Effect of pasture buffer length and pasture type on runoff water quality following prescribed burning in the Wivenhoe Catchment

Abstract

Burning of pastures is a management practice adopted by graziers worldwide. When rain falls on burnt pastures, it can lead to increased pollutant transport in runoff. However, this transport can be modified by vegetative buffers which intercept the runoff downslope of burnt areas. This study examines the effects of different pasture buffer lengths (0, 2, and 5 m) on sediment and chemical transport from two pasture sites near Wivenhoe Dam, the main water reservoir for Brisbane City. Simulated rainfall (100 mm/h) was applied to 18 plots on pasture sites after they were burnt, and insoluble and soluble components were measured in the runoff. Most eroded sediment/organic debris accumulated against the first row of the grass buffer strips or was deposited in the upslope backwater region. Buffer length had little impact on the runoff concentrations of NO3- and NO2- (NOx), total Kjeldahl nitrogen, and total nitrogen from the 5-m-length upslope plots but was significant for sediment loss rate, filterable reactive phosphate, ammonium, and total and dissolved organic carbon. Pasture type was significant for NOx, ammonium, sediment loss rate, and total organic carbon only. Burning increased enrichment ratios of nutrients and carbon in the runoff compared with unburnt plots, but a 2-m buffer strip subsequently reduced the enrichment ratio values by >30%. Buffers strips of unburnt pasture grass may provide an effective tool for post-fire erosion control following prescribed burning; however, further work including scaling to larger plot sizes and catchment level is required.