Influence of changed management of sugarcane on some soil chemical properties in the humid wet tropics of north Queensland

Abstract

A study was undertaken to quantify changes in chemical attributes and cation exchange characteristics of soils associated with two forms of changed management in sugar production systems 6–9 years after their implementation. The two production systems included a long-term green cane trash blanketed (GCTB)/burn trial and a rotation experiment established to investigate crop management strategies to reduce the effects of yield decline in long-term sugarcane monocultures. The treatments investigated in the latter trial were (a) long-term continuous GCTB, (b) grass ley, and (c) a bare fallow treatment. Under both the GCTB and grass ley systems, pH significantly declined over 6 years with an associated increase in aluminium saturation and decline in base saturation of the cation exchange complex. Although the decline in base saturation under the GCTB treatment would effectively indicate a degree of degradation, this base diminution is easily reversed through conventional management strategies. Organic carbon levels increased by 4 t/ha under the GCTB treatments compared with the burnt over a 9-year period. In contrast, under a grass ley system, organic carbon increased by 9 t/ha over a 6-year period compared with continuous cane, clearly demonstrating the superior nature of grass leys in sequestering carbon in this tropical environment. The increased net proton addition in the GCTB system relative to the burnt system was estimated to be 0.71 kmol H⁺/ha.year, and the net proton additions for the continuous cane, grass ley, and fallow systems were 1.38, 3.81, and 0.87 kmol H⁺/ha.year, respectively. The cation exchange characteristics were derived from bulk samples collected from each of the trials. The divergence between the cation exchange curves due to treatment was greatest in the surface 10 cm, but this diminished with depth, indicating that the influence of trash retention or grass leys on soil charge characteristics was confined to the surface layer. The increases in cation exchange capacity (CEC) at pH 5.5 of the GCTB treatment compared with the burnt, and of the grass ley treatment compared with the continuous cane, were estimated to be 0.67 and 0.75 cmol_c/kg over the 0–10 cm depth interval. Plots of effective CEC v. CEC clearly indicated that 9–31% of the cations measured where not associated with the exchange complex and would therefore be subject to leaching loss. Organic C oxidisable by 33 mM KMnO₄ (C1) or 333 mM KMnO₄ (C3) was highly linearly correlated with total organic C across both sites and all treatments. This suggests that the proportions of different functional organic groups were not greatly affected by burning, trash retention, or pasture.