Organic P in temperate forest mineral soils as affected by humus form and mineralogical characteristics and its relationship to the foliar P content of European beech
Introduction
Phosphorus (P) deficiency of beech (Fagus sylvatica) is widespread in Europe (Jonard et al., 2014; Talkner et al., 2015). According to the second German national forest soil inventory (conducted 2006–2008), foliar P contents of beech were below the normal range at 60% of the plots investigated (Riek et al., 2016). However, it is difficult to determine whether trees are prone to P deficiency at a given site, since foliar analysis is currently the only widely accepted tool available for assessing the P nutritional status of beech (e. g. Braun et al., 2010; Jonard et al., 2014; Merino et al., 2008). To individually assess the sustainability of forest management practices, it would be favorable to be able to predict P availability to trees from simple soil P tests or other soil parameters.
Various extractants for P have been tested to predict the P nutritional status or the growth response to P fertilization of several tree species under different climatic conditions. Generally best results have been obtained when the acid ammonium fluoride solutions of Bray and Kurtz (1945) or the alkaline NaHCO3 solution of Olsen et al. (1954) were used (Ballard and Pritchett, 1975; Ballard, 1974; Mason et al., 2010; Radwan et al., 1985; Wells et al., 1986). However, these methods did not always prove successful (Kadeba and Boyle, 1978; Mendham et al., 2002) and have only rarely been tested in temperate forest soils. A major issue of these and most other classical soil tests is that they neglect organic P.
In forest soils low in total P (TP), the supply of P to trees is dominated by biological processes related to the turnover of organic P rather than by physicochemical exchange and dilution processes (Achat et al., 2009, Achat et al., 2012, Achat et al., 2013; Bünemann et al., 2016; George et al., 2017; Lang et al., 2017). To characterize the bioavailability of organic P, the sequential fractionation method of Hedley et al. (1982) has often been applied to forest soils (Johnson et al., 2003). However, organic P fractions separated by this method are defined operationally and the fractions considered non-labile may still be important for tree P nutrition (Fox et al., 2011; Niederberger et al., 2017; Richter et al., 2006). In contrast, methods for determining total organic P (TOP) are less laborious and do not separate different fractions (Turner et al., 2005). Despite this, TOP has been found to be closely related to the P uptake of plants for some soils (Harrison, 1987). For instance, Adepetu and Corey (1976) found that the TOP content of 21 Nigerian rainforest and savanna soils was a better predictor of the P uptake of maize than the contents of total P (TP) or Bray-I-extractable P. However, to our knowledge, it has never been tested whether TOP is a useful predictor of the P nutritional status of trees in temperate forest ecosystems.
The usefulness of TOP as a predictor of the P nutritional status of trees may be limited by the fact that TOP contents in forest soils are strongly affected by mineralogical characteristics such as the contents of clay or reactive aluminum (Al) and iron (Fe) (Grand and Lavkulich, 2015; Talkner et al., 2009; Vincent et al., 2012; Werner et al., 2017). This is probably due to a reduction of the microbial degradability of organic P compounds when interacting mineral components are present (Celi and Barberis, 2005). When investigating the P nutritional status of trees in response to the TOP stock, such a reduced bioavailability of TOP may be corrected for by including soil mineralogical parameters as independent variables in multiple regression models. Using a similar approach, Achat et al. (2013) were able to predict P mineralization from the ratio of organic matter to the sum of oxalate-extractable Al (Alox) and Fe (Feox) ions in French Arenosols and Podzols.
TOP stocks in forest mineral soil layers may also be affected by factors related to the presence of different humus forms. Due to a higher degree of perturbation in mull compared to moder/mor soils, a higher proportion of litter is incorporated into the mineral soil, which may lead to an increased accumulation of soil organic carbon (SOC) (Andreetta et al., 2011; De Nicola et al., 2014) and possibly also of TOP (Harrison, 1979) in this compartment. Additionally, more favorable conditions for soil microorganisms might result in higher microbial biomass P (MBP) stocks in mineral layers of mull compared to moder/mor soils (Zederer et al., 2017). The turnover of organic P in the mineral soil might be more important for tree P nutrition at mull sites than at moder/mor sites, since trees can take up a considerable proportion of their P requirements from the forest floor when moder/mor type humus is present (Brandtberg et al., 2004; Jonard et al., 2009; Paré and Bernier, 1989a, Paré and Bernier, 1989b). Thus, when predicting the P nutritional status of trees from the TOP stock in mineral soil, the humus form might be another important independent variable.
Based on these considerations, our central objectives were to identify the main influencing factors on the organic P stock in mineral forest soils and to test its usefulness as a predictor of the foliar P content of beech. Our hypotheses were: (1) as a result of stronger perturbation, SOC, TOP and MBP in the mineral soil are higher at mull than at moder/mor sites, (2) the TOP content and the SOC:TOP ratio are affected by soil mineralogical characteristics such as Feox, Alox and clay, (3) multiple regression models that include the TOP stock, soil mineralogical parameters and the humus form as predictors are suitable to explain the variability of the foliar P content of beech.
Section snippets
Study sites, foliage and soil sampling and sample preparation
Twenty mature beech (Fagus sylvatica L.) stands located in the German states of Hesse, Lower Saxony, Schleswig-Holstein and Thuringia were selected to represent a gradient in foliar P contents (Table 1). Nine stands were located at mull sites, eight at moder sites, and three at mor sites. Stands were assigned to the two humus form groups “mull” and “moder/mor”, resulting in similar ranges of foliar P contents in both groups. Detailed information on humus forms (Zanella et al., 2011), soil types
Soil chemical and mineralogical parameters and soil P fractions
Of all soil chemical parameters investigated, soil pH differed most strongly between the humus form groups “mull” and “moder/mor”. Considering the entire sampled profile, soil pH (measured in H2O) at the mull sites was significantly higher and varied over a considerably wider range (4.5–7.2) than at the moder/mor (4.2–4.7) sites (Table 2, Fig. 1a). Mineralogical differences between the two groups were reflected in significantly higher clay contents under mull than under moder/mor at 10–50 cm
Humus form-related effects on SOC, TOP and MBP stocks
SOC stocks did not differ significantly between mull and moder/mor sites in any of the soil layers investigated (Fig. 1d), partly rejecting hypothesis (1). In contrast, considerably higher SOC stocks were found under mull than under moder in mineral topsoils in Mediterranean Italy (Andreetta et al., 2011; De Nicola et al., 2014). However, in line with our results, equal or even lower SOC stocks under mineral mull compared to mineral moder/mor soil layers were reported in temperate European
Conclusions
In contrast to our hypothesis (1), the humus form was not a useful stratifier for SOC, TOP or MBP stocks. Significant variations in the SOC:TOP ratio observed between humus form groups were at least partly related to co-varying Feox and clay contents. Thus, the humus form itself does not appear to be a major influencing factor on the contents and C:P stoichiometry of SOM in temperate mineral beech forest soils.
In line with our hypothesis (2), the TOP content was positively related to SOC, TN, Fe
Acknowledgements
We thank Winfried Klotz and Wilfried Schnadhorst for help in soil and foliage sampling and Nils König and his laboratory team of the Environmental Analysis Group of the Northwest German Forest Research Institute for analytical advice and assistance. We also would like to thank Ines Chmara of ThüringenForst for the permission to take samples from Thuringian monitoring plots. This study was supported by a DFG grant (TA 826/2-1) associated with the DFG priority program “SPP 1685 – Ecosystem
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