Consequence of litter removal on pedogenesis: A case study in Bachs and Irchel (Switzerland)
Graphical abstract
Introduction
In soil genesis, the alteration products of one individual horizon become reagents within the next horizon; this phenomenon is particularly evident in the case of podzolisation (Ugolini et al., 1988). Podzolisation consists of two main chemical components: i) mobile organic acids, which are the key proton donors that drive the soil processes in the O, E and Bhs horizons, and govern both soil pH and leaching; ii) these acids dissolve minerals and form metal–organic complexes that are nested at the Bhs/Bs interface (Ugolini et al., 1977). One or more plant litter horizons exist above these mineral horizons under natural conditions, at the interface between the forest plant biomass and soil, and represent one of the potential key compartments that serves as a C sink (Bellassen and Luyssaert, 2014, Janzen, 2004, Luyssaert et al., 2010). Fresh plant litter generally consists of distinguishable vegetal remains, leaves, needles, roots, bark, twig and wood pieces, either fragmented or whole. This organic material rapidly or slowly degrades, depending on the local climatic and biological conditions (Catoni et al., 2016). This thin, delicate layer of organic material can be easily affected by humans. For instance, forest litter raking as a replacement for straw in husbandry is an old non-timber practice in forest management that has been widespread in Europe since the seventeenth century (Bürgi et al., 2006, Bürgi and Gimmi, 2007, Gimmi et al., 2008). At its peak in 1853, an estimated 50 Tg dry litter per year was raked at the European level (McGrath et al., 2015). Local historic forest litter-raking results in a long-term reduction in C pools in soils, which is relevant for C accounting on broader scales (Gimmi et al., 2013). After long-term raking, it has been calculated that mixed and deciduous forests show soil carbon depletion by up to 20% of the potential total soil carbon sink without gathering litter (Gimmi et al., 2013). Several studies have speculated that the influence of gathering forest litter might also play a key role in soil nutrient biogeochemical cycles (Glatzel, 1991); Glatzel (1990), Dzwonko and Gawronski (2002), and Vild et al. (2015) suggested that a progressive depletion of soil nutrients as a consequence of litter removal occurs.
Several ecosystem models enable the impact of anthropogenic activities on ecosystems to be scaled up (e.g., Kaplan et al., 2012), although the timeframe within which soil carbon pools can reach equilibrium and/or fully recover remains unclear, as well as the effects on soil biogeochemical cycles. It is also unclear whether local soil biogeochemical cycles in individual specific circumstances can be realistically extrapolated, for instance, litter-raking.
Human intervention in soil processes has a considerably greater effect than natural perturbations and thus, exceeds the resiliency of soil to recover to its original condition (Amundson et al., 2015). Questions include how a soil evolves, whether human intervention alters one soil horizon and whether the soil formation process becomes slower sensu Simonson (1959) or Runge (1973). The aim of this study was to compare the properties of two broadleaf litters, to understand whether soil organic matter (SOM) develops and to develop a framework by which SOM chemistry is altered as it passes through various litter horizons towards mineral soil. Here, we present results from two beech forests; one mixed (beech and oak) and a pure (beech) forest grown under very similar environmental conditions. The aim was to understand how species influence the soil upon which they develop, and to evaluate the effect of the periodic removal of the forest litter. We postulate that litters of different composition, due to the diverse vegetation cover but built over similar soils have similar properties, and that litter removal, if not combined with other degradation factors, does not influence soil chemical quality.
Section snippets
Study sites
Litter material was collected from two mature forests in Switzerland. The first stand is at Irchel (47°32′19″N, 08°36′12″E; elevation 640 m a.s.l.) and is 70 years old and dominated by Fagus sylvatica (L.). The second forest is located in the vicinity of Bachs (47°32′02″N, 08°26′45″E; elevation 589 m a.s.l.). The stand is dominated by F. sylvatica (L.) with Quercus petraea (Matt.) and some Pinus sylvestris (L.) present as a companion species. The potential natural vegetation is Luzulo silvaticae–
Comparison of leaves (beech–oak)
The initial pH for beech and oak was above 7. During leaching, the pH dropped to 6.6 in both beech samples and to 5.5 in oak. The initial electrical conductivity was rather site-specific: EC was 180 μS cm− 1 at Bachs and 240 μS cm− 1 at Irchel and decreased to 160 μS cm− 1 within a couple of weeks, independently on the site or species (Fig. 1, left).
During leaching, the released carbon initially appeared to be relatively species-unspecific but it changed during leaching cycles. Beech, on average,
Discussion
To compare two similar litters, consisting of pure beech and beech mixed with oak, we identified the differences independently. The small initial differences were highlighted via leaching: oak leaves produced a more acidic leachate (pH 5.5) than beech (pH 6.5), although both had relatively similar ionic strengths. We did not measure significant differences in the composition of beech and oak leaves, apart from relatively more cutins and waxes in oak leaves (Table 1, Fig. 6a). The leaching
Conclusions
We investigated two broadleaved litters, beech and oak, and observed some differences in carbon losses during decomposition, in leachates in particular. However, the initial dissimilarities between the two leaves became homogenised over time and the litters became indistinguishable. When leaves fall on the soil ecosystem, they become litter and alter rapidly. As a litter horizon is altered, it becomes more stable and loses fewer elements, in gaseous or liquid form. In these stands, similar
Acknowledgements
Special thanks to Peter Blaser, who launched the entire cooperation. We gratefully acknowledge the OECD co-operative research programme on biological resources for a research fellowship on management for sustainable agricultural systems. All the WSL staffs are acknowledged for the friendship and the help in conducting the experiments. We thank C. Cosentino for providing NMR spectra. We thank the two anonymous reviewers for their meticulousness, their patience, and their extremely constructive
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