Decrease in phosphorus concentrations when P fertiliser application is reduced or omitted from grazed pasture soils

Abstract

Many intensively managed soils contain phosphorus (P) concentrations greater than required for optimum production. Soils with P concentrations in excess of the agronomic optimum can have unnecessary losses of P that can adversely affect water bodies. Reducing excessive soil-P concentrations is important for the economic and environmental sustainability of intensive agriculture, such as the Australian dairy industry. However, little is known of decreases in extractable soil-P concentrations when P fertiliser applications are reduced or omitted from soils with P concentrations and properties representative of intensive pasture grazing systems. Decreases in extractable P (calcium chloride (CaCl2), Olsen and Colwell) were monitored for up to 4.5 years for six Australian grazed pasture soils (Red Ferrosol, Brown Kurosol, Grey Dermosol, Brown Dermosol, Podosol and Hydrosol) with contrasting textures and P-buffering indices (PBI). Sixteen treatments consisting of four initial extractable-P concentrations (Pinit) paired with four ongoing P fertiliser rates (Pfert) were established for each of the six soils, except on an extremely low-PBI Podosol, where a range of Pinit concentrations could not be established. The resultant decreases in P were larger with higher Pinit concentration and lower rate of ongoing Pfert, except in the extremely low PBI Podosol where decreases in initially high CaCl2-P concentrations were large irrespective of ongoing Pfert. There was a greater proportional decrease in the environmentally extractable P compared with agronomically extractable P, with mean decreases in CaCl2-P of 57%, Olsen-P of 25%, and Colwell-P of 12%. The Pinit concentrations, which were well above agronomic optimum, remained above this target. This study advances scientific knowledge of extractable soil-P concentrations when P fertiliser inputs are withheld or reduced from grazed pasture soils, and aids land and catchment managers in estimating likely changes over time.

Integrating legacy soil phosphorus into sustainable nutrient management strategies for future food, bioenergy and water security

“…Soils with high STP will meet crop P demands over a longer time period than soils with lower STP (Table 2). Declines in STP are initially rapid on very P-rich soils, and then decline more slowly as the labile P not extracted by STP replenishes P in the soil solution (McCollum 1991;Schulte et al 2010;Coad et al 2014;Johnston et al 2014;Fig. 1c).…”

“…Current STP methods also extract vastly different amounts and forms of soil P depending on the method used, and the role of rhizosphere processes in soil P acquisition is not adequately captured. Hence there are soils that can supply adequate Pi in solution even when STP is low (Herlihy et al 2004;Paris et al 2004), and Pi in the soil solution can decline faster than STP when P fertilizer is withheld (van der Salm et al 2009;Dodd et al 2012;Coad et al 2014). Thus, yield reductions from drawing down legacy P have been observed on some soils after relatively short periods, even though STP appears adequate, or even high (Table 2).…”

Enhancing plant diversity in a novel grassland using seed addition

“…Third, the combination of elevated soil nutrient levels from fertilization and exotic species result in competitive exclusion of many native species that are adapted to lower nutrient soils (Bakker & Berendse, 1999;Prober et al, 2005;Seabloom et al, 2015). Soil nutrients, particularly phosphorus, may remain in grassland soils for decades and are difficult to remove by management (Coad, Burkitt, Dougherty, & Sparrow, 2014;Falkengren-Grerup et al, 2006). Finally, these grazed Australian temperate grasslands generally move from dominance by C 4 (usually Themeda triandra Forssk.)…”

The Ability to Reduce Soil Legacy Phosphorus at a Country Scale

“…For example, in addition to predicting the rate of decline in WEP was a function of Olsen P and ASC, Dodd et al (2013) investigated further using 33 P to show that the rate of decline in WEP was associated with P that was immediately exchangeable, while more strongly sorbed P only exchangeable over months was not. In measuring CaCl 2 -P in six Australian soils of varying soil P sorption capacity, Coad et al (2014) found that the rate of decline in CaCl 2 -P over 4 years was inversely related to soil P sorption capacity, measured as the P buffering index (Burkitt et al, 2006). Calcium chloride extractable P was used in this study a proxy to estimate the potential for P loss in leachate (McDowell and Condron, 2004).…”

“…The most extreme version of a negative P balance involves no application or returns of P to the soil. Research has shown that in grassland systems with little erosion and a similar climate this no-P scenario results in a rate of decline that reflects the initial STP concentration and ASC (Dodd et al, 2013;Coad et al, 2014). Other management strategies are available to decrease P losses while improving the chances of being profitable.…”