MR CRAIG SCANLAN Profile Craig’s fascination with dirt (and getting dirty) began as a child spending while spending holidays on family’s farm in the south-west – and is still going strong. He graduated with a Bachelor of Agribusiness with Honours from Curtin University of Technology 2001. Following graduation, Craig worked as a Research Officer for the Department of Agriculture and Food (DAFWA) in Northam for three and a half years. Biological plow or plug ? How crop roots modify water flow through soils Craig Scanlan Christoph Hinz Overview ¾ Introduction – soil-plant hydrological feedback ¾ Field observations ¾ How do roots modify soil hydraulic properties ? ¾ Experimental work ¾ Root geometry ¾ Conceptual model ¾ Current work – completing the loop Soil-plant hydrological feedback Plant available soil water Plant growth Mechanisms of Root-Induced Changes to SHP ¾ Blocking pore space ¾ Leaving macropores Decaying lucerne following decay taproots ¾ Wetting and drying cycles ¾ Root exudates Increase or decrease aggregation Support bacteria Change surface tension ¾ Fungi associated with roots Figure from McCallum et al., 2004, Aust J Exp Agric, 44:299-307 Field Observations: Lucerne ¾ Increases in infiltration rate over 2 to 3 years ¾ Attributed to decrease in plants per m2 and decay of taproots ¾ Decayed taproots form channels up to 8 mm in diameter Data from: 1 Meek et al., 1992, Soil Sci Soc Am J, 56:908-13 2 Meek et al., 1989, Soil Sci Soc Am J, 53:238-41 Field Observations: Wheat ¾ Decreasing trend in hydraulic conductivity from sowing to tillering Roots filling pore space Harvest ¾ Increasing trend from tillering to harvest Creation of macropores by decaying roots Flowering Tillering Emergence Murphy et al., 1993, Aust J Soil Res, 31:171-97 Knowledge gaps ¾ No quantitative relationship between changes to soil hydraulic properties and root properties ¾ No quantitative analysis of how root-induced changes to soil hydraulic properties affect plant growth Objectives ¾ Quantify the relationship between root induced changes to SHP and root properties Glasshouse / lab experiment ¾ Quantify how root geometry is affected by plant type, soil etc. ¾ Develop a model based upon physical principles that quantifies how roots will modify SHP ¾ Quantify the effects of root-induced changes to SHP on crop growth Experiment: Outline ¾ Aim Determine changes in SHP and root length for lifecycle of wheat ¾ Methodology Plants grown in PVC columns, 45 cm tall, 10 cm diameter in yellow sand Determine SHP functions • Collect data on soil-water behaviour using multistep outflow experiment • Inverse modelling of outflow data Experiment: Methodology Experiment: Results * Error bars are standard error Experiment: Conclusions ¾ Root induced changes to SHP relate to maturity stage of wheat ¾ Increase in Ks most likely due to decay of seminal roots ¾ Greatest effect on saturated hydraulic conductivity ¾ Near-saturated hydraulic conductivity followed same trend as saturated. Root geometry: Outline ¾ Knowledge of root diameter frequency distribution essential to quantify how roots modify pores ¾ Frequency distributions often reported Are there relationships between plant type and attributes of root diam. frequency distributions ? Root Geometry: Results Cumulative frequency of root radii ⎛ ln (r ) − μ ⎞ F (r ) = 0.5 + 0.5erf ⎜ ⎟ ⎝ σ 2 ⎠ Data from: 1 Pierret et al., 2005, New Phyt, 166:967-80 2 Qin et al.m, 2004, Agr J, 96:1523:30 Conceptual model Modified capillary bundle model ¾ Model physical effects only ¾ Neglect Microbial activity Drying Surface tension ¾ Necessary Components How does the presence of a root alter • Capillary rise • Flux • Volume Which pores are affected by roots Combined properties of soil with and without roots Pores with roots present Pore boundary Capillary Rise Root boundary φ φ 2γ cos φ hc = ρgr2 ha hc Free water surface r1 r2 2γ cos φ ha = ρg (r2 − r1 ) Flux Pore boundary Root boundary Poiseuille's law r1 l r2 Δp 2 r2 qc = 8μl ( ) r −r ⎤ Δp ⎡ 2 2 qa = ⎢r1 + r2 − ⎥ 8μl ⎣ ln(r2 / r1 ) ⎦ 2 2 Cutlip and Shacham (1999) 2 1 Capillary Rise r1 β = r2 Effect of root : pore radius on capillary rise in concentric cylinders Add root 1 βh = 1− β Flux Effect of root : pore radius on flux in concentric cylinders 1− β 2 βq = 1+ β − ln⎛⎜ 1 ⎞⎟ ⎝ β⎠ 2 Add root Model Assumptions ¾ Soil is deformable ¾ Occupied pores adjust radius to root radius and and β ¾ Pore size distribution calculated from VG Se function Parameters from Carsel and Parish (1988) ¾ Volume of each pore class determines probability of occupation by roots ¾ Roots have random direction Saturated hydraulic conductivity Water Retention RLD = ↑ Water content β = 0.75, coarse roots Connectivity c= τ root-affected soil τ unaffected soil c= c=100 c=10 c=1 Simulation Modeling ¾ Aim Identify growing conditions where a positive or negative feedback develops in wheat crops ¾ Method Combine crop growth and water flow model Build in root-induced changes to SHP Scope • Wheat • 4 soil types: sand, loam, clay, duplex 3 rainfall environments: high, medium, low Simulation Modeling: Expected Outcomes ¾ Identify soil type – rainfall combinations where feedback is negative or positive ¾ Identify situations where crop management can be used to shift from a negative to positive feedback ¾ Identify situations where a negative feedback is inevitable Summary ¾ Plow or plug ? Both. ¾ Conceptual model shows this also Dependent upon ratio of root radius to pore radius ¾ Identified processes Pore blocking / creation of macropores most influential processes ¾ Quantified effect of crop roots on soil hydraulic conductivity and water retention Experiment Conceptual model Simulation modeling in progress….. Acknowledgements ¾ Grains Research and Development Corporation ¾ Department of Agriculture and Food WA
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