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• W622078260 abstract "By definition, there is little or no pasture production benefit of plant available soil phosphorus (P)concentrations above agronomic optimum. Soil P concentrations above the agronomic optimum can result inunnecessarily elevated P concentrations in runoff that can adversely impact on water bodies. I hypothesisedthat a reduction in these excessive P concentrations in pasture soils may be achieved through applyingfertiliser P at rates that are less than those required to maintain soil P concentrations. This thesis consideredthis hypothesis by utilising six established field sites which represented the broad range of soils used forpasture production in south eastern Australia (Burkitt et al., 2001; Burkitt et al., 2006). The soils at the sitesencompassed a wide range of P buffering indices (PBI) (from 6–519) and textures (sand to clay loam). At eachsite a replicated trial consisting of four initial soil P (Pinit) concentrations combined with four on-going Pfertiliser rates (Pfert) had been previously established, and soil samples taken from these trials formed the basisfor much of the work presented in this thesis.The first experimental chapter (Chapter 2) reports on the changes in extractable P of these field soilswhich were sampled (0-10 cm) once every six months (biannually) for up to four and a half years. Phosphorusextractable in calcium chloride (CaCl2-P) was monitored using annual samples of the two lowest and the highestPinit concentrations receiving the two lowest and the highest on-going Pfert rates. Olsen- and Colwell-extractableP concentrations were monitored biannually for all sixteen treatments. Excluding a soil with an extremely lowPBI where soil P concentrations could not be increased, the study revealed decreases to be larger the greaterthe Pinit concentration, and the smaller the on-going rate of Pfert. The influence of Pfert on decreases in extractablesoil P was not as large as that of Pinit concentration. The relative decrease in the more readily available CaCl2-P (-57%) was greater than relative decreases in the agronomic measures, Olsen-P (-25%) and Colwell-P (-12%).When Pfert was withheld, soil P concentrations initially well above agronomic optimum remained above this level.This study advances the knowledge of P decline characteristics and will aid land managers in understandinglikely changes in soil P concentrations when P fertiliser inputs are reduced.The second experimental chapter (Chapter 3) reports the development of a model to assist policymakers and land managers in setting realistic timeframes to return soils with excessive P to agronomic and/orenvironmental optimum. The model utilised the original field study’s (Chapter 2) CaCl2-P, Olsen-P and Colwell-P dataset. The model assumed and confirmed the suitability of an exponential decay function and predictedthe final P concentration for CaCl2-P, Olsen-P or Colwell-P as: Final P concentration = (previously measured Pconcentration + e x P fertiliser applied) exp (-d x years since P applied); where e is the increase in soil P for eachunit of applied P and d is the decay constant representing how quickly the soil P moved from the labile tounavailable P pools. Using parameters derived across all the soils, where P exports ranged from 2.9 and 12.3 kgP/ha.yr, the model predicts it would take approximately 14 years (ranging from 11 to 20 years) for Olsen-Pconcentrations of between 34 and 44 mg/kg to decrease to an agronomic optimum of 17 mg/kg. An initialOlsen-P concentration of between 55 and 96 mg/kg would take approximately 32 years (ranging from 26 to 49years). Using soil specific parameters, the model also identified that some soils, i.e. those with high PBI, could be maintained at agronomic optimum P concentrations without the risk of environmental loss being above thespecified CaCl2-P threshold of 0.25 mg/kg (0.05 mg/L). In contrast, soils with low P sorption capacity exceededthe environmental threshold even when soil P concentrations were below agronomic optimum i.e. Olsen-P 14-17 mg/kg. Despite soil P decreases varying according to soil, a soil term was not incorporated into the model asthere were only six soils studied. Further work incorporating more soils and various P exports is required toexamine the influence of soil properties such as P sorption, and P export, on decreases in soil P.Chapter 4 reports on an investigation of the changes in further soil P pools as extractable Pconcentrations of selected treatments of Chapter 2 decreased. These treatments included the two lowest andthe highest Pinit concentrations which received no on-going Pfert treatment. This research revealed that themajority of the P was not being exported or sorbed, but inorganic P (Pi) was being converted to organic P (Po).A longer period of monitoring is required to examine if the Po concentration will stabilise once a maximum Poconcentration is approached. Relationships between the various P measures allowed us to calculate a degreeof P sorption saturation (DPSS) for these soils. Degree of P sorption saturation was relatively low for the twolowest PBI soils when compared to higher PBI soils, despite CaCl2-P concentrations being some of the largestreported in the literature. Further correlation of DPSS with P losses from Australian soils is required as wesuggest that the thresholds will vary for soils with extreme P sorption capacities. Alternative methods ofcalculating DPSS, using Australian measures such as P sorbed after the addition of 1000 mg P/kg, as measuredas part of the PBI method, and Colwell-P, without the requirement of an alpha value, were proposed.Chapter 5 reports the findings of an incubation study and the longer-term effect of drying andrewetting soils. It was found that extremely low PBI soils with high organic matter (OM) contents may be athigh risk of P loss to the surrounding environment. Chapter 5 showed that drying and rewetting the soils withthe lowest PBI produced large increases in CaCl2-P and Olsen-P, hypothesised to be of microbial and OM origin.Thus, irrespective of fertiliser management, there are large risks of P loss from such soils, even when Olsen-Pconcentrations are below agronomic optimum. In comparison, P released from soils of higher PBI did not resultin large increases in CaCl2-P, which remained below the threshold of environmental concern, probably due tochemical sorption.The large potential for P loss from soils of extremely low PBI was further highlighted in the finalexperimental chapter (Chapter 6). Chapter 6 compared extractable soil P concentrations of the lowest andhighest PBI soils to a depth of 100 cm. It was revealed that soil with an extremely low PBI was susceptible tolarge P losses, with the majority of applied fertiliser P i.e. 81% (406 kg P/ha) not recovered in this deepsampling zone. Such a large potential loss of applied P brings into question whether such soils should be usedfor agricultural purposes, especially if soluble P fertiliser is applied, unless the soil’s ability to sorb P is firstlyincreased through the application of a suitable amendment.Until preferred soil P extractions and thresholds have been accepted for determining the risk of P lossaccording to soil P concentration and buffering capacity, the depletion of soil P concentrations of intensively grazed pastures to agronomic optimum must be encouraged. Only once agronomic optimum soil Pconcentrations are reached should the appropriate maintenance P fertiliser application, to account for soilsorption and P exported in produce, be supplied. However, the currently defined agronomic optimum i.e.Colwell-P of 23 mg/kg for PBI <15, may be too high for extremely low PBI soils. Phosphorus fertilisermanagement for low PBI soils requires more precision than higher PBI soils to limit environmental loss of P.Indeed, intensive grazing of extremely low PBI soils is questionable until their PBI’s are increased throughapplication of amendments." @default.
• W622078260 created "2016-06-24" @default.
• W622078260 creator A5078716876 @default.
• W622078260 date "2013-08-01" @default.
• W622078260 modified "2023-09-27" @default.
• W622078260 title "Managing phosphorus in intensive pasture soils to improve the long-term environmental sustainability of the Dairy Industry" @default.
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