Biological Functions
Nutrient release from applied organic amendments is dependent on decomposition processes. The soil fauna and flora which collectively are termed the microbial biomass (MB) are considered the primary and secondary consumers in the food chain, cycling nutrients from dead and decaying plant debris and other microfauna through the soil (Brady and Weil, 1999). Research conducted on surface applied biosolids to shrubland and grassland in the United States found an increase in MB six years after application (Barbarick et al., 2004). However, Boyle and Paul (1989) found a significant decrease in MB after 20 weeks of incubating sludge treated soils. The soils were gathered from sites three years after eight years continued sludge application.
Brendecke et al. (1993) found that after applying anaerobically digested sludge at rates between 8.0 and 24 Mg/ha each year for four years to an arid soil, there were no
significant adverse affects to microbial populations or their activity. They also found no significant changes to soil physical and chemical properties aside from elevated PO4 –
P, giving rise to their suggestion that microbial activity was not a good predictor of soil fertility. Plate counts of bacteria and fungi were also conducted and found to not be significantly affected by biosolids (Brendecke et al., 1993). In their study of soil fungal and bacterial changes resulting from the addition of organic materials with different C/N ratios, Johannes and Erland (2007) found that bacteria increased with a lower C/N ratio material (alfalfa) whilst fungi increased with a higher C/N ratio material (barley straw). However, when they added N to balance the C/N ratio of the barley straw, fungi
increased and bacteria was inhibited, against expectations (Johannes and Erland, 2007). This would suggest that the composition of organic amendments, other than the C/N ratio, may impact on bacterial and fungal activity and subsequent biological function. In a five year glasshouse experiment conducted in Italy by Marchesini et al. (1988), fertiliser and compost treatments were applied to a sandy soil (85% sand) to examine possible benefits of long-term compost treatment of soil. Findings were that although yields were more for the compost treatments than the fertiliser treatments, microbial activity was similar across all treatments. However, it was suggested that the microflora developed in the composted mixes consisted of qualitatively different populations
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offering more beneficial conditions for plant growth (hence, higher yields) and at the same time excluding the development of harmful organisms (Marchesini et al., 1988). In another greenhouse trial by Lupwayi et al. (1998), microbial biomass and bacterial diversity were analysed from residues of wheat, barley, canola, field peas and red clover incorporated into potted soils at equivalent rates of 5 tons an acre. The results indicated that fertiliser alone (without a carbon source) did not stimulate microbial growth, but rather, had an indirect effect by promoting growth, but only if plant residues were then incorporated into the soil (Lupwayi et al., 1998). The authors also suggested that soil microorganisms perform many agriculturally important functions including decomposition and recycling of nutrients from dead organic material, nitrogen fixation, and maintenance of soil structure and detoxification of agrochemicals.
A study by Falih and Wainwright (1996) into the incorporation of sugar beet into soil found that the easily available carbon (from the beet) stimulated microbial and enzymatic activity . They suggested that amendments such as this could stimulate beneficial microbial processes in soil, such as P solubilisation. However, although it was considered that nitrification and S oxidation were mediated solely by chemo- autotrophic bacteria and hence not directly influenced by addition of C substrates, the authors concluded that the stimulated microbial activity by sugar beet could lead to nitrification, leaching and S immobilisation (Falih and Wainwright, 1996).
In a further study conducted by Cooper and Warman (1997) to assess microbial activity within both composted and fertilised plots, they found that compost application increased dehydrogenase enzyme activity DHA and organic C, whereas the fertiliser treatments resulted in a decrease in DHA without a corresponding decrease in organic C or pH. Conclusions were that microbial activity in the fertiliser treatments was possibly affected by factors other than organic C and pH. Various mechanisms that were suggested for decreased microbial activity in N fertilised plots included:-
• a direct inhibiting effect of nitrogenous compounds making C less available • N increasing the retention of C and
• a partial sterilisation effect from the raised osmotic potential of the soil solution
due to fertiliser salts (Soderstrom et al., 1983 cited by Cooper and Warman (1997)).
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The results of combined treatments referred to herein appear to support the theory that compost amendments resulted in increased qualitative microbial activity. However, it has not been so evident that inorganic fertilisers had a detrimental effect on the microflora (Marchesini et al., 1988). These findings were in direct contrast to an older study by Pettersson and Wistinghausen (1979) who were of the view that to add mineral salts in soluble inorganic form essentially by-passed the activity of micro-organisms, considered by some as the most important function of soil, rendering them, superfluous. They continued to suggest that this inactivation of vital soil processes would lead to reduced product quality and plant dependence on a precisely regulated supply of minerals from the outside.
Soil Chemical Function
The soil chemical functions that may be affected by additions of organic materials include complexing of cations, CEC and pH. Cation complexes can enhance P
availability, reduce concentrations of toxic cations, and promote the binding of organic matter to soil minerals (Baldock and Skjemstad, 1999). Adding compost and manures was shown by Schefe et al. (2008) and Whalen and Chang (2001) to increase P
availability, although Pritchard et al. (2004) suggested that the increase in soil P availability after applying biosolids was from the release of inorganic P from biosolids and not soil organic P. Basta (2001) found that using alkaline organic amendments for rehabilitating mine sites reduced human exposure to Cd and Pb. However, Hue et al. (2001) found that Mn toxicity increased with the addition of organic amendments and Whatmuff (2002) found an increase in plant uptake of Cd and Zn after applying sewage sludge.
Soil salinity and sodicity are soil chemical attributes that can lead to a decline in soil physical function. Salinity is the presence of dissolved salts in the soil or water within the root zone (Shaw, 1999) and, although it can improve aggregation, it can inhibit plant growth through osmotic stress (Fitzpatrick et al., 1994). Sodicity on the other hand is the presence of Na+ ions within the soil matrix that when exposed to excess water can lead to a breakdown of soil aggregation and subsequent reduction in hydraulic
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Sodosols by definition are texture contrast soils that are only sodic and not acidic in the upper B horizon (Isbell, 2002). Mitigation of sodicity has generally been through the use of gypsum (Tillman and Surapaneni, 2002), which is problematic in Tasmania where (a) gypsum is expensive and not readily available and (b) it needs to be placed in the subsoil. However, Alan et al. (2008) have shown that problems associated with soil salinity and sodicity may be alleviated with the use of composts. Their research
suggested that Ca2+, Mg2+ and K+ in the compost treatments occupied the cation exchange sites on soil particles, which minimised adsorption and enhanced leaching of Na+ with rainfall/irrigation. This contrasts with Aoyama et al. (2006), who found an increase in EC1:5 with lime treated sludge and surmised the cause as increased soluble
salt from elevated levels of Ca2+. Limited research has been conducted on the affect of
biosolids on soils exhibiting saline and sodic properties, although some research has been conducted using sodic irrigation water or irrigated effluent (Tillman and
Surapaneni, 2002) while others have investigated subsoil sodic remediation (Clark et al., 2009).
Soil Physical Function
A decrease in soil organic matter (SOM) or soil organic carbon (SOC) can negatively affect soil physical functions including structural stability, water holding capacity and thermal properties (Baldock and Skjemstad, 1999). Such decreases can be exacerbated through conventional tillage and irrigation (Gwenzi et al., 2009). However, applying organic amendments such as biosolids over a longer term has been shown to increase soil C within aggregates with subsequent improvement of aggregation and soil water retention (Wallace et al., 2009). After five years, Tester (1990) found that penetration resistance and bulk density (soil properties that affect soil function) were less for biosolids and compost treatments compared to soil treated only with inorganic
fertilisers. Mohammad et al.(2007) also found that organic amendments decreased bulk density and increased soil carbon. They observed cumulative effects on soil physical properties with continued organic additions, even though experiments were conducted in a hot humid tropical environment; normally associated with accelerated
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Armstrong et al. (2007a) found improvements to soil physical properties of Sodosols specifically, with the addition of composted pig bedding litter. Small unstable
aggregates decreased concomitant with an increase in larger aggregates (Armstrong et al., 2007a). After four years of applying urban waste compost Giusquiani et al. (1995) found an increase in porosity, increased water retention, a decrease in bulk density and improved soil structure. They concluded that soil structure was stabilised by a thin protective coating over the elongated pores, which subsequently prevented water damage and enhanced soil pore function.