Abstract

Increased deposition of reactive nitrogen (N) since pre-industrial times has dramatically altered the conditions for forest growth and decomposition of organic material in Germany. The second National Forest Soil Inventory (NFSI II, 2006–2008) shows the status of N accumulation in forest soils. A median N stock of 6.3 t ha–1 has been found in the soil profile down to a depth of maximum 90 cm, whereof 50% is stored in the upper 30 cm of the mineral soil. The high regional variability of N stocks is explained by forest type, parent material, soil acidity, annual mean temperature, and adjacent agricultural land use. C/N ratios of the top soil were on average higher (24.0) than those during NFSI I (22.4, 1989–1992), which may be seen as a first effect of slowly decreasing deposition rates. Observations on limed plots suggest that acidification inhibits soil biological activity and thereby reduces N-storage in the mineral soil. The median annual N balance for German forest soils between NFSI I and II varies between +2.9 and +7 kg ha–1, depending on the harvest regime assumed. Negative N balances occurred mainly in mountain ranges like the Black Forest or the Rhenish Slate Mountains. N stocks in the upper 30 cm of the soil generally increased, while there are indications for losses of N from deeper soil layers, potentially linked to progressing acidification in these layers. Irrespective of existing measurement uncertainties, the findings indicate the vulnerability of forest N stocks under changing conditions. Further reductions of N deposition should be strived for to reduce the risk of nitrate leaching from forest soils.

Highlights

  • Nitrogen (N) in forest soils is a key variable to assess the state of forest ecosystems due to the large effects it has on forest growth, ecosystem integrity, and human health

  • Since atmospheric nitrogen (N2) is a very stable molecule, requiring high amounts of energy for the transformation into reactive N species that may contribute to plant-available N, forest trees always depended on the recycling of organic N from the decomposition of dead organic matter in the soil, where organic molecules are with 95% the prevailing form of N (Rohmann and Sontheimer 1985)

  • Alternative insight into the status and dynamics of N stocks in German forest soils between NFSI I and NFSI II may only be derived from Intensive Forest Monitoring plots (IFM plots) of the ICP Forests Level II network and additional plots operated according to the same methodology (UNECE ICP Forests Programme Co-ordinating Centre 2016), as the only available independent dataset from approximately the same area and within nearly the same time frame

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Summary

Introduction

Nitrogen (N) in forest soils is a key variable to assess the state of forest ecosystems due to the large effects it has on forest growth, ecosystem integrity, and human health. Without consideration of human activities, there is only limited exchange of this ecosystem-internal cycle with the environment (Larcher 2001): The natural sources for additional plant-available N (lightnings and N2 fixing microbes, redistribution of N by moving water or animals) are scarce; tree species show mechanisms for minimizing N losses from the forest ecosystem due to leaching or gaseous emission These include to build up dense fine root and mycorrhizal networks close to the origin of newly mineralized N compounds or immediate uptake of any plantavailable N, preferentially NH4+ (Ek et al 1994; Posch et al 2015). These trend calculations are followed by the discussion of methods and a final discussion of the results

Nitrogen Stocks in Forest Soils
Gradient of Nitrogen Stocks with Depth in the Soil Profile
Nitrogen Stocks in the Organic Layer
Nitrogen Stocks in the Soil Profile
C/N Ratios in the Top Soil
Comparison to C/N Ratios of NFSI I
Forest Type
Parent Material and Soil Acidity
Annual Mean Temperature
Agricultural Land Use
Nitrogen Stock Changes
Nitrogen Stock Difference on NFSI Plots
Nitrogen Stock Difference on IFM Plots
Nitrogen Balance Estimation
Atmospheric Nitrogen Deposition
Gaseous Nitrogen Emissions
Nitrogen Leaching
Net Nitrogen Uptake for Different Harvest Scenarios
Discussion of Estimated Balances
Discussion of Methods
Spatial Variability
Uncertainty from Analytical Errors
Treatment of Very Low Concentrations
Findings
Plot Selection Effects
Summary and Conclusions
Full Text
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