Dissolved and colloidal phosphorus affect P cycling in calcareous forest soils
Introduction
Natural colloids in the size range 1–1000 nm (Hartland et al., 2013) have been identified as important vectors governing phosphorus (P) mobility (Haygarth et al., 1997, Rick and Arai, 2011) and bioavailability (Montalvo et al., 2015) in terrestrial ecosystems. In riverine systems, a significant proportion of P may not be truly dissolved but rather bound to colloids (Gottselig et al., 2017). In soils, colloid-associated P together with P dissolved in the soil solution represents the mobile P fraction (Shand et al., 2000, Sinaj et al., 1998). Soil colloids primarily consist of low molecular weight breakdown products of soil organic matter and weathering-derived minerals (Hartland et al., 2013). The binding of organic molecules to mineral fractions reduces the overall surface activity and can stabilize them against further aggregation (Tsao et al., 2011).
The significance of colloids in P cycling results from the large P binding capacity per mass unit due to their high specific surface area, and also from their high mobility because of small size (Gottselig et al., 2014, Missong et al., 2017). It has been suggested that colloidal P contributes up to 75% of transportable P in arable soils (de Jonge et al., 2004). Colloid-facilitated P loss from agricultural or grassland soils to surface water (Heathwaite and Dils, 2000, Heathwaite et al., 2005, Uusitalo et al., 2001) is an important non-point source of eutrophication (Bol et al., 2018). Moreover, colloidal P transport also leads to spatial redistribution within soil profiles (Bol et al., 2016, Missong et al., 2018). 31P nuclear magnetic resonance spectroscopy (NMR) has been employed for the study of colloidal and dissolved P in arable soil (Jiang et al., 2015, Liu et al., 2014), grassland soil (Jiang et al., 2017), and silicate forest soil (Missong et al., 2016). Colloidal fractions from acidic forest soils were shown to be enriched in organic P forms compared to bulk soils and soil solution (Bol et al., 2016, Missong et al., 2017, Missong et al., 2016). In a previous NMR study, we observed a substantial P accumulation in lower horizons of Tangelhumus soils in Bavarian forests after long-term soil formation (Wang et al., 2019). One hypothesis is that this could have been the result of downward P redistribution in either colloidal or dissolved form.
The current research was a follow-up study on forest soils with calcareous bedrock, focusing on depth- and site-dependent P distribution in colloids and soil solution. The objectives of the present study were i) to reveal the P composition in colloids and soil solution; ii) to understand the origin of colloids by comparing their chemical composition with that of corresponding soil layers, iii) to identify environmental factors resulting in the site-dependent difference in colloid formation; and iv) to demonstrate the role of colloids and soil solution as potential vectors for vertical P redistribution in calcareous forest soils. To this end, soil samples were taken from four calcareous forest sites at various horizons. Soil colloids (less than 300 nm) were then extracted from soils via a protocol involving sedimentation and centrifugation (Séquaris and Lewandowski, 2003). Colloids obtained in this way, technically termed water-dispersible colloids (WDC) (Jiang et al., 2013, Séquaris et al., 2013, Séquaris and Lewandowski, 2003), were considered to be representative of natural colloids in soil leachates (Missong et al., 2018). The P forms in WDC, soil solution, and bulk soils at each depth were analyzed by 31P NMR after NaOH-EDTA extraction. Moreover, contents of organic carbon and other elements in WDC were determined by the asymmetric field flow fractionation technique (AF4) (Wahlund and Giddings, 1987) coupled with organic carbon detector (Huber and Frimmel, 1991) (OCD) and ICP-MS (Gottselig et al., 2017).
Section snippets
Sites and soils
Besides the two previously studied WETT1 and WETT2 Tangelhumus profiles (Histosol, the detailed site description refers to (Prietzel et al., 2013)) from Wettersteinwald forest in the Bavarian Alps, two additional calcareous forest sites with a different soil type (Rendzic Leptosols) were selected for comparison. The L, Of, Oh, and Ah layers from Tuttlingen and Mangfall forests were taken and denoted as TUT and MAN, respectively. Soil horizon designation follows the German soil classification
Elemental composition and size fractionation of WDC
WDC generally represented a small fraction of the total soil mass (in Table S2). More WDC were extracted from L and Of than from Oh and Ah layers. The results of the elemental analysis (Fig. 1) suggested that WDC were predominantly, an average of 56% by mass, composed of organic C (Corg). The concentration of WDC-associated organic C (WDC-Corg), shown as a dotted line along the right axis in Fig. 1, generally displayed a decrease from the L layer to the deeper soil horizons in WETT soils, while
P composition of soils, colloidal, and dissolved fractions
A previous study (Wang et al., 2019) suggested that the transformation of organic P in WETT1 and 2 soils was characterized by the continuous degradation of large P-containing biopolymers along with a gradual accumulation of P monoesters with depth. In TUT and MAN soils, the proportion of monoesters relative to total P also increased with depth and accounted for 66% and 72% in the lowermost Ah layer, respectively. In the organic L and Of layers of WETT sites, monoester signals detected by NMR
Conclusion
AF4 and NMR results suggest that colloids are formed in L and Of horizons of calcareous forest soils, where microorganisms continuously degrade plant litters and release P-containing organic compounds mainly in the form of diesters. Dissolved Ca2+ from the deposition of eolian dust support the formation of P-containing colloids by bridging effect. Site-dependent difference in WDC formation can be explained by the impact of pH, temperature, soil aeration, and litter type. Moreover, soil solution
Funding
This work was supported by the China Scholarship Council (grant number 201508320227); and Deutsche Forschungsgemeinschaft Priority Programme (grant number SPP 1685).
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors are thankful to the China Scholarship Council (CSC, 201508320227), Deutsche Forschungsgemeinschaft Priority Programme (SPP1685) for financial support of personnel and lab costs, respectively. We acknowledge Dr. Volker Nischwitz, Mrs. Claudia Walraf, and Mrs. Daniela Gesekus for lab support. We gratefully appreciate Prof. Jan Siemens for his inputs to improve the manuscript during revision.
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