Minor response of gross N turnover and N leaching to drying, rewetting and irrigation in the topsoil of a Norway spruce forest

Abstract

Forest floors in the temperate climate zone are frequently subjected to strong changes in soil moisture, but the consequences for the soil N cycle are poorly known. In a field experiment we tested the hypotheses that soil drying leads to a decrease of gross N turnover and that natural rewetting causes a pulse of gross N turnover and an increase of N leaching from the forest floor. A further hypothesis was that optimal water availability induced by irrigation causes maximum N turnover and N leaching. Replicated control, throughfall exclusion and irrigation plots were established in a Norway spruce forest to simulate different precipitation patterns during a growing season. Gross N turnover rates were determined in undisturbed soil cores from Oi + Oe and Oa + EA horizons by the 15N pool dilution technique. Forest floor percolates were periodically collected by suction plates. After 142 mm throughfall was excluded, the median soil water potential at the throughfall exclusion plots increased from pF 1.9 to 4.5 in the Oi + Oe horizon and from pF 1.8 to 3.8 in the Oa + EA horizon. Gross ammonification ranged from 14 to 45 mg N kg−1 soil day−1 in the Oi + Oe horizon and from 4.6 to 11.4 mg N kg−1 soil day−1 in the Oa + EA horizon. Gross ammonification of both horizons was smallest in the throughfall exclusion plots during the manipulation, but the differences between all treatments were not statistically significant. Gross nitrification in both horizons was very small, ranging from 1.6 to 11.1 mg N kg−1 soil day−1. No effects of decreasing water potential and rewetting on gross nitrification rates were observed because of the small rates and huge spatial variations. Irrigation had no effect as the differences from the control in soil water potential remained small. N leaching from the forest floor was not affected by the treatments. Our findings suggest that ammonification in forest floors continues at considerable rates even at small water potentials. The hypotheses of increased N turnover and N leaching following rewetting of dry forest floor or irrigation were not confirmed.