Anderson, 2005), and to increase quantities of limiting nutrients such as nitrogen and phosphorus (Sterritt and Lester, 1980).
2.9.1 Soil physical properties
The addition of sewage sludge to soils reduces soil bulk density, increases soil porosity and increases the stability of aggregates in a variety of soil types. Soil porosity increases in the small to medium pore size range (30 – 50 µm and 50 – 500 µm) with the addition of sewage sludge (Pagliai et al., 1983), which can lead to an increase in water infiltration rates. The increase in aggregate stability is more pronounced in soils with a higher clay content, perhaps due to an already higher tendency to aggregate (García-Orenes et al., 2005), or due to an already low organic matter content within these soils. The increased percentage of stable aggregates initially improves the water holding capacity of the soil, although Epstein (1975) found that while the stable aggregate percentage remained higher in sewage sludge amended soil (28 – 35% for amended soils, compared to 17% of the control soils), the saturated hydraulic conductivity of the soil returned to that of the original soil after 50 to 80 days (Epstein, 1975). As relatively recalcitrant organic matter (Larney and Angers, 2012), the effect of sewage sludge application on saturated hydraulic conductivity may be expected to last longer than Epstein reported, although other studies have found that the benefits persist for at least four years (Wallace et al., 2009; Lindsay and Logan, 1996). However it is important to consider that studies reporting on the longevity of sewage sludges in soil may be dependent on the application methods. Some studies are based on a single, large application of sludge for carbon capture or fertility purposes, which is a very different application regime to that of the uses of sewage sludge as a soil amendment.
These contradictory results regarding the longevity of sewage sludge in soil can potentially be explained by the wide level of heterogeneity between sludges, depending on where they are produced. Sludge composition is affected, not only on the wastewater composition in the area (Sommers, 1977), but also on the processing methods used by the production plant. Different forms of sewage sludge (liquid, dewatered or dry solids) have different nitrogen availability (Smith et al., 1998). Additionally, there is evidence that when sewage sludges are thermally dried at higher temperature, the organic matter is more recalcitrant, and so less available for breakdown by soil organisms (Case et al., 2016). This would result in the sewage sludges remaining in the soil for a longer period of time, and so impacting upon soil structure for longer. However, the environmental conditions of the site of application will also have an impact on the rates of sewage
(Khaleel et al., 1981), and better aerated sites decomposing almost 50% of the waste carbon within 90 days (Mathers and Stewart, 1970).
2.9.2 Soil pH
There is a great amount of variability in the literature as to the effect of the addition of sewage sludge on soil pH. While some papers have found that the addition of sewage sludge reduced the pH of the soil from 6.2 unamended to 5.2 after three years of sludge application (Neilsen et al., 1998), others have found that the soil pH increased towards neutrality, whether the soil began at low pH values of 4.86 or already close to neutral values of 6.30 or 7.09 (Tsadilas et al., 1995). Variability in the content of the sludges, along with variations in the soil themselves, may lead to highly varied effects of the application of sewage sludge on soil pH. As sludges are produced in a variety of processing plants, their composition will be highly dependent on the wastewater produced in the local area. While N, P and K levels may be relatively homogenous, the concentrations of trace elements are more variable (Sommers, 1977). This variation in base material and trace elements may well transfer into the variation observed across various experiments. Variation in sludge pH has been observed across countries, with sewage sludge pHs ranging from 6.8 in Thailand (Parkpain et al., 1998) to 8.6 in Spain (Martinez et al., 2002, in Singh and Agrawal, 2008), and within countries, with sludges in Kolkata, India, ranging in pH from 5.96 to 7.14 (Saha et al., 2018).
2.9.3 Soil fauna
The addition of sewage sludge also impacts soil microbial populations. Bastida et al. (2008) found that, seventeen years after single applications of medium (a soil organic matter content increase of 1%), high (an increase of 1.5%) or very high (2%) quantities of sewage sludges, the presence of the phospho-lipid fatty acid (PLFA) profiles of both soil bacteria and soil fungi was significantly higher than unamended soils, with bacterial and fungal PLFA concentrations in amended soils double those observed in control soils (Bastida et al., 2008). In addition to increasing the microbial populations of soils, the application of sewage sludge also increases the populations of earthworms (Hamilton and Dindal, 1989), although this seems to be highly dependent on the earthworm ecotypes present (Coors et al., 2016), with L. terrestris being observed to grow at a faster rate in soil amended with sewage sludge (Hartenstein and Amico, 1983). While most of the existing research regarding interactions between sewage sludge application and earthworms focuses on the uptake of metals and other environmental pollutants,
the reported changes in earthworm populations and growth rates, with the anecic L. terrestris seeming to respond positively to the presence of sludge, while Hamilton and Dindal (1989) found that populations of the epigeic E. fetida reduced in its presence, many laboratory based experiments have found that many species of earthworm avoid sewage sludge when other food sources such as leaf litter are available (Le Bayon and Binet, 2006; Doube et al., 1997; Artuso et al., 2011). Other studies have found that earthworms willshow a preference for soil that has been amended with sewage sludge as a source of organic matter compared to an OECD artificial soil composed of Sphagnum peat mixed with kaolinite clay and sand quartz (Moreira et al., 2008), or over a range of unamended control soils (Bouldin et al., 2016).
2.10 Earthworms, flooding, and sewage sludge: The knowledge gaps
This thesis is part of the research being carried out by the BIOSAS research group. The BIOSAS group is investigating the BIOchemical-physical-biological function of Sludge in Agricultural Soils, and this thesis is investigating the interactions of sludge amended soils, flooding, and earthworm populations.
From the literature, we know that earthworm populations are highly responsive to flooding, with earthworm population structures being highly influenced by the flooding regime at the ecotype level. We also know that there are several advantages to the application of sewage sludge to farmland soil. However, most existing research focuses on the metal content of sludges and the effect that may have on earthworm populations.
There are three key questions that therefore arise for this thesis:
1. Does regular flooding impact earthworm populations in arable and pasture soils differently, given the already low earthworm populations in arable soils?
2. Does increasing organic matter content, in the form of sewage sludge application, lead to faster rates of oxygen depletion when soils floods?
3. Are there species specific responses of earthworms to flooding? And, if so, how might these change with the application of sewage sludge to soil?