Effects of forest management on soil N cycling in beech forests stocking on calcareous soils

The effects of forest management (thinning) on gross and net N conversion, the balance of inorganic N production and consumption, inorganic N concentrations and on soil microbial biomass in the Ah layer were studied in situ during eight intensive field measuring campaigns in the years 2002-2004 at three beech (Fagus sylvatica L.) forest sites. At all sites adjacent thinning plots ("T'') and untreated control plots ("C'') were established. Since the sites are characterized either by cool-moist microclimate (NE site and NW site) or by warm-dry microclimate (SW site) and thinning took place in the year 1999 at the NE and SW sites and in the year 2003 at the NW site the experimental design allowed to evaluate (1) short- term effects (years 1-2) of thinning at the NW site and (2) medium-term effects (years 4-6) of thinning under different microclimate at the SW and NE site. Microbial biomass N was consistently higher at the thinning plots of all sites during most of the field campaigns and was overall significantly higher at the SWT and NWT plots as compared to the corresponding untreated control plots. The size of the microbial biomass N pool was found to correlate positively with both gross ammonification and gross nitrification as well as with extractable soil NO3- concentrations. At the SW site neither gross ammonification, gross nitrification, gross ammonium (NH4+) immobilization and gross nitrate (NO3-) immobilization nor net ammonification, net nitrification and extractable NH4+ and NO3- contents were significantly different between control and thinning plot. At the NET plot lower gross ammonification and gross NH4+ immobilization in conjunction with constant nitrification rates coincided with higher net nitrification and significantly higher extractable NO3- concentrations. Thus, the medium-term effects of thinning varied with different microclimate. The most striking thinning effects were found at the newly thinned NW site, where gross ammonification and gross NH4+ immobilization were dramatically higher immediately after thinning. However, they subsequently tended to decrease in favor of gross nitrification, which was significantly higher at the NWT plot as compared to the NWC plot during all field campaigns after thinning except for April 2004. This increase in gross nitrification at the NWT plot (1.73 mg N kg(-1) sdw day(-1) versus 0.48 mg N kg(-1) sdw day(-1) at the NWC plot) coincided with significantly higher extractable NO3- concentrations (4.59 mg N kg(-1) sdw at the NWT plot versus 0.96 mg N kg(-1) sdw at the NWC plot). Pronounced differences in relative N retention (the ratio of gross NH4+ immobilization + gross NO3- immobilization to gross ammonification + gross nitrification) were found across the six research plots investigated and could be positively correlated to the soil C/N ratio (R = 0.94; p = 0.005). In sum, the results obtained in this study show that (1) thinning can lead to a shift in the balance of microbial inorganic N production and consumption causing a clear decrease in the N retention capacity in the monitored forest soils especially in the first two years after thinning, (2) the resistance of the investigated forest ecosystems to disturbances of N cycling by thinning may vary with different soil C contents and C N ratios, e. g. caused by differences in microclimate, (3) thinning effects tend to decline with the growth of understorey vegetation in the years 4-6 after thinning.