Biochemical Processes Controlling Soil Nitrogen Mineralization under Waterlogged Conditions

Abstract

Changes in precipitation patterns in recent years have resulted in flooding or ponding of many farmlands in the North Central region of the United States. In this work, 11 field‐moist surface soils (0–15 cm) and their air‐dried counterparts were incubated at 30°C under waterlogged conditions for times ranging from 3 to 15 d, and the amounts of NH4–N released were determined. Results showed that, presumably because of slaking effects, the rates of NH4–N release were greater in air‐dried than in field‐moist soils. The mineralization rates of the field‐moist soils were significantly correlated with organic C (r = 0.59, P < 0.05), organic N (r = 0.69, P < 0.05), and microbial biomass C (r = 0.62, P < 0.05) and N (r = 0.63, P < 0.05). The corresponding r values for the air‐dried soils were 0.76 (P < 0.01), 0.87 (P < 0.01), 0.84 (P < 0.01), and 0.85 (P < 0.001), respectively. Estimation of the amounts of N mineralized per hectare of field‐moist soils in 1 d ranged from 2.2 to 4.9 kg (average = 3.3). Those values for air‐dried soils ranged from 5.2 to 26.2 kg (average = 15.5). The rates of hydrolysis of six enzyme substrates studied were significantly correlated with the rates of NH4–N release in the field‐moist soils. The relationships were amidase (r = 0.82, P < 0.01), arginase (r = 0.90, P < 0.001), asparaginase (r = 0.86, P < 0.001), glutaminase (r = 0.88, P < 0.001), β‐glucosidase (r = 0.83, P < 0.01), and β‐glucosaminidase (r = 0.84, P < 0.01). Temperature coefficient (Q10) values ranged from 1.2 to 1.4 (average = 1.3) for the field‐moist soils and from 1.0 to 1.3 (average = 1.1) for their air‐dried counterparts. This study showed that microbial biomass, a number of hydrolases, and slaking seem to play a significant role in hydrolysis of N in temporarily flooded soils.