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.
Mycobiome of Cysts of the Soybean Cyst Nematode Under Long Term Crop Rotation