Extreme events and grassland ecosystem function: The impact of flooding on P mobilization

Abstract

Improved grasslands make up an important part of UK agriculture and in addition to providing forage for livestock, grasslands provide additional and important ecosystem services including the regulation of water quantity and quality. In the context of global climate change the most recent IPCC report predicts greater uncertainty in weather patterns and an increased incidence of extreme weather events, such as heat waves, drought, heavy rains and storms¹. It has been predicted that these episodic extreme events pose a bigger threat to European agroecosystems than progressive climate change². Despite the large risks posed, our understanding of how extreme events will impact on plant and soil functioning and the downstream benefits/impacts remains poor. One vital ecosystem service provided by grasslands is the regulation of nutrient cycling to facilitate plant production and control the release of nutrients, including phosphorus (P) runoff. To investigate the impact of extreme climatic events on grassland ecosystems we have set-up a plot-scale field trial on an improved lowland Welsh sheep grazed pasture. The trial design consists of 20 field plots 3 m by 3 m which will be subjected to five different treatment (stress) regimes with four replicates as follows; (i) control – natural climatic conditions, (ii) spring flood, (iii) summer drought, (iv) spring flood + summer drought and (v) summer flood. This presentation will focus on the results of the spring flood initiated in April 2016. The flood plots were flooded to a depth of 30 cm with water from the adjacent river. The flood will be maintained for 8 weeks and the grass sward will be completely submerged throughout. Large pulses of P release have been observed following inundation on dry soils³ as a result of both abiotic desorption, dissolution and biotic release related to the soil microbial pool. In this presentation we will report the changes in the flood and soil water chemistry over the course of the 8 weeks under flooding and the following month of recovery. Analyses will include dissolved reactive P (DRP) and dissolved organic P (DOP), Fe(II)/Fe(III) and DOC concentrations. In addition, we will report the water extractable P concentrations (both inorganic and organic forms), and the microbial community structure (from PLFA analysis) from soil cores taken immediately before and after the flood event and at the end of the month’s recovery period.