Trends in soil solution dissolved organic carbon (DOC) concentrations across European forests

Marta Camino‐Serrano ,Josep Peñuelas,Lars Vesterdal,Walter Seidling,Kasia Sawicka,Zora Lachmanová,Jim Johnson,Guia Cecchini,Elisabeth Graf Pannatier,Philippe Ciais,Zoran Galić,Laure Gandois,Sebastiaan Luyssaert,Nicholas Clarke,Henning Meesenburg,Arne Verstraeten,Bertrand Guenet,Mathieu Jonard,Tiina M Nieminen,Uwe Klinck,Ivan A Janssens,Jordi Sardans,Karin Hansen,Tanja G M Sanders,Sophia Etzold,Peter Waldner,Anne Thimonier,Elena Vanguelova,Bert Gielen,Sara Vicca,Antti‐Jussi Lindroos

doi:10.5194/bg-13-5567-2016

Abstract

Abstract. Dissolved organic carbon (DOC) in surface waters is connected to DOC in soil solution through hydrological pathways. Therefore, it is expected that long-term dynamics of DOC in surface waters reflect DOC trends in soil solution. However, a multitude of site studies have failed so far to establish consistent trends in soil solution DOC, whereas increasing concentrations in European surface waters over the past decades appear to be the norm, possibly as a result of recovery from acidification. The objectives of this study were therefore to understand the long-term trends of soil solution DOC from a large number of European forests (ICP Forests Level II plots) and determine their main physico-chemical and biological controls. We applied trend analysis at two levels: (1) to the entire European dataset and (2) to the individual time series and related trends with plot characteristics, i.e., soil and vegetation properties, soil solution chemistry and atmospheric deposition loads. Analyses of the entire dataset showed an overall increasing trend in DOC concentrations in the organic layers, but, at individual plots and depths, there was no clear overall trend in soil solution DOC. The rate change in soil solution DOC ranged between −16.8 and +23 % yr−1 (median = +0.4 % yr−1) across Europe. The non-significant trends (40 %) outnumbered the increasing (35 %) and decreasing trends (25 %) across the 97 ICP Forests Level II sites. By means of multivariate statistics, we found increasing trends in DOC concentrations with increasing mean nitrate (NO3−) deposition and increasing trends in DOC concentrations with decreasing mean sulfate (SO42−) deposition, with the magnitude of these relationships depending on plot deposition history. While the attribution of increasing trends in DOC to the reduction of SO42− deposition could be confirmed in low to medium N deposition areas, in agreement with observations in surface waters, this was not the case in high N deposition areas. In conclusion, long-term trends of soil solution DOC reflected the interactions between controls acting at local (soil and vegetation properties) and regional (atmospheric deposition of SO42− and inorganic N) scales.

Highlights

Dissolved organic carbon (DOC) in soil solution is the source of much of the terrestrially derived DOC in surface waters (Battin et al, 2009; Bianchi, 2011; Regnier et al, 2013)
A significantly increasing DOC trend (p < 0.05) in soil solution collected with tension lysimeters was found in deep mineral soil (> 80 cm) for all sites, mainly for coniferous forest sites (Table 1), but this trend is based on a limited number of plots which are not especially well distributed in Europe (75 % of German plots)
Different monotonic long-term trends of soil solution DOC have been found across European forests at plot scale, with the largest trends for specific plots and depths not being statistically significant for specific plots and depths not being statistically significant (40 %), followed by significantly positive (35 %) and significantly negative trends (25 %)

Summary

Introduction

Dissolved organic carbon (DOC) in soil solution is the source of much of the terrestrially derived DOC in surface waters (Battin et al, 2009; Bianchi, 2011; Regnier et al, 2013). Increasing air temperatures warm the soil, stimulating soil organic matter (SOM) decomposition through greater microbial activity (Davidson and Janssens, 2006; Hartley and Ineson, 2008; Kalbitz et al, 2000). Other drivers, such as increased atmospheric CO2 and the accumulation of atmospherically deposited inorganic nitrogen, are thought to increase the sources of DOC by enhancing primary plant productivity (i.e., through stimulating root exudates or increased litterfall) (de Vries et al, 2014; Ferretti et al, 2014; Sucker and Krause, 2010). Land use and management (e.g. drainage of peatlands, changes in forest management or grazing systems) may alter the flux of DOC leaving the ecosystem, but no consistent trends in the hydrologic regime or land use changes have been detected in areas where increasing DOC trends have been observed (Monteith et al, 2007)

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Conclusion