Relationship between soil structure and runoff/soil loss after 24 years of conservation tillage
Introduction
For centuries tillage has been used to prepare suitable seedbeds to grow crops. However, numerous workers have demonstrated that conventional system of excessive tillage was seriously reducing soil organic matter and degrading soil structure, and as a result, accelerating soil erosion (Angers and Mehuys, 1989, Tisdall and Oades, 1982, Elliott, 1986, Kay, 1990, Papendick and Parr, 1997). Soil erosion caused by water is primarily due to particle detachment and transport by rainfall and runoff (Ellison, 1947). The stability of aggregates at the immediate soil surface greatly influences the soil susceptibility to erosion. Raindrop impact on freshly tilled soil causes breakdown of surface aggregates. This breakdown affects infiltration, surface sealing (Loch and Foley, 1994), and soil detachment (Reichert and Norton, 1994).
Results of some studies concerned with erosion control indicated that no-tillage systems were an effective way of controlling erosion (Uri et al., 1999). Therefore, conservation tillage systems, rather than plow-based methods of seedbed preparation, have the potential to provide more sustainable use of soil resources. There have been numerous reports of reduced runoff and soil loss under conservation tillage (e.g. Packer et al., 1992, Bradford and Huang, 1994). However, many of these changes were found within a short time after conversion to conservation tillage and therefore were simply due to the temporary protective effect of residue cover (Freebairn and Wockner, 1986). There is much less information on the effect of long-term changes in soil structure on runoff and soil erosion hazards. There is evidence, particularly in low rainfall areas which suggests that while significant decreases in runoff was detected under reduced tilled and no-till after a season, significant changes in bulk density, organic carbon and water stable aggregation were not detected even after 8 years (Packer et al., 1992).
The application of conservation tillage, in particular direct drill (no-tillage) system has been reported to lead to higher soil organic matter level and higher aggregate stability than that of the conventional tillage (Kern and Johnson, 1993, Carter and Rennie, 1982, Bear et al., 1994, Lal et al., 1994, Hamblin, 1980). However, there is less information on the extent that changes in soil physical properties (porosity, hydraulic conductivity), brought about by different tillage systems can affect permanent change in soil infiltration and erosion susceptibility.
The objective of this research was to characterize the impacts of two different tillage systems (conservation tillage versus conventional tillage) on surface soil structure and structural stability of an Oxic Paleustalf in Wagga Wagga, Australia and to test the validity of relationships between soil surface structure and runoff/soil erosion.
Section snippets
Experimental design
The long-term rotation/tillage experiment began in 1979 at the Wagga Wagga Agricultural Institute, Wagga Wagga (35°05′S, 147°20′E), NSW, Australia. In this study, we evaluated two tillage/stubble treatments under a wheat (Triticum aestivum)/lupin (Lupinus angustifolus) rotation, namely:
- 1.
direct drilled and stubble retained—DD/SR,
- 2.
conventional cultivation and stubble burnt—CC/SB.
For the tillage treatments, DD refers to no cultivation prior to sowing and CC to three cultivations to 0.1 m depth with a
Soil organic carbon
The total organic carbon content of 0–5 cm layer was significantly higher in DD/SR than in the CC/SB (Table 1). DD/SR soil had 69% more TOC than the CC/SB. The higher organic carbon in the DD/SR treatment was probably caused by reduced rate of oxidation of organic matter and the absence of soil redistribution as well as the increased carbon input due to stubble retention under direct drilling. Chan et al. (2002) reported that tillage had a much greater effect in reducing total soil organic
Conclusions
After 24 years of different tillage and stubble management, significant differences in topsoil structure and structural stability were detectable in the 0–5 cm depth. Avoiding soil disturbance (no-tillage) and retaining residues resulted in increasing the organic carbon level and aggregate stability. The higher aggregate stability was associated with lower bulk density and higher macroporosity (pores of diameter >60 μm) in the 0–5 cm soil layer of the direct drill and stubble retained system.
Acknowledgement
We thank the ATSE Crawford Fund, Australia for financial support.
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