exp. Stn.

Received for publication July 27, 1987.

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