Integral energy of conventional available water, least limiting water range and integral water capacity for better characterization of water availability and soil physical quality

Abstract

Different approaches have been proposed for quantification of soil water availability for plants but mostly they do not fully describe how water is released from the soil to be absorbed by the plant roots. A new concept of integral energy ( E I) was suggested by Minasny and McBratney (Minasny, B., McBratney, A.B. 2003. Integral energy as a measure of soil-water availability. Plant and Soil 249, 253–262) to quantify the energy required for plants to take up a unit mass of soil water over a defined water content range. This study was conducted to explore the E I concept in association with other new approaches for soil water availability including the least limiting water range (LLWR) and the integral water capacity (IWC) besides conventional plant available water (PAW). We also examined the relationship between E I and Dexter's index of soil physical quality ( S-value). Twelve agricultural soils were selected from different regions in Hamadan province, western Iran. Soil water retention and penetration resistance, Q, were measured on undisturbed samples taken from the 5–10 cm layer. The PAW, LLWR and IWC were calculated with two matric suctions ( h) of 100 and 330 hPa for field capacity (FC), and then the E I values were calculated for PAW, LLWR and IWC. There were significant differences ( P < 0.01) between the E I values calculated for PAW 100, PAW 330, LLWR 100, LLWR 330 and IWC. The highest (319.0 J kg −1) and the lowest (160.7 J kg −1) means of E I were found for the E I(IWC) and E I(PAW330), respectively. The E I values calculated for PAW 100, LLWR 100 and LLWR 330 were 225.6, 177.9 and 254.1 J kg −1, respectively. The mean value of E I(PAW330) was almost twice as large as the mean of E I(IWC) showing that IWC is mostly located at lower h values when compared with PAW 330. Significant relationships were obtained between E I(IWC) and h at Q = 1.5 MPa, and E I(LLWR100) or E I(LLWR330) and h at Q = 2 MPa indicating strong dependency of E I on soil strength in the dry range. We did not find significant relationships between E I(PAW100) or E I(PAW330) and bulk density ( ρ b) or relative ρ b ( ρ b-rel). However, E I(LLWR100) or E I(LLWR330) was negatively and significantly affected by ρ b and ρ b-rel. Both E I(PAW100) and E I(PAW330) increased with increasing clay content showing that a plant must use more energy to absorb a unit mass of PAW from a clay soil than from a sandy soil. High negative correlations were found between E I(PAW100) or E I(PAW330) and the shape parameter ( n) of the van Genuchten function showing that soils with steep water retention curves (coarse-textured or well-structured) will have lower E I(PAW). Negative and significant relations between E I(PAW100) or E I(PAW330) and S were obtained showing the possibility of using S to predict the energy that must be used by plants to take up a unit mass of water in the PAW range. Our findings show that E I can be used as an index of soil physical quality in addition to the PAW, LLWR, IWC and S approaches.