Management practices for control of runoff losses from cotton furrows under storm rainfall. III. Cover and wheel traffic effects on nutrients (N and P) in runoff from a black Vertosol.(Report)

Introduction

Cotton (Gossypium hirsutum L.) was grown on some 550 000 ha in inland New South Wales and Queensland in 1998-99, with 82% of the area irrigated, predominantly by flood irrigation in furrows (Cotton Yearbook 1999). Given the high levels of fertiliser use in cotton production, there is the potential for considerable transport of nutrients in runoff, as found by Rummenie anal Noble (1996) and Waters and Noble (1999) in runoff on cotton farms (furrows and taildrains) and in main drains in the Emerald Irrigation Area (EIA). Median concentrations of total nitrogen (N) and phosphorus (P) in EIA drainage water reported by these studies were well above environmental guidelines (QEPA 2006). The effects continued to be evident downstream in the Nogoa River at Duckponds after EIA drainage enters the Nogoa, despite considerable dilution with better quality water released from Fairbairn Dam. Median total N and P concentrations at Duckponds reported by DNR (1997) exceeded respective guideline levels of 0.5 and 0.05 mg/L now recommended for protection of aquatic ecosystems in this region (QEPA 2006).

The cotton industry has recognised the need for improved farm management and environmental performance and is encouraging growers to adopt 'best management practices' (BMPs), including strategies to enhance downstream water quality through better water, fertiliser, and pesticide use and improved management of erosion and runoff (Productivity Commission 2003). A BMP program (Williams and Williams 2000) including a land and water module has been implemented across the cotton industry since the rime of this study (1994) and of the studies cited above. It is important to note that most cotton growers/irrigators already retain all irrigation tailwater for re-use and capture some proportion of rainfall runoff.

Concerns about management of pesticides in the cotton industry and possible impacts on the riverine environment have recently been investigated (Schofield and Edge 1998). Large reductions in pesticide use have resulted from the use of genetically modified cotton, advances in integrated pest management, and implementation of BMPs. However, there has been less research in Australia into runoff transport of nutrients from cotton and other crops or into suitable management options for nutrients. Management options include: (i) controlling nutrient levels in the soil surface (Baker and Laflen 1983) through fertiliser amount, placement, and timing (to just meet crop requirements); (ii) soil and water management within the field; and (iii) managing runoff after it leaves the field. In developing within-field BMPs for control of nutrient runoff, many general principles of soil and water conservation developed in dryland cropping (Freebaim et al. 1996) should be applicable. One of the most effective and robust of these management principles is retention of crop residues to provide surface cover. The benefits of cover in reducing soil erosion from cropping land have been clearly demonstrated in a variety of farming systems in Australia (Freebaim et al. 1996), including furrow-irrigated cotton (Silburn et al. 1998; Waters et al. 1999; Waters 2001; Silburn and Glanville 2002). However, few studies have measured effects of surface cover, or other management practices, on nutrient transport in runoff from cropping lands in Australia. Two Australian studies by Rose and Dalal (1988) and Palis et al. (1990) estimated and measured, respectively, significant reductions in runoff losses of total N with cover. As total N is mainly organic N, which is often strongly sorbed to soil particles (Walter et al. 1979), both results reflect the strong effect of cover on erosion.

In contrast, other factors may determine the effectiveness of conservation practices in reducing losses of more soluble chemicals such as nitrate-N (N[O.sub.3]-N). These include the extent to which the practice reduces the amount of runoff, the dominant mechanism for reducing transport of soluble nutrients. Conservation practices that retain surface cover generally have more effect on sediment losses than on the amount of runoff (Freebairn et al. 1996). Conservation farming practices that retain surface cover usually significantly reduce both concentrations and total losses of sediment-associated chemicals (McDowell and McGregor 1984; Chichester and Richardson 1992; Silburn et al. 2002) due to their effectiveness in controlling sediment movement. However, for nutrients transported in runoff in the dissolved phase such as N[O.sub.3]-N, ammonium-N (N[H.sub.4]-N), and filterable reactive phosphorus (FRP), a review of studies in the United States (Baker and Laflen 1983) found many examples where conservation tillage systems gave significantly higher concentrations and losses than tilled systems. Baker and Laflen (1983) stressed the importance of fertiliser placement and point out that conservation tillage, particularly no-till, can have negative consequences for water quality if it involves surface (non-incorporated) fertiliser application. Our review of studies of effects of conservation tillage on dissolved nutrients in runoff found that many (but not all) studies where dissolved N and P in runoff increased with increased cover had involved surface-applied fertiliser which was then incorporated, with less incorporation for higher cover treatments. Where fertiliser was subsurface-applied for all treatments, concentrations of dissolved N and P in runoff were often similar, or decreased, with increasing cover (e.g. Mueller et al. 1984; Andraski et al. 1985; Chichester and Richardson 1992).

Losses of dissolved N and P in runoff are often a small proportion of total losses from erosive situations such as cropping (Burwell et al. 1975; Lal 1976; Barisas et al. 1978; Johnson et al. 1979; Baker and Laflen 1983; McDowell and McGregor 1984), being outweighed by losses associated with sediment. Runoff losses of dissolved nutrient forms are limited because, firstly, they are commonly a small proportion of the total N or P in the soil (Rose and Dalal 1988), and secondly, dissolved chemicals are often leached below the soil surface before runoff begins. Concentrations of solutes in runoff can be 1-2 orders of magnitude lower where leaching is unrestricted compared with restricted leaching (Ahuja and Lehman 1983). Even so, dissolved N and P may be of concern, as they are more readily bio-available forms of nutrients (and so may pose a greater risk to aquatic systems, at least in the short-term); they are also less likely to settle out than are nutrients attached to sediment. Dissolved concentrations and losses can be considerably greater where interflow or subsurface lateral flow contributes to runoff (Burwell et al. 1977; Baker and Laflen 1983; Cox and Pitman 2001). Thus, the main issue with dissolved nutrients is avoiding these particular situations.

Controlled traffic is a practice widely used in the Australian cotton industry to minimise compaction and soil structural degradation. In a row crop such as cotton, controlled traffic involves using selected inter-rows (furrows) for all wheel traffic and avoiding traffic on others. Trafficked soil is more compacted and gives more runoff and soil loss than non-trafficked soil (Tullberg et al. 2001; Silburn and Glanville 2002) and greater runoff losses of pesticides (Baker and Laflen 1979; Silburn et al. 2002). The behaviour of pesticides gives some indication of potential behaviour of nutrients. Hulugalle et al. (2002) found that 2-m beds resulted in lower runoff and soil erosion than 1-m beds. Two-metre beds reduce the proportion of area that is compacted. However, there have been few studies of the effects of traffic practices on runoff of nutrients.