Denitrification Rates in Relation to Total and Extractable Soil Carbon
1975
Stanford, George | Vander Pol, Roger A. | Dzienia, Stanislaw
Denitrification rates were studied under near-anaerobic conditions in 30 soils of diverse origin that differed widely in pH, organic C contents, and other characteristics. Soils with added NO₃-N were submerged in water and containers were sealed to prevent further oxygen intake during incubation. Disappearance of NO₃-N and production of NH₄-N were determined at 1-day intervals or longer over a 10-day period. Since soils were not shaken during incubation, denitrification rates were influenced by diffusion of nitrate from the liquid to the soil layer. In most soils, amounts of NO₃-N declined exponentially with time of incubation. Thus, under the experimental conditions, the loss of nitrate was depicted better by log NO₃-N vs. time (t, hours) than ppm NO₃-N vs. t. The apparent first-order rate constants (k), denoting the fractional loss of NO₃-N/hour, ranged from about 0.001 to 0.04 hour⁻¹. Correlations of k with total soil organic C and with soil “glucose-C,” extracted by boiling soils for 1 hour in 0.01M CaCl₂, were highly significant. However, the extractable glucose-C (an index of readily decomposable C sources) provided a more reliable basis for predicting k than did total organic C. The regression of k on glucose C (X) for 30 soils is as follows: k = 0.188X − 0.00093, (r² × 100 = 82%). For the corresponding regression involving total organic C, r² × 100 = 69%. Within 24 to 48 hours, appreciable amounts of Mn appeared in solution and usually continued to increase with time. Reduced Fe did not appear until most of the NO₃-N had disappeared. After 48 hours of incubation, the multiple regression of Mn (Y), in solution, on NH₄-N produced (X₁) and initial soil pH (X₂), with associated statistics, was as follows: Y = 13.8 + 3.5X₁ − 13.5X₂ (R = 0.77), rY₁.₂ = 0.48. Values are significant at the 1% level.
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