The role of soil phosphorus sorption characteristics in the functioning and stability of lowland heath ecosystems

Previous research indicates that transition between lowland heath and scrub ecosystems depends upon soil phosphorus (P) sorption capacity (PSC). Experimental work found a positive relationship between P availability and tree invasion but the relationship between PSC, P availability and scrub invasion is poorly understood making it difficult to clearly link landscape invasion patterns with small-scale experimental findings. Using a combination of descriptive and experimental studies we re-examined the relationship between PSC and tree invasion and investigated the hypothesis that PSC is a key determinant of P retention and therefore the P available to scrub colonists. In a statistical model fitted to soil data from three regions soil organic matter (SOM) content explained most of the variation in available P but PSC also accounted for a significant portion of the variance. Additional models suggest that soil P saturation and the proportion of available P in water desorbable form, both indicators of leaching losses, are strongly dependent on PSC. These findings are supported by experimental results; there was greater retention of added P, in plant available form, on high PSC soils and low PSC soils saturated at lower levels of addition. When synthesized with existing data, these results demonstrate that the relationship between PSC and P availability operates via a variety of mechanisms and at several spatio-temporal scales. PSC may for instance, influence post-disturbance SOM accumulation rates. Therefore PSC, by controlling P-availability and ecosystem development, may control the propensity of a site to either heath or scrub.