Saturday, 15 July 2006

Drying and Rewetting Cycles and Phosphorus Dynamics.

Clayton R. Butterly1, Petra Marschner1, Ann McNeill1, and Jeff Baldock2. (1) The Univ of Adelaide, Waite Campus, PMB 1 Glen Osmond, Adelaide 5064, Australia, (2) CSIRO Land and Water, PMB 2 Glen Osmond SA, Adelaide, 5064, Australia

A significant proportion (up to 60%) of Phosphorus (P) within Australian soils is in organic form. Despite this, dynamics of soil organic P (Po) remains poorly understood. Considering that a large proportion of total soil P and fertiliser P may be unavailable to crops and pastures, a better understanding of Po mineralisation processes and the effects of environmental parameters and management practices will contribute to a better understanding of the potential plant-available P supply capacity of soils. Soils within southern Australia are frequently subjected to drying and rewetting cycles, which are known to result in changes in nutrient availability. The ‘flush' of available nutrients following rewetting of a dry soil is thought to occur from either the release of contents from cells killed by osmotic shock, export of osmo-regulatory substrates from living cells and/or the release of occluded organic matter from breakdown of soil aggregates. These processes release organic substrate that is subsequently mineralised by the microbial biomass resulting in an increase in available inorganic nutrients. The aim of this project is to assess the effects of Drying and Rewetting (DRW) on P cycling. In particular, to quantify the P flush that occurs during DRW in agricultural soils of southern Australia, to understand the mechanisms involved (role of microbial biomass, influence of repeat DRW cycles and interaction of C, N and P) and to quantify how this process influences plant available P. The current study was designed to screen a range of soils, representing common soil types across southern Australia, for flushes in respiration activity and changes in available nutrients (P, C and N). Since the flush will be a function of soil properties such as texture and organic carbon content it is anticipated that this study will identify which of these parameters are important in predicting occurrence and size of flushes following DRW. Results indicate that the peak of respiration occurred after 6h in all soils. However both the magnitude of this peak and the rate of decline after 6h differed. The effect of DRW on changes in total P, inorganic P, nitrogen and dissolved organic carbon will be presented.

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