Long-term changes in soil carbon and nitrogen under trash blanketing

Although trash blanketing has been widely adopted in the sugar industry, there is little information on the magnitude or time scales of changes in soil nitrogen (N) and carbon (C), important determinants of soil N fertility, in trash-blanketed soils. This information was obtained by measuring total Nand C, mineral N, and N mineralisation potential (assessed from laboratory incubations) in soil profiles sampled from conventional burnt and green cane-trash blanketed (GCTB) management treatments at five sites. The sites were located from northern NSW to the wet tropics and had the treatments imposed for time ranging from 1 to 17 years. At the oldest site, Abergowrie in the Herbert River Valley, a cropping systems model was also used to examine likely longer-term (35 year) changes in total Nand C and impacts on sustainable production. Differences in total N and C were confined to the surface 2 or 5 cm, with Nand C being higher for GCTB in all but the youngest site. However, there was no relationship between the magnitude of the difference and the period over which the treatments had been imposed. There were no consistent treatment differences in mineral N at any depth and no significant differences in potential N mineralisation. Trends in simulated total soil C (0-20 cm) and cane yields agreed well with measurements previously made at the Abergowrie site over 15 years (two crop cycles). In the 35 year simulations, total soil C increased continually in the GCTB treatment and decreased slightly to an equilibrium value in the burnt treatment. The simulations suggest that there would be no adverse production or environmental consequences of maintaining 'conventional' N management for several decades after the initiation of trash blanketing at this site. The study illustrates the complexity of the impacts of trash management on soil Nand C. Responses to trash blanketing will be the product of soil and climate interactions, and systems modelling will be a useful tool for efficiently determining the likely outcome of these interactions. In practical terms, this means adjustment to nitrogen management in response to trash blanketing needs to be conservative and supported by good monitoring of the impacts on cane yield and CCS.