Trash management - consequences for soil carbon and nitrogen

When sugarcane (Sacharum spp.) trash is burnt, organic matter and nutrients are lost; whereas under the green cane trash blanketing (GCTB) system, they are retained. This has led to speculation that soil organic matter and nutrient levels may be increasing in GCTB systems, allowing fertiliser applications to be reduced. We investigated the effects of sugarcane trash management (burnt or GCTB) on indicators of short- and long-term soil carbon (C) and nitrogen (N) cycling. The burnt and GCTB systems were compared in 5 field experiments at 3 locations with widely differing climates\. Soil and trash were sampled eight to 10 times over one (12-month) season in each experiment. At the time of this study, the experiments had been running for between one and six years. Under GCTB, trash containing 7-12 t dry matter/ha, 3000--5000 kg C/ha, and 30--60 kg N/ha was deposited on the soil surface at harvest. Most of the dry matter (>85%) was decomposed during the subsequent 12 months. In the oldest experiments, 3-6 years of GCTB increased total N, organic C and microbial biomass C in the soiL These changes were largely restricted to the upper 20 or 50 mm. In the youngest experiments, 1-2 years of GCTB had no effect on total or microbial C and N levels. Short-term laboratory incubations of soil showed that microbial respiration was greater under GCTB, particularly in the older experiments. Concomitant net mineralisation of N, however, generally did not differ significantly between trash management treatments. The ratio (C mineralisation : N mineralisation) indicated greater immobilisation of N in GCTB than in burnt soils. The effect of GCTB on soil properties increased with increasing cumulative returns of C and N. Approximately 10--20% of cumulative C returns were measured as soil organic C and 40--100% of cumulative N returns measured as total soil N. We combined measured rates of decomposition and accumulation of C and N with assumptions about the mineralisation of C and N in trash left from previous years, and calculated equilibrium C and N balances for two soils that differed in total C and N. These calculations suggested that GCTB could cause soil organic C to increase by 1.5-14% and soil total N to increase by 1.5-21% in the top 250 mm of these soils. The increases in total C and N and time to equilibrium were highly dependent on the rate of trash decomposition.