Management of sugarcane harvest residues: consequences for soil carbon and nitrogen
Fiona A. Robertson A and Peter J. Thorburn BA Corresponding author: CRC for Sustainable Sugar Production and BSES Ltd, 50 Meiers Road, Indooroopilly, Qld 4068, Australia. Present Address: Department of Primary Industries, PIRVic., Private Bag 105, Hamilton, Vic. 3300, Australia. Email: fiona.robertson@dpi.vic.gov.au
B CSIRO Sustainable Ecosystems and CRC for Sustainable Sugar Production, Queensland Bioscience Precinct, 306 Carmody Road, St Lucia, Qld 4067, Australia.
Australian Journal of Soil Research 45(1) 13-23 https://doi.org/10.1071/SR06080
Submitted: 3 July 2006 Accepted: 11 December 2006 Published: 14 February 2007
Abstract
The Australian sugar industry is moving away from the practice of burning the crop before harvest to a system of green cane trash blanketing (GCTB). Since the residues that would have been lost in the fire are returned to the soil, nutrients and organic matter may be accumulating under trash blanketing. There is a need to know if this is the case, to better manage fertiliser inputs and maintain soil fertility. The objective of this work was to determine whether conversion from a burning to a GCTB trash management system is likely to affect soil fertility in terms of C and N. Indicators of short- and long-term soil C and N cycling were measured in 5 field experiments in contrasting climatic conditions.
The effects of GCTB varied among experiments. Experiments that had been running for 1–2 years (Harwood) showed no significant trash management effects. In experiments that had been running for 3–6 years (Mackay and Tully), soil organic C and total N were up to 21% greater under trash blanketing than under burning, to 0.10 or 0.25 m depth (most of this effect being in the top 50 mm). Soil microbial activity (CO2 production) and soil microbial biomass also increased under GCTB, presumably as a consequence of the improved C availability. Most of the trash C was respired by the microbial biomass and lost from the system as CO2. The stimulation of microbial activity in these relatively short-term GCTB systems was not accompanied by increased net mineralisation of soil N, probably because of the greatly increased net immobilisation of N. It was calculated that, with standard fertiliser applications, the entire trash blanket could be decomposed without compromising the supply of N to the crop. Calculations of possible long-term effects of converting from a burnt to a GCTB production system suggested that, at the sites studied, soil organic C could increase by 8–15%, total soil N could increase by 9–24%, and inorganic soil N could increase by 37 kg/ha.year, and that it would take 20–30 years for the soils to approach this new equilibrium. The results suggest that fertiliser N application should not be reduced in the first 6 years after adoption of GCTB, but small reductions may be possible in the longer term (>15 years).
Additional keywords: residue retention, microbial biomass, N mineralisation, C mineralisation, C sequestration.
Acknowledgments
Thank you to Graham Kingston, Alan Hurney, and Les Chapman for allowing this study to be superimposed on their field experiments and providing supporting data. Thank you to Ross Mitchell for providing data for Table 4. Thank you to Kaylene Harris, Ruth Mitchell, Kylee Sankowsky, Patricia Nelson and Jody Biggs for assistance with the field and laboratory work. We acknowledge funding from the Australian Government and Sugarcane Industry through the CRC for Sustainable Sugar Production, BSES Ltd, and the Sugar Research and Development Corporation.
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