We are using laboratory microcosms to investigate how soil and plants respond to flooding under a wide range of environmental conditions. Specifically, this approach allows us to screen a wide range of soil types under a range of simulated weather and management regimes. This approach also provides us with a deeper mechanistic understanding of how soil ecosystems respond to and recover from flood events.
The main aims of the experiments are:
- To investigate the impact of temperature (5, 15, and 25°C) on the response of soil biological and chemical quality indicators to flooding
- To investigate the impact of flood residence time and depth on soil biological and chemical quality indicators
- To investigate how the cycling of N and P is impacted upon by flooding
- To investigate how flooding impacts upon the release of greenhouse gases from soil
- To investigate the impact of cereal crop residues on the response of soil biological and chemical quality indicators to flooding
- To investigate the impact of animal wastes applied to the soil on the response of soil biological and chemical quality indicators to flooding
- To investigate the influence of floodwater aeration and light on a range of soil biological and chemical quality indicators
- To investigate how different soil types respond to flooding
These microcosms consist of 2.5 litre plastic boxes each of which contain ca. 1 kg of soil. In most experiments we are placing intact blocks of grassland soil (and associated vegetation) in the microcosms, however, in other experiments we have arable soil without vegetation (i.e. to simulate fallow conditions).
The soil used in the experiment are mostly Eutric Cambisols, but with a range of textures. After placing the soil in microcosm boxes a Rhizon soil solution sampler is inserted in the soil and the boxes placed in a climate-controlled growth room to acclimatise for 2 weeks. The boxes have gas-tight lids that allow the collection of greenhouse gases.
After 2 weeks floodwater (river water) is added to the soil to a depth of 10 cm. The floodwater is normally maintained for 8-12 weeks after which it is removed to monitor soil and plant recovery (4-8 weeks).
Measurements made during the experiment include soil solution Fe, Mn, DOC, P, NO3-, NH4+, pH, EC, soil redox status, greenhouse gas emissions (N2O, CO2, CH4), root and shoot growth, microbial community structure (PLFAs) and microbial community functional profiling (GeoChip).
Soil solution Fe: Flooding causes a rapid and large increase in soluble Fe2+ in both the soil and floodwater. These levels fall rapidly once the floodwater is removed. Fe3+ plaques are typically seen around the roots showing signs of O2 release from the roots into the soil. As expected, prolonged flooding caused a change in soil colour (from brown to grey) in the bulk soil.
Soil solution P: Contrary to expectation, flooding has little effect on the release of P into solution from either the soil itself or from newly introduced 33P-labelled fertiliser.
Soil solution NO3-: Flooding causes a rapid reduction in soil nitrate levels. These quickly recover after removal of the floodwater (due to the conversion of accumulated NH4+ to NO3-).
Soil solution NH4+: Flooding causes a rapid increase in soil ammonium levels with the amount produced proportional to flooding time. These ammoniums levels quickly decrease after removal of the floodwater.
pH: This is relatively unaffected by flooding.
EC: This increases in response to flooding (due to electrolyte leakage from senescing roots, foliage and due to the lack of plant nutrient sink).
Soil redox status: This declines from +300 mV to reach values of -100 to -300 mV after prolonged periods of flooding. The presence of organic induces a lower redox potential in the soil. Redox potentials of +300 mV are rapidly restored after removal of the floodwater.
Greenhouse gas emissions (N2O, CO2, CH4): Generally, these markedly increase under flooding and are enhanced in the presence of plant and animal residues.
Root and shoot growth: Shoot growth is relatively unaffected if it is held above the floodwater. Even submerged vegetation recovers well after flooding if light can penetrate through the floodwater.
Earthworms: These survive if the floodwater is at the soil surface. If the floodwater also submerges the vegetation then the earthworm population disappears completely.
Microbial community structure functioning: Initial PLFA analysis suggests that flooding causes no major shift in microbial community structure and the presence of functional genes. We have not yet tested the transcript levels of key functional genes.