Sugarcane in Australia is increasingly grown under the green cane trash blanket system where harvest residues (trash) are retained on the soil surface instead of being burnt. This is considered a more sustainable system, but relatively little is knownabout its effects on soil carbon (C) and nitrogen (N). As part of a study to understand the effects of trash retention on soil C and N dynamics, we measured the composition and decomposition of sugarcane trash in terms of dry matter (DM), C, and N in 5field experiments in contrasting climatic conditions in Queensland and New South Wales. The trash from newly harvested sugarcane contained large quantities of DM (7-12t/ha) and C (3-5 t/ha), which could be estimated from cane yield, and significant quantities of N (28-54 kg/ha), which could not be predicted from cane yield. Trash quality was low (C : N ratio >70) and it took a year for most of the trash to decompose. Cumulative thermal time was the variable most closely associated with cumulative DM andC decomposition. Variation in the rate of trash DM and C decomposition between sampling dates was partially related to temperature and rainfall at 2 of the 3 sites, but was considered to be influenced by other factors (such as soil, trash, and management) as much as by climate. There were 2 phases of decomposition: an early phase when C : N ratios were high and variable and net N loss or gain was not related to C loss; and a late phase when C ; N ratios were much lower and similar across experiments and net N loss was related to C loss. The rate of N loss from trash during the first 12 months was slow (1-5 kg/month), which would have been of little immediate significance for plant growth. The potential value of trash for soil N supply lies in cumulative effects over the medium-long term.
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