Impacts of landscape position, seasonality and flooding on biogeochemical function of soils in a floodplain forest

Mark Farrell, Mark Thomas, Dave Huber, Steve Szarvas, Thomas Carter, Athina Puccini, Brendan Malone, Senani Karunaratne, Helen Glanville

Research output: Book/ReportOther bookpeer-review

Abstract

The Koondrook-Perricoota State Forest is the second largest river red gum forest in the world, covering a length of the northern bank of the River Murray in New South Wales of ~60 km, from Barham in the north west to near Moama in the south east. It is a floodplain forest, and has evolved in the presence of regular spring flooding events due to snowmelt in the headwaters of the River Murray and its tributaries. The forest is one of The Living Murray Icon sites, recognised for its importance as an inland wetland. The forest is dominated by river red gum (Eucalyptus camaldulensis L.), which is harvested by the Forestry Corporation of NSW, as well as black box (Eucalyptus largiflorens) and grey box (Eucalyptus macrocarpa) towards the drier extremities. Since management of the River Murray’s flows began, the forest’s natural flooding pattern has been altered. In particular, The ‘Millennium Drought’ around the turn of the 21st century led to the recognition that a means of delivering environmental water flows to the forest would be required to ensure its sustainability. Consequently, substantial levee and channel works were completed, enabling the forest managers to deliver water in a controlled manner. The purpose of this research project was to describe the soil health of the forest and to understand how this varied through space and time. Specifically, the project examined landscape- and local-scale variation in soil chemical and biological properties including soil carbon and its fractions, nutrient concentrations and fluxes, and microbial community size and functional structure. A subset of soil properties were also studied over five sampling episodes across two years in order to understand seasonal variation. Lastly, in spring 2019, an environmental flow event delivered 500 ML water d-1 to the forest for a period of 60 days, and the impact of this on the sites it inundated was also studied. The main findings of this work are that there was systematic local-scale variation in soil health, with higher soil organic matter, nutrients, microbial activity and biomass at the higher sites relative to those at the bottom of the toposequences studied. Further analysis revealed that the organic matter at these higher sites was likely less vulnerable to loss, having been subjected to greater processing. These findings are contrary to what might have been expected, and are likely due to the sluggish way in which water slowly backfills away from low points, depositing fine suspended loads higher in the landscape before draining. Though we found substantial variation though time, related to classical seasonal cycles, little impact on soil properties was observed from of the individual flood event that occurred during the study period. This suggests that the differences seen along toposequences are a result of successive inundation events over preceding decades and centuries, many of which are known to have been greater in volume and length than the spring 2019 event.
Original languageEnglish
PublisherCSIRO Australia
Commissioning bodyMurray Darling Basin Authority
DOIs
Publication statusPublished - 1 Feb 2022
Externally publishedYes

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