The Role of Rapid Flow Paths for Nitrogen Transformation in a Forest Soil A Field Study with Micro Suction Cups

Preferential flow is a common phenomenon in soils. This study was conducted to investigate the significance of rapid flow paths for N transformation in a forested Humaquept in central Switzerland. Fifty micro suction cups, each with a surface area of 12 mm 2 , were installed in a regular grid in the uppermost 5 cm. First, the location of each “microcup” relative to main flow paths was estimated based on the response to applications of a dye, SO 2− 4 , and Cl − Then, a N‐addition experiment was carried out to study the N transformation at locations along flow paths and within the soil matrix. Only 23 of 50 microcups responded to the application of the dye within the first 24 h, which indicates that a large portion of the soil volume is not in contact with the infiltrating rainwater. Those microcups which responded to the added dye were regarded to be located along flow paths. At depths below 2 cm, under temporarily reducing conditions, sampling locations in or near flow paths had higher NO − 3 concentrations (20–25 μM) than those of the soil matrix (below 12 μM). Within 24 h after a simulated rainfall, the ratio decreased more in the flow paths (between −2.4 and −4.9 mol mol −1 ) than in the soil matrix (−0.7 to −0.8 mol mol −1 ), which indicates an enhanced denitrification at these locations. In the subsequent dry period, nitrification started 2 d earlier and was more pronounced along flow paths. The results of this study suggest that flow paths are microhabitats with an increased N transformation compared with the soil matrix.