Phosphorus enrichment from point to catchment scale following fire in eucalypt forests

Erosion studies at multiple scales have shown selectivity in particle transport and delivery can be inferred from nutrient enrichment. Most studies reveal the enrichment ratio (ER) of phosphorus on sediment is high, demonstrating the deposition of coarser particles over the transport pathway. Experimental work on burnt landscapes often takes place at the hillslope plot scale (1–10 m2) by necessity, with inferences made on system sediment and nutrient responses from small areas. Scale effects on generation and delivery need to be considered in such cases. In a study in wet eucalypt forests burnt by wildfire, phosphorus concentration on mineral sediment and organic material at the point (0.01 m2), plot (1–10 m2) and catchment (106 m2) was used to estimate ERs over two scales. The data revealed ERs of 1.5 from point to plot and 2 from plot to catchment. These ratios are relatively low compared with other studies. We suggest the principal reason is short transport pathways which act to decrease deposition, due to the spatial heterogeneity of post-burn soil hydraulic properties. The association of total phosphorus with mineral material was slightly higher than with organic matter. The study suggests that using plot scale hillslope experiments to infer sediment and nutrient loss after wildfire would have overestimated losses by around 100%.

Research highlights► P enrichment ratios were 1.5 from point–plot and 2 from plot–catchment. ► The association of P with mineral material was greater than with organic matter. ► The low enrichment ratios infer short transport lengths. ► Plot (1–10 m2) hillslope experiments only would overestimate losses by 100%.