Throughfall and output fluxes of Mg in European forest ecosystems: a regional assessment

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Abstract

The biogeochemistry of Mg is of special importance in terrestrial ecology since Mg deficiency is a widespread phenomenon in European forests. Here, we investigate regional patterns of annual Mg fluxes with throughfall in relation to Mg outputs with seepage and runoff using published data from 71 forest ecosystems. Throughfall fluxes of Mg in this dataset are between 0.24 and 22.4 kg ha−1 yr−1 and are related to two major factors: the distance to the sea and the amount of dust in deposition. The gradient of Mg input from the sea may extend to 200 km inland. Output fluxes of Mg are on an average higher than throughfall input fluxes, with an average net loss of Mg from most ecosystems of 0.5–1.5 kg ha−1 yr−1 despite widespread Mg deficiency in European forest ecosystems. The amount of Mg leached in runoff or seepage water is largely dependent on the amount of Mg in throughfall. The influence of soil conditions and bedrock type on Mg output are surprisingly minor in this database indicating that the Mg cycle in many European forest ecosystems is to a great extent influenced by atmospheric inputs of Mg.

Introduction

Magnesium deficiency of trees has played a prominent role in recent research on forest ecosystem, functioning as influenced by acidic deposition. Symptoms of acute Mg deficiency (yellowing of older needles; intercostal chlorosis in leaves) have been observed in forests in many regions of Europe and North America (Hüttl, 1991). Foliar analysis is a common method for objective quantifying Mg deficiency. Ranges and threshold values for foliar Mg content of different tree species are reported (Ende and Evers, 1997). In Germany, 57% of the Norway spruce sites are presently classified as Mg deficient (Riek and Wolff, 1998). The spatial patterns of deficiency symptoms are related to the exchangeable Mg pool in the soils (Ende and Evers, 1997), to the Mg concentration of the soil solution (Matzner, 1989), to soil parent material and climatic stress (Landmann et al., 1997). In German forest soils, the exchangeable Mg pools on non-calcareous soils are generally low with 60% of sites having exchangeable pools lower than 200 kg ha−1 yr−1 at soil depths of 0–30 cm (BMELF, 1997). The annual requirement of Mg by forest stands for biomass production and canopy leaching is in the range of 3–30 kg ha−1 yr−1 (Kaupenjohann, 1997). Often the Mg storage in above-ground biomass exceeds the exchangeable Mg pool in the soil. Given this situation, the input of Mg becomes crucial for the overall cycling of Mg in forest ecosystems.

The major sources of Mg to unfertilized forest soils are from atmospheric deposition and silicate weathering (Feger, 1997). There are two main sources of Mg in the atmosphere: (i) sea salt and (ii) mineral particles originating from soil dust or from mining, combustion and other industrial processes. Decreased emission of these particles due to emission controls has led to steep decline in the atmospheric deposition of Ca and Mg in Europe and North America (Matzner and Meiwes, 1994, Hedin et al., 1994). In long-term ecosystem case studies in Germany, the decreasing input of Mg caused by the decrease of atmospheric deposition has been shown to influence the Mg output fluxes with seepage and runoff as well as the Mg nutrition of trees (Alewell et al., 2000, Matzner, 1989), suggesting a strong influence of Mg deposition on the Mg cycle in forest ecosystems.

The Mg input by silicate weathering should depend on the mineral composition of the soil and is not easily measured under field conditions (van der Salm, 1999). Mg inputs by weathering in an acid forest soil on sandstone were estimated at about 4 kg ha−1 yr−1 (Wesselink et al., 1994) and about 0.4 kg ha−1 yr−1 in an acid forest soil on an extremely Mg-poor granite (Stahr et al., 1998).

Under central European climatic conditions, the major loss of Mg from forest ecosystems is generally with seepage and runoff water, followed by biomass export, especially if whole-tree harvesting is practiced (Feger, 1997). In order to predict long-term Mg outputs with seepage and runoff, one may use deterministic soil chemical models with cation exchange as the dominant process. Such models have been developed to predict the effects of acidic deposition on soils and streamwater (e.g. MAGIC: Cosby et al., 1985; SAFE/PROFILE: Sverdrup et al., 1995; SMART/RESAM/NUCSAM: van der Salm et al., 1995). To our knowledge, however, the Mg cycle has not been specifically addressed by modeling efforts at either the local or the regional scale.

Here, we follow an empirical approach to identify factors specifically correlated to the Mg output from 71 forested plots and catchments across Europe. Because of the critical importance of Mg to the onset of forest decline symptoms, we are primarily interested in: (i) the rates and regional patterns of Mg fluxes in throughfall and output, (ii) whether the Mg output fluxes with seepage and runoff are related to the Mg fluxes with throughfall at a regional scale and (iii) to what extent other ecosystem parameters (e.g. soil conditions, tree species, acid inputs, plot versus catchment) modify the Mg output.

Section snippets

Materials and methods

A subset of the IFEF (Indicators of Forest Ecosystem Functioning; Dise et al., 1998) database of European forested plots and catchments was used for this study. This database was compiled from published reports and by contacting individual research groups across Europe (Table 1). The database contains average annual fluxes of major mineral elements with bulk precipitation, throughfall (canopydrip+stemflow), output with seepage below the rooting zone (in case of plot studies) and output with

Results

Throughfall fluxes of magnesium range from 0.24 to 22.4 kg ha−1 yr−1 with the majority of the sites less than 5 kg ha−1 yr−1. The highest fluxes of Mg mostly occur at sites close to the sea (Fig. 1) indicating the marine source of Mg. The highest fluxes of three coast sites (>18 kg ha−1 yr−1) are presumably caused by two reasons: (i) very high amounts of precipitation (Great Britain and Ireland); (ii) longest fetch of open sea (Denmark and Ireland). The marine Mg component in throughfall shows a strong

Discussion

Analyzing regional relationships and trends from data compilations is inevitably subject to uncertainty. The data we use come from different studies, sources and time periods and often methods (sampling, experimental layout, analytical methods) and definition of ecosystem components differ. This situation results in a variability of the data that may complicate the derivation of empirical relationships. In this study we use, if available, chloride budgets and charge balances to improve data

Acknowledgements

Funding was provided by the German Ministry of Education, Science and Technology (BMBF) grant No. BEO 0339476C and to N.B. Dise while working as a guest scientist at BITÖK, University of Bayreuth. Furthermore, by the Open University Research Development Fund, Milton Keynes, UK to J. MacDonald. We gratefully acknowledge the contributions of the many researchers from individual experimental sites towards the database.

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    Tel.: +49-35203-381803; fax: +49-35203-381809.

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    Present address: Scottish Environmental Protection Agency (SEPA), Stirling FK94TR, UK.

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