Addendum for RRRD009: Runoff nitrogen, phosphorus and sediment generation rates from pasture legumes: An enhancement to reef catchment modelling

Craig M. Thornton and Amanda E. Elledge

Department of Natural Mines and Resources, Rockhampton

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Executive Summary

Nitrogen, phosphorus and sediment were monitored in runoff from virgin brigalow scrub, grass pasture and leguminous pastures from 2010 to 2012 at the Brigalow Catchment Study, located in the Fitzroy Basin. Brigalow scrub is representative of the landscape in its pre-European condition. It was hypothesised that nutrient and sediment loads from a newly established ley pasture (previously cropping) would decline over time as plant cover and biomass increased. The data did not clearly demonstrate this, with trends confounded due to record breaking rainfall and runoff. Consequently the applicability of this data to reflect catchment responses in more typical seasons was unknown. Thus, an additional two years of monitoring was undertaken to capture water quality responses to less extreme climatic sequences.

Rainfall in the period 2013 to 2014 was much closer to the long-term annual average (660 mm) and did not exceed the 60th percentile in either year. Runoff during 2013 supported the 2010 to 2012 result that clearing brigalow scrub for either cropping or grazing increased runoff. Grass pasture continued to display this trend in 2014; however, butterfly pea ley pasture had similar runoff to brigalow scrub. Loads of total, oxidised and dissolved inorganic nitrogen from butterfly pea, grass and leucaena pastures were all lower than virgin brigalow scrub. However, the greatest load of dissolved inorganic phosphorus came from butterfly pea in both years.

There was little change in the relativity of loads between brigalow scrub, grass pasture and leucaena pasture between 2010 and 2014; however, loads from butterfly pea ley pasture showed quiet different dynamics. This catchment typically had the highest nutrient and sediment loads during 2010 to 2012. Conversely, loads from butterfly pea during 2013 and 2014 were similar to the other pasture land uses with loads of total, oxidised and dissolved inorganic nitrogen, total phosphorus and total suspended sediment all less than brigalow scrub.

No temporal trends were detected in the event mean concentrations of nutrients or sediment during 2010 to 2014 from brigalow scrub, grass pasture or leucaena pasture. However, a declining trend was observed for total, oxidised and dissolved inorganic nitrogen and total suspended sediment from butterfly pea ley pasture.

These findings support the hypothesis that higher nutrient and sediment loads are exported from ley pasture during the development phase and then decline over time towards that of long-term grazed landscapes. Loads of nutrients and sediment from long-term grazed landscapes were lower than that of virgin brigalow scrub. No temporal trends were detected in the event mean concentration of nutrients and sediment from brigalow scrub or the established grass and leucaena pastures from wet to dry years. This indicates that not only do these land uses maintain their specific flow signatures in extreme wet seasons, but they also maintain their specific water quality signatures.

The dynamics of dissolved inorganic nitrogen in runoff from established legume and non-legume pastures is still not clear. The risk posed to water quality is likely to be of concern given the concentration of dissolved inorganic nitrogen in runoff from butterfly pea is equal to that reported for some sugar cane systems. However, these concentrations are typically an order of magnitude less than those from brigalow scrub.

When considering on-ground management action, this study indicates that the establishment stage of a ley pasture is, not unexpectedly, the period of greatest risk to water quality. Conservative grazing management combined with spelling should be promoted in the first year to coincide with the highest risk of total, oxidised and dissolved inorganic nitrogen loss in runoff. Continued management for high cover and biomass will deliver reductions in nutrients and sediment loads past the first year.

The incorporation of a legume ley pasture into a farming system compared to a more permanent legume pasture, such as leucaena, needs to be carefully considered from both an economic and environmental perspective. Switching in and out of legume pastures, particularly ley pastures in cropping enterprises, is a substantial financial investment with the establishment phase proving the greatest risk to water quality.