Nitrate accumulation under cropping in the Ferrosols of Far North Queensland wet tropics

Abstract

Recent research on the fate of applied fertiliser N in the Ferrosols of the wet tropics of Far North Queensland (FNQ) has shown that the nitrate leaching below the crop root-zone is a major pathway of N loss from paddocks. Information on the fate of this nitrate is essential to develop best N fertiliser management practices and for the long-term sustainability of land and water resources. Because of the ability of Ferrosols to adsorb anions in the soil matrix, it was speculated that the leached nitrate may be accumulating at depth in the Ferrosol profiles. The objectives of this study were to (i) verify whether the leached nitrate has been accumulating in the Ferrosols under the major cropping systems in the Johnstone River Catchment (JRC) of FNQ, and (ii) provide preliminary estimates for nitrate retention capacity of the Ferrosols. Soil cores to a depth of 10 m were taken from under sugarcane (Saccharum officinarum-S), banana (Musa (AAA group, Cavendish subgroup) cv. Williams), dairy pasture, and rainforest in JRC during August 1995. The cores were segmented at 0.5-m depth increments and soil samples were analysed for nitrate- and ammonium-N, cation- (CEC) and anion- (AEC) exchange capacities, pH, Ca²⁺ , Mg ²⁺ , K ⁺ , Na ⁺ , and Cl ^– . Nitrate-N concentration under sugarcane was as high as 33 mg/kg, compared with 6.9 mg/kg for banana, 0.3 mg/kg under rainforest, and that under pasture was below detection limit. Nitrate-N load in the top 10 m of the profiles under sugarcane ranged from 345 to 1875 kg nitrate-N/ha compared with 145 kg/ ha for banana, and 21 kg/ha under rainforest. Most of the nitrate accumulation was found between 2 and 8 m, i.e. well below the crop root-zone. From 7% to 70% of the nitrate that leached below crop root-zone was retained at depths >1 m. In general, Cl ^– and total cation (TC = sum of Ca²⁺ , Mg ²⁺ , K ⁺ , and Na ⁺ ) concentrations in the profiles under cropping were higher than those under rainforest, and the pH under sugarcane was more acidic. Simple correlation analysis indicated associations existed between the accumulated nitrate and Cl ^– , pH, AEC, or TC. The estimated nitrate holding capacity of the Ferrosols ranged from 17 to 32 t N/ha. The results show that large quantities of the nitrate that leached below crop root-zone have accumulated at depth under long-term sugarcane and banana cropping in the Ferrosols of FNQ.