Nitrogen turnover and N2O production in the forest floor of beech stands as influenced by forest management

Effects of forest management (thinning) on gross ammonification, net ammonification, net nitrification, microbial biomass, and N2O production were studied in the forest floor of adjacent untreated control ("C") and thinned ("T") plots in three beech (Fagus sylvatica L.) stands in the Swabian Jura (Southern Germany) during three intensive field campaigns in the year 2004. The investigated sites are located less than 1 km apart on the slopes of a narrow valley. Due to different exposure (southwest, northeast, northwest), the three sites are characterized by warm-dry microclimate (southwest site, SW) and cool-moist microclimate (northeast site, NE; and northwest site, NW). Measurements at the NW site covered the second year (13 to 20 months) after thinning, and measurements at the SW and NE sites covered the sixth year (61 to 68 months) after thinning. Mean gross ammonification varied insignificantly across the six plots (range: 37.5 +/- 31.2 to 51.0 +/- 10.5 mg N (kg dry soil)(-1) d(-1)). The SW site was characterized by very low net nitrification and nitrate (NO3-) concentrations that were not significantly different between control and thinned plot. In contrast, for the thinned plot at the NE site (NET), significantly increased mean net nitrification (2.3 +/- 1.2 mg N (kg dry soil)(-1) d-1 at the NET plot vs. 0.4 +/- 0.2 mg N (kg dry soil)(-1) d(-1) at the NEC plot) and mean extractable NO3- concentrations (43.9 +/- 22.8 mg N (kg dry soil)(-1) at the NET plot vs. 4.1 +/- 0.8 mg N (kg dry soil)(-1) at the NEC plot) were observed. The differences in net nitrification and NO3- concentrations across the research plots were related to differences in the forest-floor C : N ratios: net nitrification increased exponentially below a threshold C : N value of about 25. The results of this study indicate that the forest floor of the warm-dry SW site is very resistant to N loss triggered by thinning due to high C : N ratios around 30. Under the cool-moist microclimate of the NE site, a significantly lower C : N ratio of 22.1 at the thinned plot (control plot: 26.7) coincided with significantly increased net nitrification. Thus, different responses of net nitrification to thinning under different microclimate appear to be triggered by different C : N ratios. Nitrous oxide production was mainly governed by forest-floor water content, and since differences in water content at adjacent control and thinned plots were low, N2O production was not significantly different between adjacent control and thinned plots.