Nitrate reduction in agricultural acid sulfate soil

Abstract

A significant proportion of Australia's sugarcane crop is grown on east-coast estuarine floodplains underlain by pyritic gel-clay subsoils. At the current study site these agricultural acid sulfate soils are typically characterised by a topsoil horizon of river alluvium, a subsoil of oxidised actual acid sulfate soil (AASS), a zone of partially oxidised AASS and a deep sulfidic horizon of pyritic potential acid sulfate soil (PASS). Addition of nitrogenous fertiliser at key points in the sugarcane cropping cycle can create soil nitrogen levels in excess of immediate soil flora/fauna and crop requirements. In high rainfall tropical and sub-tropical regions conditions are thus suitable for nitrate, a strong oxidising agent, to leach down to the sulfidic soil layers with the consequent risk of pyrite oxidation. Little information is available on the fate of nitrogenous fertilisers in these pyritic subsoils. The purpose of this field and laboratory study was to evaluate the potential for nitrate reduction to occur in the presence of pyrite in sugarcane soils in the Tweed River valley, northern NSW, Australia. The study focus was on examining the soil profile hydrology including leaching mechanisms and nitrate concentrations down the profile to the AASS/PASS interface, as well as evaluating the potential for nitrate to increase the rate of pyrite oxidation in this generally anoxic soil zone.

Following an investigative nitrogen field trial to gather initial data, a second replicated urea fertiliser treatment trial with a nil-treatment control plot and three nitrogen (N) treatments was set up on a plant-cane-block in collaboration with a Tweed region cane grower, Robert Quirk. Installed loggers recorded rainfall, air and soil temperature, soil moisture and watertable data. Separate surveys and analytical work characterised selected soil physical, morphological and geochemical aspects. Soil profile sampling on four occasions over the twelve month crop cycle was analysed for N-species, NH{u2084}{u207A} and N0{u2083}{u207B}.

Hydraulic data analysis showed the watertable generally varying between 0.2 and 1.4 m below ground level with observed strong and rapid responses to rainfall events greater than approximately 15 mm per day. This and associated data supports the postulate that soil nitrate could move down the profile under even moderate precipitation events in these soils. Temperature, pH, redox potential and biological substrate soil data demonstrated the biogeochemical suitability of these subsoil zones to support nitrate reduction. Soil-N analysis revealed significant differences between N-trial treatments using urea fertiliser and also significant nitrogen transformation and movement within the soil profile.

Over a period of weeks, the urea fertiliser was rapidly transformed and appeared in the upper profile as elevated levels of ammonium and nitrate ions. The initial high ammonium levels quickly declined to be replaced almost completely by nitrate in the upper layers of the cane soil. Subsequently, increasing soil nitrate concentrations were evident deeper in the soil profile on higher nitrogen treatment plots during the middle phase of the crop cycle. In no instances were significant levels of nitrate detected below the soil redoxcline (the oxic-anoxic boundary) at around 1.0 m depth, nor was nitrate pooling evident anywhere in the AASS transition zone. Laboratory experimental work was undertaken to evaluate nitrate reduction coupled with pyrite oxidation under the biogeochemical conditions existing in the AASS transition zone. Results indicated that nitrate reduction associated with pyrite oxidation does take place in pyritic gel clay from the field site.