The phosphorus enrichment ratio of overfertilized soils
Symposium no. 37Paper no. 1118Presentation: oral The phosphorus enrichment ratio of overfertilized soils SCALENGHE Riccardo (1), BARBERIS Elisabetta (2) and EDWARDS Anthony
C. (3)
(1)Università di Palermo ACEP -Viale delle Scienze Palermo I-90128 Italy
(2)Università di Torino, DIVAPRA-44, via Leonardo da Vinci, I-10095 Grugliasco,
Italy
(3)The Macaulay Land Use Research Institute, Craigiebuckler AB15 8QH, Aberdeen
Scotland, UK
Abstract
Phosphorus (P) associated with inorganic and organic soil material eroded during high flow storm events can constitute a large proportion (>60 percent) of the P transported in surface runoff from cultivated land. The size selective nature of erosive transport results in the preferential loss of the finer clay sized particles which also tend to show the highest accumulations of P. In situations where clay particles are predominantly composed of phyllosilicates and iron and aluminium oxides a large specific surface area of the finer fraction is common. The associated surface charge depends greatly upon the surrounding environmental conditions such as the soil solution’ ionic strength and pH. These conditions can be strongly modified by sorption of anions such as P which has important consequences for the extent of particle dispersion/flocculation degree.
Recently it has been demonstrated that adsorption of P onto iron oxides, phyllosilicates and calcite can lead to an increase of the net negative charge and a consequent dispersion of particles, organic P was observed to cause dispersion more than the inorganic form because of the higher charge density and steric stabilisation effects. Thus, while on the one hand high rates of P sorption can decrease P concentrations in soil solution reducing the potential leaching to surface waters, while perhaps enhancing nutrient transfer by increasing the mobilisation of colloidal particles that can act as carriers. To test this hypothesis clay (<2 μm), silt (2-20 μm) and sand (20-200μm) fractions of twelve overfertilized soils belonging to six World Reference Base groups were separated using progressively stronger dispersion methods. In all cases total P content was highest in the clay fraction ranging from 1020 to 11430 mg P kg-1. The highest contents of Feo, Fed and OC were also found in the clay fraction. Phosphorus enrichment ratio (PER), defined as the ratio of total P content of separates to that of soil, is always >1 in clay (indicating a concentration). PER shows an inverse trend with clay content and this is probably due to the high P sorption saturation occurred on the clay fraction.
Keywords:P losses, adsorption, desorption, P forms, dispersibility, clay
Introduction
The major P losses are related to eroded clay particles, where the highest P accumulation occurs. The mobilisation of these particles is correlated to
dispersion/flocculation phenomena that in turn depend on their properties and on reactions that can occur at the surface. Clay particles are mainly constituted by phyllosilicates and iron and aluminium oxides which are characterised by high specific surface areas and electrical charge that depend on the soil solution’ ionic strength and pH and can be strongly modified by sorption of anions such as P with important consequences for the particle dispersion/flocculation degree.
It has been observed that adsorption of P on iron oxides, phyllosilicates and calcite leads to a net increase of the negative charge and a consequent dispersion of particles, organic P was observed to disperse more than the inorganic form because of the higher charge density and steric stabilisation effects (Celi et al., 1999; Celi et al., 2000). Thus, if on one hand P sorption can decrease nutrient levels in the soil solution and limit their input to waters, on the other hand can enhance nutrient transfer by increasing the mobilisation of colloidal particles that can act as carriers. Similar findings have been shown by Uusitalo et al. (2000) reporting significative relationships between particulate P and total suspended solids in surface ruoff waters of southwestern Finland.
The purpose of this paper is to fully characterize the P content, forms and availability of various soil size separates and investigate the extent of any relationship between dispersibility and P content of soil particles.
Materials and Methods
Soils
The twelve selected soils belong to seven World Reference Base groupings Chernozems, Luvisols, Gleysols, Fluvisols, Vertisols, Umbrisols and Cambisols (FAO/ISRIC/ISSS, 1998). When considered together the twelve sites and soils vary widely in climate, cropping, and fertilisation history. Although the full characterisation of these soils is reported elsewhere (Barberis et al., 1996), the most relevant properties are summarized in Table 1. Mineralogical, physical and chemical properties related to P cycle differ widely, but all soils have at least twice the optimum concentration of available P as estimated using the official analytical methods of its relevant country and can thus be considered overfertilized.
Table1Soil general characteristics.
Group
Soil pH OC Fe d Fe o P tot P olsen P resin P water Texture CaCl2------ g kg-1 ------------------ mg kg-1 ----------C
E17.87 4.00.71400501411sandy loam
E27.79 2.80.870624820clay
I37.612 6.1 1.7103535730clay loam SA
D1 6.424 6.0 2.011681102416silty loam
D2 6.7138.6 3.1890591251loam
E3 6.8815.8 1.133320420sandy clay
I2 5.7411.5 4.010********sandy loam AOMR
G3 5.43914.48.01920401141sandy loam
G6 4.63312.7 5.61593751911sandy loam
G9 5.42914.8 5.61331501331sandy loam ALT
D3 5.19 2.2 1.798812130013sand
I1 5.2129.6 4.970948920loam
For convenience, the soils can be grouped into (1) calcareous, C, (E1, E2 and I3), (2) slightly acid, SA, (D1, D2, E3 and I2), (3) acid and rich in organic matter, AOMR, (G3, G6 and G9), and (4) acid and light-textured, ALT, (D3 and I1) (Delgado and Torrent, 1997). The latter group contains two soils with extreme properties: D3 is a sandy soil with a large amount of extractable P and I1 was sampled from a rice growing region, and therefore will have experienced periodic flooding as a routine agronomic practice.
Fractionation of soils
Three particle size fractions, hereinafter named separates, <2μm (clay) 2-200 μm (silt) and >200 μm (sand), were obtained by centrifugation and sieving after mechanical shaking in water for 24 hours (mechanical dispersion treatment: MD).
Organic carbon (OC) content of separates was determined by wet oxidation, correcting for calcium carbonate; iron was extracted by ammonium oxalate (Fe o) (Schwertmann, 1964) and citrate-bicarbonate-dithionite (Fe d) (Mehra and Jackson, 1960). Fe was analyzed by AAS. Total P (P tot) was determined after digestion with HClO4-H2SO4 (Bray and Kurz, 1945), P olsen was determined by the method of Olsen et al. (1954), extraction of P by anionic resin (P resin) was carried out according Sibbesen (1977) with a resin:separate ratio of 1:4.4 for clays and 1:1.1 for silt and sand, and with a resin:solution ratio of 1:80. For the water extraction (P water), 5 g of soil were shaken for 15 min., centrifuged and the supernatant filtered through a 0.22 μm membrane. For all samples, the concentration of P was determined colorimetrically by the molybdate-blue method (Murphy and Riley, 1962). Acid or alkali solutions were neutralized prior to P determination.
Other two sets of separates were obtained using respectively a mild and a strong dispersion treatment: (1) Water Dispersion: (WD): wet sieving in water and (2) Chemical Dispersion (CD): the soil samples were treated as in MD but with addition of NaOH at pH=9 overnight. P soluble in NaOH, determined as a control of P losses, accounts for less than 44 μg P g-1 of separates.
Results and Discussion
The soils widely varied in the yield of separates (Figure 1) obtained after the MD treatment. The distribution of OC, Fe o and Fe d in the separates reflects their close association with the finest particles (Figure 2). In some samples the high content of OC, Fe o and Fe d in the sand separates confirmed the presence of finer particles in this fraction which are cemented by iron oxides or organic matter.
Total P content (Table 2) was generally highest in the clay fraction except in calcareous samples very poor in sand (2%) where the total P was highest in the coarsest separates (E2 and I3). As well labile P, evaluated by extracting separates with water (P water), NaHCO3 (P olsen) and resin (P resin) was generally highest in clay separates.
Phosphorus buffer capacity, calculated using the quotient P resin/P water, for clay sized separates is higher than PBC for soils except in E2 calcareous sample sand poor.
Enrichment ratios were calculated (Table 3) according to Sharpley (1985) for some soil properties (OC, Fe d and Fe o, P tot) as the ratio of separate content to soil. PER has been regressed against clay, CER, FeoER, FedER: only the oxalate-extractable iron enrichment ratio seem affect the phosphorus enrichment ratio PER = 0.967 FeoER + 0.405 (n = 36, R2 0.935, p <0.006).
20406080100E1E2I3D1D2D2E3E3I2G3G6G9D3I1
Figure 1Yield of separates (%) obtained after the MD treatment. Black bars are clay,
grey silt and empty bars indicate sand.
0510*******
1
2
clay silt sand
O C (%)F e d (%)F e o (%)
Figure 2Box and whisker plots summarising the OC, Fe d and Fe o contents of each size
fraction for Mechanically Dispersed samples for all twelve soils. Where the line represents the median, and the box indicates the lower and upper quartiles respectively, outliers are denoted by an asterix.
Table2Phosphorus in soil separates (MD).
P tot P water P olsen P resin clay silt sand clay silt sand clay silt sand clay silt sand
---------------------------------------------- mg kg-1 of fraction ---------------------------------------------
E138607781015 1.50.8 1.382272451555118 E2116077712350.50.2n.d.1419n.d.12543323 I3179097422780.70.2n.d.59323618646111 D1249580716587.3 5.0n.d.1735480715146379 D22720657598 2.1 1.6 2.17926304356059
E31020272360.10.20.332127170113
I24490741308 3.0 3.4 2.222933124558041
G3558017445970.10.10.26436163877837
G657901380621 1.60.30.784732965510353
G9465010852350.20.20.68339123636315
D311430611320n.d. 6.4 6.43804733166812075
I124056207060.20.10.797353739557109 Table3Enrichment ratios, calculated as the ratio of the OC, Fe d, Fe o or P content of clay separates (MD) to that of soil.
CER FedER FeoER PER
E1 2.2 3.1 3.2 2.8
E20.8 1.6 1.6 1.6
I3 1.3 2.8 1.9 1.7
D1 2.0 3.0 2.7 2.1
D2 2.2 4.0 2.7 3.1
E3 1.4 2.8 1.8 3.1
I2 3.7 4.9 3.4 4.2
G3 2.5 2.8 2.5 2.9
G6 3.1 3.3 2.8 3.6
G9 3.4 3.1 2.4 3.5
D318.917.013.011.6
I1 1.3 5.2 2.6 3.4 Water dispersion (WD) produced the smaller amount of clay (Figure 3) and higher of sand than MD indicating that the latter fraction still contained clay-sized particles. In terms of clay and sand content WD appeared to be the mildest treatment, MD medium while CD the strongest. Comparing the two mild treatments, water and mechanical (Figure 4), the stronger treatment produces a larger amount of finest particles but total P is not showing a clear trend. Nevertheless comparing with the strongest treatments clay-sized WD or MD separates (Figure 5) were always richer in total P than those obtained by CD. The fact that the most dispersible clay is the richest in P can be explained considering that when clay is not flocculated in the soil its surfaces are most accessible
to P. So, comparing clay-sized separates PER obtained by WD or CD in respect to MD (Figure 6) a linear correlation is found and CD clay, silt and sand sized are less enriched in P confirming an accumulation in true finer particles.
clay silt sand
10
3050701030507010305070c h e m i c a l d i s p e r s i o n
m e c h a n i c a l d i s p e r s i o n w a t e r d i s p e r s i o n Figure 3Yield of separates (%) obtained after the three treatments. Where the line
represents the median, and the box indicates the lower and upper quartiles,respectively.
-1500
-1000-500
05001000D i f f e r e n c e i n P t o t (m g k g -1
)
-40
-30-20-1001020
3040E1E2I3D1D2E3I2G3G6G9D3I1
D i f f e r e n c e (%)
Note - clay fraction for D3 total P requires to be multiplied by 3.
Figure 4Difference in proportion of clay (black), silt (empty) and sand (grey) sized
material a) and total P content b) of WD and equivalent MD size fractions (a negative number indicates a greater amount in the MD fractions).
clay P tot(mg kg-1)
E2
I3
E3
I2
I1
Figure5Total P content of clay sized material obtaine from water (filled bars), mechanical (grey bars) and chemical (empty bars) treatments.
Figure6Phosphorus Enrichment Ratio of water (empty diamonds) and chemical (filled squares) separates vs clay (%) obtained by chemical dispersion.
Conclusion
Owning to the selective erosion of fine particles and P enrichment in fine earth separates, particulate P plays a role as a carrier of P from agricultural fields.
As previously demonstated by Celi et al. (1999, 2000) for binary systems, even in soils P sorption onto clay particles produces not only an increase in P concentration of erodible particles but also an increase of their dispersibility. As a potential source of
nutrients, soil particles have to be targeted for managing water quality problems both in terms of soil loss and sediment delivery and their native properties.
Acknowledgements
This work was funded by the European Community (AIR3 CT92-0303).
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