Nitrate retention under sugarcane in wet tropical Queensland deep soil profiles

Abstract

Nitrate leaching below the crop root-zone in variable charge soils may be adsorbed at anion exchange sites, thereby temporarily reducing the risk of contamination of water bodies. The objectives of this study were (i) to investigate whether nitrate adsorption, accumulation, and retention in the Johnstone River Catchment of Far North Queensland wet tropics is widespread; (ii) to assess the capacity of soil in the Johnstone River Catchment to retain nitrate; and (iii) to deduce the consequences of nitrate adsorption/desorption on contamination of water bodies. Soil cores ranging from 8 to 12.5 m depth were taken from 28 sites across the catchment, representing 9 Ferrosol soil types under sugarcane (Saccharum officinarum-S) cultivation for at least 50 years and from rainforest. The cores were segmented at 0.5-m depth increments and subsamples were analysed for nitrate-N, cation and anion exchange capacities, pH, exchangeable cations (Ca, Mg, K, Na), soil organic C, electrical conductivity, sulfate-S, and chloride. Nitrate-N concentration under sugarcane ranged from 0 to 72.5 mg/kg, compared with 0 to 0.31 mg/kg under rainforest, both Pin Gin soils. The average N load in 1–12 m depth across 19 highly oxidic profiles of the Pin Gin soil series was 1550 kg/ha, compared with 185 kg/ha under 8 non-Pin Gin soils and 11 kg/ha in rainforest on a Pin Gin soil. Most of the nitrate retention was observed at depth of 2–12 m, particularly at 4–10 m, indicating that the accumulation was well below the crop root-zone. The average maximum potential nitrate retention capacity was 10.8 t/ha for the Pin Gin and 4.7 t/ha for the non-Pin Gin soil. Compared with the current N load, the soils still possess a large capacity to adsorb and retain nitrate in profiles. Retention of large quantities of the leached nitrate deep in most of the profiles has reduced the risk of contamination of water bodies. However, computations show that substantial quantities of the nitrate leached below the root-zone were not adsorbed and remain unaccounted for. This unaccounted nitrate might have entered both on- and off-site water bodies and/or have been denitrified.

The availability of nitrogen from sugarcane trash on contrasting soils in the wet tropics of North Queensland

“…While our results indicate that nitrate occupied a small proportion of anion exchange capacity (Fig. 6), substantial quantities of nitrate have accumulated under sugarcane at depths below 4 m in other Ferrosols in this region in contrast to native vegetation (Rasiah et al 2003).…”

Impact of pre-existing sulphate on retention of imported chloride and nitrate in variable charge soil profiles

“…The Ferrosols in the far-north Queensland's wet tropics possess the capacity (Rasiah et al, 2003a;Rasiah and Armour, 2001;Gillman and Abel, 1987) and have been found to retain large quantities of the nitrate leached below the root-zone (Rasiah et al, 2003a;Rasiah and Armour, 2001) However, the Ferrosols in this environment may contain considerable sulphate (Ayers and Manton, 1991;Douglas, 1968), which may limit the competitive adsorption of nitrate at anion exchange sites (Katou et al, 1996). Furthermore, the competitive anion adsorption at anion exchange sites is in the order sulphate>chlor-ide>nitrate (Katou et al, 1996).…”

“…The leached nitrate might be adsorbed at anion exchange sites (Rasiah et al, 2003a;Rasiah and Armour, 2001), denitrify, and/or enter streams (Rasiah et al, 2003b). The retention and accumulation of the leached nitrate in soil matrix in this region is considered as advantageous, because the nitrate leaching below the crop root-zone could otherwise find its way into streams (Rasiah et al, 2003) and finally to the Great Barrier Reef (GBR) lagoon.…”

The expected impact of climate change on nitrogen losses from wet tropical sugarcane production in the Great Barrier Reef region

“…Not all applied nitrogen is utilised by the sugarcane crop (Thorburn et al 2003b), and losses of nitrogen from sugarcane production systems have been detected in surface water (Reghenzani et al 1996;Bengtson et al 1998;Ng Kee Kwong et al 2002) and groundwater (Rasiah et al 2003a(Rasiah et al , 2003bThorburn et al 2003a;Stewart et al 2006).…”