The role of rapid flow paths for nitrogen transformation in a forest soil:A field study with micro suction cups

Preferential now 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, eac h 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 now paths was estimated based on the response to applications of a dye, SO42-, and Cl-. Then, a N-addition experiment was carried out to study the N tran sformation at locations along flow paths and within the soil matrix. Only 2 3 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 con tact with the infiltrating rainwater, Those microcups which responded to th e added dye were regarded to be located along flow paths, At depths below 2 cm, under temporarily reducing conditions, sampling locations in or near f low paths had higher NO3- concentrations (20-25 mu M) than those of the soi l matrix (below 12 mu M). Within 24 h after a simulated rainfall, the NO3-/ Cl- ratio decreased more in the now 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 e nhanced denitrification at these locations. In the subsequent dry period, n itrification started 2 d earlier and was more pronounced along flow paths. The results of this study suggest that now paths are microhabitats with an increased N transformation compared with the soil matrix.