Laboratory drying of organic-matter rich soils: Phosphorus solubility effects, influence of soil characteristics, and consequences for environmental interpretation

Abstract

Agronomic soil test P (STP) and soil type data may be used to predict diffuse soluble P loss from soil to water (e.g. Withers, P.J.A., Dils, R.M., Hodgkinson, R.A., 1999. Transfer of phosphorus from small agricultural basins with variable soil types and land use. p. 41. In: Impact of Land use change on nutrient loads from different sources. IAHS publication no. 257; Daly, K., Mills, P., Coulter, B., McGarrigle, M., 2002. Modelling phosphorus concentrations in Irish rivers using land use, soil type, and soil phosphorus data. J. Environ. Qual., 31: 590–599). However, previous studies quantifying the effect of STP and soil type on P solubility have usually relied on laboratory analyses of dried soil samples to quantify P solubility. Drying soil samples has been found to substantially increase P solubility (e.g. Bartlett, R., James, B., 1980. Studying dried, stored soil samples—some pitfalls. Soil Sci. Soc. Am. J., 44: 721–724; Turner, B.L., Haygarth, P.M., 2001. Phosphorus solubilization in rewetted soils. Nature, 411: 258–258), and microbial biomass and organic matter (OM) have been implicated in this (e.g. Chepkwony, C.K., Haynes, R.J., Swift, R.S., Harrison, R., 2001. Mineralization of soil organic P induced by drying and rewetting as a source of plant-available P in limed and unlimed samples of an acid soil. Plant Soil, 234: 83–90; Turner, B.L., Driessen, J.P., Haygarth, P.M., McKelvie, I.D., 2003. Potential contribution of lysed bacterial cells to phosphorus solubilisation in two rewetted Australian pasture soils. Soil Biol. Biochem., 35: 187–189). Many grassland soils in western Ireland contain high OM and moisture contents, and may therefore be particularly susceptible to drying-induced soluble P release. This study quantifies drying-induced changes in P solubility, by measuring water-extractable P (P w) and desorbable P (P feo) contents in moist and dried samples of a range of 33, mainly grassland, soils from western Ireland. Molybdate analyses of P w extracts before and after acid digestion were used to quantify dissolved reactive P (P wDRP), taken to represent predominantly readily soluble inorganic P, and dissolved molybdate-unreactive P (MUP), taken to represent predominantly readily soluble organic P. Dried soil samples were analysed for a wide range of general and P-specific characteristics, including STP (Morgan P and Olsen P) and the degree of sorption saturation (DPSS). Based on OM content, soils were divided into peat (> 30% OM) and predominantly mineral (< 30% OM) soils after Daly et al. (2000). Almost all soils exhibited large increases in P solubility after drying, with the size of drying-induced soluble P increases increasing in the order P wDRP < P wMUP< P feo. Absolute drying-induced increases were statistically similar between peat and mineral soil groups (e.g. P feo mean increases of 9.06 and 13.81 for peat and mineral soil groups, respectively). However, when expressed as a proportion of moist sample P solubility, all soluble P increases were significantly greater for peat compared with mineral soils (e.g. P feo percentage increases 735% and 155% for peat and mineral soil groups, respectively). Consequently, some differentiation in P solubility between peat and mineral soil groups was lost after drying. Drying-induced P w increases appeared to be related to soil OM and moisture contents, indicating drying-induced P release through microbial cell lysis and OM destabilisation. Increases were not related to moist sample P solubility or STP, resulting in the loss of some differentiation in P solubility between high and low STP soils. Drying-induced P feo increases were positively related to DPSS, reflecting chemical equilibrium control of desorbable P released from dried samples. It was concluded that the standard practice of drying soils prior to P solubility analyses reduces the inferred importance of soil type and STP as risk factors in diffuse soluble P losses.