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impacts of ASS (Fitzpatrick et al. 2008). Unless the negative impacts are successfully managed, soil characteristics, water quality and biodiversity, human health ...
Amelioration of sulfuric soil using organic matter with varying nitrogen content Patrick S. MichaelA,B,C, Rob ReidB,C, Rob Fitzpatrick B,C A

Department of Agriculture, PNG University of Technology, PMB, LAE, MP 411, Papua New Guinea, [email protected]. ac.pg School of Earth and Environmental Sciences, The University of Adelaide, SA 5005, Australia C Acid Sulfate Soil Centre (ASSC), CSIRO Land and Water, Urrbrae, South Australia B

Abstract This paper reports the findings of two studies conducted to investigate the effects on pH and Eh of acid sulfate soil following addition of organic carbon and nitrogen. The first study compared the responses to simple carbon sources (glucose, sodium acetate and molasses) with complex organic matter in the form of chopped Phragmites. The second experiment considered the effect of nitrogen by testing organic matter with varying N content. Changes in Eh and sulfate concentrations induced by these treatments mirrored the changes in pH. These experiments showed that organic carbon alone was ineffective in treating sulfur soil, and that nitrogen was needed. Lucerne hay which had the highest N content produced the largest increase in pH, and that the pH changes of pea straw and wheat straw were roughly proportional to their N content. It was proposed that the alkalinising effect of the treatments was mediated by anaerobic microbial metabolism which required sources of nitrogen as well as organic carbon. Introduction Acid sulfate soil (ASS) are naturally occurring soils or sediments formed under reducing and water-logged (anaerobic) conditions that either contain sulfuric acid or have the potential to form it, in an amount that can have serious negative impacts on the health of human and the environment (Ljung et al. 2009). When ASS materials are below the natural water table, no threat is posed unless the water table is lowered as a result of changes in land use or hydrological regimes (Reid and Butcher 2011), leading to the exposure and oxidation of sulfidic (potential ASS) materials containing sulfidic minerals (e.g. pyrite, FeS 2). Oxidation of sulfidic minerals results in production of sulfuric acid. Release of the sulfuric acid produced in turn leads to solubilisation of soil matrices in which potentially toxic constituents (metals and metalloids) are held. Production of sulfuric acid, mobilisation and accumulation of toxic soil constituents such as As and Al, coupled with deoxygenation, and production of monosulfidic black ooze are the major causes of the negative impacts of ASS (Fitzpatrick et al. 2008). Unless the negative impacts are successfully managed, soil characteristics, water quality and biodiversity, human health, commercial and recreational fisheries, engineered and community infrastructure, scenic amenity and tourism, and agricultural productivity are seriously affected. The seriousness of the impacts has made management of ASS materials an important issue, and several management strategies have been proposed, focusing on two key principles. The first principle is to neutralise the actual acidity and to manage the by-products of oxidation. The second is to curtail oxidation of sulfidic soil materials and prevent exposure. Among the strategies proposed and tested, application of an alkaline material, e.g. agricultural lime to neutralise sulfuric soil materials and, minimise disturbance of the potential ASS materials are the most preferred management strategies (Ljung et al. 2009). An alternative strategy that has received equal attention in managing acidified environments caused by acidic soil materials other than ASS (e.g. acid mine drainage) with varying success is the application of organic matter, preferably containing high carbon to create alkalinity using microbial processes. The principle is to create an anoxic environment by microbial depletion of oxygen, in order to stimulate the activity of sulfur reducing microbes to generate alkalinity to neutralise the soil (Frömmichen et al. 2004). This strategy has the potential to both ameliorate sulfuric soils and to prevent the oxidation of sulfidic soils by utilizing the biogenic alkalinity produced during microbial decomposition of the organic matter. In comparison to lime, which is expensive and may not be readily accessible in poor economies (Powell and Martens 2005), organic matter is relatively cheap and readily available. This strategy, however, is currently not widely used in ASS management, in part due to contradictory results on its efficacy. The purpose of the current study was to investigate the effects of organic matter with varying nitrogen content on the pH and Eh of sulfuric soil. Additionally, the effect of these treatments on sulfate content was measured. 1

Materials and Methods Soil Sulfidic soil collected from the Finniss River (35˚24’28.28’’S; 138˚49’54.37’’E) in South Australia under less than 1 m of water was oxidised to be used as sulfuric soil. The average pH in water (pHw 1:5 w/w) of the sulfidic soil was 6.7, with a field capacity of 49.2%. The average organic matter content estimated by weight loss-on-ignition (Schulte and Hopkins 1996) was 10.6%. More detailed geochemistry and metal concentrations of the collection site are given in (Fitzpatrick et al. 2008), the sites being identified as AA26.3 and FIN 26. Nitrogen and carbon sources Organic carbon was supplied either as simple compounds glucose, sodium acetate, and unsulfured molasses, or in complex form in ground (