Plant Soil DOI 10.1007/s11104-014-2222-6
REGULAR ARTICLE
Soil fungi appear to have a retarding rather than a stimulating role on soil apatite weathering Mark M. Smits & Leif Johansson & Håkan Wallander
Received: 7 April 2014 / Accepted: 25 July 2014 # Springer International Publishing Switzerland 2014
Abstract Aims Vegetation stimulates, in general, soil mineral weathering. It has been hypothesized that plantassociated microorganisms, especially ectomycorrhizal fungi play a major role in this process. We studied apatite dissolution in a vegetation gradient in southern Norway to test the role of ectomycorrhizal vegetation on mineral weathering. Methods A natural occurring lead contamination, probably present since the last glaciation, caused a gradient from bare soil, via sparse grass to healthy spruce forest. We measured apatite content, soil solution chemistry, δ 13 C, δ 15N, C, N and ergosterol content in soil profiles along the gradient. Results The apatite loss for each soil depth could be described by the same proton-based, dissolution
function over the whole vegetation gradient. The deviation from the 30–40 cm depth pH model showed, in the top 20 cm, a negative correlation with ergosterol, and a positive correlation with δ 13C. These correlations could reflect an inhibiting effect of biotic activity through the production of large weight organic acids and degradation of low molecular weight organic acids. Conclusions Vegetation accelerates apatite dissolution by acidifying the soil solution, but soil fungi appeared to have a retarding, rather than an enhancing effect on this process. Keywords Apatite . Fungi . Vegetation . Weathering . DOM
Introduction Responsible Editor: Thom W. Kuyper.. Electronic supplementary material The online version of this article (doi:10.1007/s11104-014-2222-6) contains supplementary material, which is available to authorized users. M. M. Smits (*) Centre for Environmental Sciences, Environmental Biology Group, Hasselt University, Agoralaan Building D, 3590 Diepenbeek, Belgium e-mail:
[email protected] M. M. Smits : H. Wallander Department of Microbial Ecology, Lund University, Ecology Building, 223 62 Lund, Sweden L. Johansson Department of Geology, Lund University, Sölvegatan 12, 223 62 Lund, Sweden
Rock-forming minerals are the primary source of most plant nutrients (Marschner 1995). But mineral weathering is a slow process. In most soils nutrient input from mineral weathering into the available nutrient pool is small compared to recycling of organically bound nutrients (Schlesinger 1991), but essential to compensate for nutrient losses from the ecosystem. In this light it is important to know the drivers of mineral weathering in soils. Several field studies have demonstrated the stimulating effect of vegetation, and forests in particular, on mineral dissolution rates (Moulton et al. 2000; Nezat et al. 2004; Taylor et al. 2009). As most plants live in symbiotic relationship with mycorrhizal fungi (Smith and Read 2008), associated with their roots, these fungi
Plant Soil
have to be taken into account in relation with plantmineral interactions. Especially one group, the ectomycorrhizal (EcM) fungi, have been put forward as important weathering agents (for reviews see Landeweert et al. 2001; Hoffland et al. 2004; Finlay et al. 2009). Indeed, experiments under laboratory conditions have demonstrated a stimulating effect of EcM fungi on mineral dissolution rates (Van Schöll et al. 2006; Smits et al. 2008; Smits et al. 2012). Mineral dissolution is essentially a liquid-surface chemistry process, involving metal-proton exchange reactions and the formation of organic-metal complexes on the mineral surface (Schott et al. 2009). In the literature there has been strong contrasting opinions about how much soil biota, including EcM fungi, contribute to mineral dissolution (e.g. Drever and Stillings 1997; Finlay et al. 2009; Jongmans et al. 1997; Smits et al. 2008; Sverdrup et al. 2002, Sverdrup 2009). This discussion has been mainly focussed on the boreal forest zone. Based on soil solution chemistry, it is likely that mineral weathering mechanisms are dominated by water- and proton-metal exchange reactions in most boreal forest soils (Sverdrup 2009). On the other hand, EcM fungi interact with minerals on the scale of individual hyphae, i.e. a smaller scale than most soil mineral grains, probably creating locally very chemically distinct environments (Smits et al. 2008). Also, nanoscale observations show distinct physical and chemical weathering at locations of fungal-mineral contact (Balogh-Brunstad et al. 2008; Bonneville et al. 2009; Saccone et al. 2012). The mineral apatite is of special interest for several reasons: (1) it is the primary source of P, one of the key plant nutrients and essential to all life; (2) laboratory experiments demonstrate strong preferential colonization of apatite crystals over other minerals (Rogers et al. 1998; Smits et al. 2012) and (3) a mycorrhizal explicit soil organic matter degradation model (Orwin et al. 2011) suggests P-limitation in soil micro-organisms, even under conditions where the vegetation is Nlimited (Orwin, personal communication). Microcosm experiments have demonstrated increasing apatite dissolution rates when EcM fungi are present (Wallander 2000; Smits et al. 2012), but field evidence is lacking. Mesh bags with apatite, incubated in boreal forest soils enhance fungal growth more than other minerals like biotite (Hagerberg et al. 2003; Nilsson and Wallander 2003), but the effect diminishes after P fertilization (Wallander and Thelin 2008), giving an indication that
soil fungi do recognize apatite as a P source. In contrast, a recent field incubation study (Koele et al. 2014) contradicts the proposed role of EcM fungi in apatite weathering. Apatite dissolution rate was similar in EcM and non-EcM ecosystems. The aim of this study is to test if vegetation has a stimulating effect on apatite dissolution, and if this effect is enhanced by vegetation forming EcM symbiosis. Here we present field measurements of apatite weathering, at different soil depths, in a vegetation gradient in southern Norway. A natural occurring lead contamination, probably present since the last glaciation, caused a vegetation gradient with EcM vegetation absent in the most affected parts of the gradient. We measured apatite content, pH, ergosterol content and C/N-isotope composition in soil profile samples in the different vegetation zones of the gradient.
Methods Study site The study area is located close to Kastad, 6.5 km north of Gjøvik, southern Norway. On a slightly sloping terrain, weathering of an outcrop of Galena-rich quartzite has created a natural Pb gradient in the down slope glacial till, which is likely present since the retreat of the last glacial sheeth (Låg et al. 1969). Due to the extreme high Pb concentrations in the topsoil, vegetation is completely absent on an area of around 100 m2 (Fig. 1). From the border of the bare area the vegetation shows a short gradient (5–10 m). We recognized the following stages: (1) “bare soil”: only bare soil and boulders covered by lichens, (2) “grass”: sparse grass vegetation (mainly Festuca rubra), (3) “Poor tree”: stunted growing Picea abies trees, (4) “No Vaccinium”: the border where Vaccinium myrtillus starts to grow (Fig. 1), (5) unaffected vegetation of P. abies trees, with Vaccinium myrtillus understory: “Control A” uphill from the contaminated site, and “Control B” downhill. The soil in the surrounding forest and in the tree vegetated part of the gradient (“Control”, “No Vaccinium” and “Poor tree”) can be described as a haplic podzol (WRB classification), although the distinct stratification is less pronounced in the “poor tree” sites. The tree-less part of the gradient (“grass” and “bare soil”) both show some soil development with organic matter mixing in the top mineral soil. These
Plant Soil Fig. 1 Overview of the vegetation gradient near Gjøvik, Norway. The picture is faced uphill. The different vegetation types are labelled
Control
Vaccinium border
Bare soil Poor Tree
Grass
soils can be classified as a cambisol (WRB classification). The gradient is on a gentle slope (
