Microbiological and geochemical response of biosolid amended soils to flooding

More frequent extreme weather events induced by climate change could pose new challenges to agricultural production. Soil organic inputs, such as biosolids, could be an effective way to improve soil and crop resilience to short-term flooding. Biosolids are a valuable and sustainable organic input, containing high N, P and organic C contents which can act as a nutrient source for crops and soil microbes. However, these benefits could be offset if the contained N and P is lost to floodwater, leading to an increased eutrophication risk. To address this question a series of controlled laboratory growth box experiments were conducted to determine the impact of varying biosolid application rates and a 10-day flood on soil geochemistry, microbiology and crop performance. Biosolid applications caused increased soil N, P and organic matter which mitigated poor crop establishment. However, there was no apparent impact on crop flood resilience, though at high application rates plants had greater dry weight biomass. The increased soil N content led to increased NO3- loss to floodwater, but no increase was observed in floodwater P with increased biosolid application despite higher soil P. This was attributed to the presence on NO3- in biosolid-applied soils acting as a terminal electron acceptor, preventing highly reducing conditions in soil porewater which would favour soluble P release. This effect meant biosolid-applied soils had no greater loss of soluble P than unapplied control soils. Soil bacterial populations characterised using 16s rRNA gene sequencing showed biosolid-applied soil populations still closely resembled those of control soils, though increased nutrient availability induced some changes in phylum relative abundances. There was evidence to show that some bacterial genera were transferred from biosolids to soils, and that flooding allowed some of these genera to increase in abundance. However, bacterial populations returned to pre-flood profiles after a 20-day recovery period.