An in situ lysimeter experiment on soil moisture influence on inorganic nitrogen discharge from forest soil

Abstract

The influences of the soil moisture condition on ammonification, nitrification and denitrification were examined by in situ experiments on forest floors using soil column lysimeters. Distinct differences were detected in the discharge concentrations of NH 4 + and NO 3 − from lysimeters with different soil water content. Three types of lysimeters (diameter 19.5 cm, depth 30.0 cm) were prepared by containing undisturbed soil samples. Inorganic nitrogen discharged from lysimeters under natural rainfall was measured approximately once a week from 5 July to 29 November 1991. Soil moisture conditions of the three lysimeters were controlled using an average pressure head at the bottom of each column at -268.6 hPa, 0 hPa, and 19.8 hPa. These three moisture conditions corresponded approximately to the conditions in the upper or mid-upper hillslopes with relatively dry soil, lower hillslopes with relatively wet soil, and riparian zones around spring points and streams with saturated soil, respectively. For these soil moisture conditions, three different types of inorganic nitrogen discharges were observed; (1) ammonium and nitrate nitrogen; (2) nitrate nitrogen; (3) neither ammonium nor nitrate nitrogen. The result clearly showed the restraint of nitrification by soil moisture deficit and the reduction of NO 3 − by denitrification under saturated conditions. This suggests that the different stages of the process in nitrogen dynamics could occur owing to only the influence of soil moisture variations even in the shallow soil layer. The nature of inorganic nitrogen discharge from each lysimeter helped to explain the spatial heterogeneity of the nitrogen dynamics caused by distributed soil moisture conditions within a watershed system.