Abstract
PurposeRecent research suggests that Swedish organic arable soils have been under-recognized as a potential source of phosphorus (P) loading to water bodies. The aim of this study was to compare P losses through leaching from organic and high-fertility mineral soils. In addition, the effectiveness of a magnesium-salt-coated biochar applied below the topsoil as a mitigation strategy for reducing P losses was evaluated.Materials and methodsPhosphorus leaching was measured from four medium- to high-P arable soils, two Typic Haplosaprists (organic 1 and 2), a Typic Hapludalf (sand), and an unclassified loam textured soil (loam), in a 17-month field study utilizing 90-cm-long lysimeters. A magnesium-salt-coated biochar was produced and characterized using X-ray powder diffraction (XPD), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), and X-ray adsorption (XANES) spectroscopy, and its phosphate adsorption capacity was determined at laboratory scale. It was also applied as a 3-cm layer, 27 cm below the soil surface of the same lysimeters and examined as a mitigation measure to reduce P leaching.Results and discussionTotal-P loads from the 17-month, unamended lysimeters were in the order of organic 2 (1.2 kg ha−1) > organic 1 (1.0 kg ha−1) > sand (0.3 kg ha−1) > loam (0.2 kg ha−1). Macropore flow, humic matter competition for sorption sites, and fewer sorption sites likely caused higher P losses from the organic soils. Analysis by XRD and SEM revealed magnesium was primarily deposited as periclase (MgO) on the biochar surface but hydrated to brucite (Mg(OH)2) in water. The Langmuir maximum adsorption capacity (Qmax) of the coated biochar was 65.4 mg P g−1. Lysimeters produced mixed results, with a 74% (P < 0.05), 51% (NS), and 30% (NS) reduction in phosphate-P from the organic 1, organic 2, and sand, respectively, while P leaching increased by 230% (NS) from the loam.ConclusionsThe findings of this study indicate that P leached from organic arable soils can be greater than from mineral soils, and therefore, these organic soils require further investigation into reducing their P losses. Metal-enriched biochar, applied as an adsorptive layer below the topsoil, has the potential to reduce P losses from medium- to high-P organic soils but appear to be less useful in mineral soils.
Highlights
Agricultural intensification has led to an increase in phosphorus (P) loading to many surface water bodies worldwide (McDowell and Sharpley 2001; OECD 2008; Kronvang et al 2009)
The findings of this study indicate that P leached from organic arable soils can be greater than from mineral soils, and these organic soils require further investigation into reducing their P losses
The degree of P saturation (DPSox), a measure of potential P release in soils (Beauchemin et al 1998), was calculated in that same study and correlated well (R = 0.84) with P leaching from the organic soil, suggesting that the presence of aluminum and ironoxides, which are often limited in these soils, is important for controlling losses
Summary
Agricultural intensification has led to an increase in phosphorus (P) loading to many surface water bodies worldwide (McDowell and Sharpley 2001; OECD 2008; Kronvang et al 2009). Organic soils are less common than their mineral soil counterparts, representing < 1% of potential total arable land worldwide (Blume et al 2015). Organic soils are desirable arable soils, especially for root vegetable production, due to the ease of cultivation and harvest. These former wetlands are often cultivated and heavily fertilized, resulting in an accumulation of P in the surface layers (Shenker et al 2005; Riddle et al 2018a). High soil-P concentrations combined with low sorption capacity (Daly et al 2001) can increase the leaching potential of phosphate, which is often the most common form of P lost from organic soils (Riddle et al 2018a)
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