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Huth N.I.,CSIRO | Huth N.I.,Agricultural Production Systems Research Unit | Thorburn P.J.,CSIRO | Radford B.J.,LMB | Thornton C.M.,LMB
Agriculture, Ecosystems and Environment | Year: 2010

There is increasing focus on greenhouse gas emissions from agricultural systems. One suggested method for increasing the sequestration of carbon (C) within agricultural soils is to increase crop productivity and therefore C input into the soil. However, if enhanced production is achieved via nitrogenous fertilisers, there is a potential tradeoff between decreased C emissions and increased nitrous oxide (N2O) emissions due to the increased soil C and nitrogen (N). An alternative is to incorporate leguminous crops into cereal cropping rotations to provide a biological source of N. However, the likely production of N2O from N released during the decomposition of leguminous residues is unknown as is the impact on C input into the soil when some cereal crops are replaced with grain legumes. Consequently, an analysis of the likely impacts has been undertaken for a subtropical dryland cropping system in Queensland, Australia where soil, climate and management are conducive to denitrification losses. A series of scenarios embracing a range of cropping rotations, N fertilisers and leguminous crops was tested using the Agricultural Production Systems Simulator (APSIM). The model configuration was tested using long term data from the Brigalow Catchment Study site near Theodore, Queensland, Australia (24.81°S, 149.80°E). A wide range of data was used in testing the model for the major terms in the C, N and water balances. Scenario analyses of alternative management systems including the use of fertiliser or legume grain or forage crops within cereal rotations demonstrated that soil C can be managed to some degree via simple changes in agronomic practice. The use of legumes within cereal rotations was not always as effective in reducing N2O emissions as improved fertiliser practice. For example, replacing wheat with chickpea did not reduce N2O emission relative to fertilised systems and did not assist in increasing soil C due to impacts on stubble cover over the important summer months. The fact that some interventions proved counterproductive due to complex feedback mechanisms highlights the need for detailed models which capture the links between water, C, N and management. Crown Copyright © 2010. Source


Silburn D.M.,Agricultural Production Systems Research Unit | Silburn D.M.,University of Canberra
Soil Research | Year: 2011

The use of simple models of soil erosion which represent the main effects of management in grazing lands in northern Australia is limited by a lack of measured parameter values. In particular, parameters are needed for erosion models (sediment concentration v. cover equations) used in daily soil-water balance models. For this research, we specifically avoided equations that use rainfall and runoff rates (e.g. peak flow), as current daily models are limited in their ability to estimate these rates. The resulting models will therefore give poor estimates of soil losses for individual events, but should give good estimates of long-term average erosion and management influences. Runoff and erosion data were available for 7 years on 12 hillslope plots with cover of 10-80%, with and without grazing, with and without tree canopy cover, on a variety of soils according to various soil classification systems. Soils were grouped into those derived from sandstone (SS), mudstone (MS), and eroded mudstone (MSe). These data were used to determine two parameters, i.e. (i) efficiency of entrainment for bare soil and (ii) a cover factor, for simple models of bedload and suspended sediment concentrations. Methods used to fit parameters affected the results; optimising to obtain the minimum sum of squares of errors in soil losses gave better results than fitting an exponential equation to sediment concentration-cover data. The use of a linear slope factor in the sediment concentration models was confirmed with data from plots with slopes 4-8%. Parameters for the bedload sediment concentration model were the same for SS, MS, and MSe soils. Parameters for the suspended sediment concentration model were the same for SS and MS soils, but the MSe soil had a greater efficiency of entrainment for bare soil (about double). The sediment concentration-cover relationships and fitted cover factors were different for suspended and bedload sediment. Thus, the resulting modelled proportion of sediment as suspended load changed with cover, from ~0.3 for bare soil to 0.9 at 80% cover, mimicking the measured data. The cover factor was lower than published values for cultivated soils, indicating less reduction in sediment concentration with greater cover. A compilation of parameter values for the sediment concentration model from published and unpublished sources in grazing and cropping lands is provided. © CSIRO 2011. Source


Silburn D.M.,Agricultural Production Systems Research Unit | Silburn D.M.,University of Canberra
Soil Research | Year: 2011

Measured Universal Soil Loss Equation (USLE) soil erodibility (K) values are not available for soils in grazing lands in northern Australia. The K values extrapolated from croplands are used in national and river-basin scale assessments of hillslope erosion, using an assumption that the cover factor (C) equals 0.45 for undisturbed (uncultivated) bare soil. Thus, the K needed for input into the models is the measured K for undisturbed soil (KU) divided by 0.45. Runoff and erosion data were available for 7 years on 12 hillslope plots with cover of 10-80%, with and without grazing, with and without tree canopy cover, on a variety of soils according to various soil classification systems. Soils were grouped into those derived from sandstone (SS), mudstone (MS), and eroded mudstone (MSe). These data were used to determine USLE KU, K, and C factor-cover relationships. Methods used to fit the parameters affected the results; minimising the sum of squares of errors in soil losses gave better results than fitting an exponential equation. The USLE LS (length-slope) factor explained the increase in measured average annual soil loss with slope, for plots with low cover. Erodibility (K) was 0.042 for SS and MS soils, irrespective of Australian Soil Classification (Chromosol, Kandosol, Rudosol, Sodosol, Tenosol); K was 0.062 for exposed, decomposing mudstone (MSe Leptic Rudosol). The measured K factor for SS and MS soils was close to that used in catchment-wide soil loss estimation for the site (0.039). This indicates that the method used for estimating K from soil properties (derived from cultivated soils) gave a reasonable estimate of K for the main duplex soils at the study site, as long as the correction for undisturbed soil is used in deriving K from measured data and in applying the USLE model. A 20% increase in K (0.050) for SS and MS soils may be warranted for heavy grazing by cattle. The C factor-cover relationship was different from the standard revised USLE (RUSLE) relationship, requiring a greater exponent ('bcov') of 0.075, rather than the default for cropland of 0.035. Increasing cover is therefore more effective at the site than suggested by the USLE. Parameters of USLE were also derived for bedload, allowing suspended load to be calculated by subtracting bedload from total soil loss. © CSIRO 2011. Source


Tolmie P.E.,Agricultural Production Systems Research Unit | Silburn D.M.,Agricultural Production Systems Research Unit | Silburn D.M.,ater Cooperative Research Center | Biggs A.J.W.,University of Queensland
Soil Research | Year: 2011

Increases in deep drainage below the root-zone can lead to secondary salinity. Few data were available for drainage under dryland cropping and pastures in the Queensland MurrayDarling Basin (QMDB) before this study. Modelled estimates were available; however, without measured drainage these could not be validated. Soil chloride (Cl) mass-balance was used to provide an extensive survey of deep drainage. The method is 'backward-looking' and can detect low rates of drainage over longer times. Soil Cl and other soil properties were collated for a number of soils, mostly Vertosols and Sodosols, for paired native vegetation, cropped and sometimes pasture sites, from historical data and new soil sampling. Large amounts of salt and Cl had accumulated under native vegetation (Cl mean 25t/ha, range 654, in 2.4m depth), due to low rates of drainage. Steady-state Cl balances for native vegetation gave average drainage of 1.2mm/year at wetter, eastern sites and 0.3mm/year for Sodosols and Grey Vertosols in drier, western areas. Chloride profiles were mostly of a shape indicating matrix/piston flow. One site (Hermitage fallow trial) appeared to be affected by diffusion of Cl to a watertable. The Cl profiles from 14 longer term cropping sites (18-70 years), mainly used for winter cropping/summer fallow, indicate: (i) large losses of Cl since clearing (mean 50%, range 13-85% for 01.5m soil); and (ii) drainage rates from transient Cl balance are a relatively low percentage of rainfall but are considerably higher than under native vegetation. Drainage averaged 8mm/year and ranged from 2 to 18mm/year. This variation is partly explained by rainfall (R 2 = 0.63) (500-730mm/year) and soil plant-available water capacity (R 2 = 0.77) (80-300mm). Deep drainage increases with increasing rainfall and with decreasing available water capacity. Drainage under pasture was less than under cropping but greater than under native vegetation. The deep drainage water (leachate) was of poor quality and will increase salinity if added to good quality groundwater. Leachate at nine sites was too saline to be used (undiluted) for irrigation (>2500mgCl/L) and was marginal at the remainder of sites (∼800mg Cl/L). Cropping areas in the QMDB have the precursors for secondary salinity developmenthigh salt loads and an increase in drainage after clearing. The Vertosols and Sodosols studied occur in 90% of croplands in the QMDB. Salinisation will depend on the properties of the underlying regolith and groundwater systems. © CSIRO 2011. Source


Silburn D.M.,Agricultural Production Systems Research Unit | Tolmie P.E.,Agricultural Production Systems Research Unit | Biggs A.J.W.,University of Queensland | Whish J.P.M.,CSIRO | French V.,Australian Department of Primary Industries and Fisheries
Soil Research | Year: 2011

Changes in land use can affect the soil water balance and mobilise primary salinity. This paper examines changes in soil chloride (Cl) and deep drainage under pasture and annual cropping on five gilgaied Grey Vertosols in southern inland Queensland, Australia, comparing them to remnant native vegetation. Transient soil Cl mass-balance (CMB) was used for crop and pasture sites, as it is suitable for determining the long-term, low rates of drainage since clearing some 40-50 years ago. Steady-state CMB was used for native vegetation. Large masses of salts and Cl were stored under native vegetation (31-103t/ha of Cl to 3.2m), and deep drainage was low (0.10-0.27mm/year). The Cl profiles were generally of a normal shape for matrix flow (e.g. no bypass flow). Soil Cl was lost under cropping (average 65% lost to 1.4m) and pasture (32%) compared with native vegetation. This lost Cl was not stored within the top 4-5m of soil, indicating movement of water below 4-5m. Deep drainage averaged 10mm/year under cropping for both gilgai mounds and depressions (range 2.7-25mm/year), and 3.3 and 5.1mm/year under pasture for mounds and depressions, respectively. Subsoil (depth 1.5-4+m) was generally dry under native vegetation and wetter under cropping and pasture. Deep drainage over the last 40-50 years was stored in the unsaturated zone (to deeper than 4+m), indicating a long time lag between land-use change and groundwater response. Steady-state CMB greatly underestimated drainage for crop and pasture sites. © CSIRO 2011. Source

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