Effects of forest management on soil N cycling in

Authors: Michael Dannenmann Rainer Gasche Astrid Ledebuhr Hans Papen

Publish Date: 2006/08/31

Volume: 287, Issue: 1-2, Pages: 279-300

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Abstract

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 coolmoist microclimate NE site and NW site or by warmdry 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 shortterm effects years 1–2 of thinning at the NW site and 2 mediumterm 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 NO 3 − concentrations At the SW site neither gross ammonification gross nitrification gross ammonium NH 4 + immobilization and gross nitrate NO 3 − immobilization nor net ammonification net nitrification and extractable NH 4 + and NO 3 − contents were significantly different between control and thinning plot At the NET plot lower gross ammonification and gross NH 4 + immobilization in conjunction with constant nitrification rates coincided with higher net nitrification and significantly higher extractable NO 3 − concentrations Thus the mediumterm 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 NH 4 + 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 173 mg N kg−1 sdw day−1 versus 048 mg N kg−1 sdw day−1 at the NWC plot coincided with significantly higher extractable NO 3 − concentrations 459 mg N kg−1 sdw at the NWT plot versus 096 mg N kg−1 sdw at the NWC plot Pronounced differences in relative N retention the ratio of gross NH 4 + immobilization + gross NO 3 − 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 = 094 p = 0005 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 thinningThe authors thank K ButterbachBahl N Brüggemann R Kiese G Willibald P Rosenkranz C Werner C Brümmer for expert technical assistance and scientific advice We are very grateful to P Ambus for performing the IRMS analyses We also thank S Wienholdt P Haas and G Werl for the perfect support during the field campaigns We appreciate very valuable comments on an earlier draft of this manuscript by K ButterbachBahl and N Brüggemann We appreciate the valuable comments of two anonymous referees who greatly contributed to improving the manuscript Generous funding of this work by the Deutsche Forschungsgemeinschaft DFG under contract number PA 442/42 is gratefully acknowledged

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