Phosphorus and carbon in soil particle size fractions: A synthesis

Marie Spohn

doi:10.1007/s10533-019-00633-x

Abstract

Despite the importance of phosphorus (P) as a macronutrient, the factors controlling the pool sizes of organic and inorganic P (OP and IP) in soils are not yet well understood. Therefore, the aim of this study was to gain insights into the pools sizes of OP, IP and organic carbon (OC) in soils and soil particle size fractions. For this purpose, I analyzed the distribution of OP, IP, and OC among particle size fractions depending on geographical location, climate, soil depth, and land use, based on published data. The clay size fraction contained on average 8.8 times more OP than the sand size fraction and 3.9 and 3.2 times more IP and OC, respectively. The OP concentrations of the silt and clay size fraction were both negatively correlated with mean annual temperature (R2 = 0.30 and 0.31, respectively, p < 0.001). The OC:OP ratios of the silt and clay size fraction were negatively correlated with latitude (R2 = 0.49 and 0.34, respectively, p < 0.001). Yet, the OC:OP ratio of the clay size fraction changed less markedly with latitude than the OC:OP ratio of the silt and the sand size fraction. The OC concentrations of all three particle size fractions were significantly (p < 0.05) lower in soils converted to cropland than in adjacent soils under natural vegetation. In contrast, the OP concentration was only significantly (p < 0.05) decreased in the sand size fraction but not in the other two particle size fractions due to land-use change. Thus, the findings suggest that OP is more persistent in soil than OC, which is most likely due to strong sorptive stabilization of OP compounds to mineral surfaces.

Highlights

Phosphorus (P) is an essential macronutrient necessary to all living organisms that controls primary production in many ecosystems (Aerts and Chapin 1999; Reich and Oleksyn 2004; Goll et al 2012)
The total organic phosphorus (TOP) concentration of the bulk soil was strongly correlated with the TOC concentration (R2 = 0.80, p \ 0.001), and the mean molar TOC:TOP ratio of the bulk soil amounted to 250
The four hypotheses are approved because it was found that (i) organic P (OP) was more strongly enriched in the clay size fraction with respect to the sand size fraction than organic carbon (OC) and inorganic P (IP), (ii) the OC:OP ratio changed less in the clay than in the sand size fraction with latitude, (iii) the OP content of the clay size fraction was less affected by land-use change than the OP content of the sand and silt size fraction, and (iv) the OP content of the clay size fraction was less affected by land-use change than the OC content of the same fraction

Summary

Introduction

Phosphorus (P) is an essential macronutrient necessary to all living organisms that controls primary production in many ecosystems (Aerts and Chapin 1999; Reich and Oleksyn 2004; Goll et al 2012). Phosphorus is present in soils in organic and inorganic forms that strongly interact with mineral surfaces and metal cations. Phosphates sorb to positively charged surfaces in soil such as Fe and Al oxides and hydroxides as well as to positively charged binding sites on OM and at the edges of phyllosilicates (Hinsinger 2001; Celi and Barberis 2005; Gerard 2016). They can be bound to mineral surfaces through ligand exchange (specific sorption). Phosphates can sorb to negatively charged surfaces through polyvalent metal cations (Kleber et al 2007)

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