Biosolids are a source of plant available nitrogen, phosphorus and other major nutrients. They also contain valuable quantities of stable organic matter, which can provide long-term benefits to soil structure and fertility. This paper reports findings from a project to assess the soil quality impacts of biosolids recycling to agricultural land. Increases in soil organic matter (SOM) and ‘light fraction’ SOM following long-term (20 year) repeated biosolids additions illustrate that applying biosolids to agricultural land is an important means of replenishing and maintaining SOM levels. A number of improvements were also measured in soil physical properties (available water capacity, water infiltration and aggregate stability), soil nutrient (nitrogen, phosphorus and sulphur) supply and soil biological properties (earthworm numbers and weight). The results from this study provide valuable evidence towards maintaining a sustainable agricultural landbank for biosolids recycling.
Keywords: Agriculture, biosolids, recycling, soil fertility, soil quality.
There are around 12 million hectares of agricultural land in the United Kingdom providing a large potential landbank for biosolids (treated sewage sludge) recycling. It is therefore not surprising that 77% of the 1.4 million tonnes (dry weight) of biosolids currently produced in the UK are recycled to agricultural land (Water UK 2010). At an average application rate of 6.5 t/ha dry solids this equates to c.170,000 ha of agricultural land receiving biosolids annually.
Biosolids are a valuable source of nitrogen (N) and phosphorus (P), as well as other major and minor plant nutrients, and as a result of some conditioning processes can have value as a liming material. Notably, biosolids contain valuable quantities of stable organic matter, and are therefore an important means of replenishing/maintaining soil organic matter levels, which are intimately linked to soil properties that have an important influence on soil quality and fertility, and hence on sustainable crop production. The importance of maintaining and improving soil quality was highlighted in the Government’s Food 2030 strategy (Defra 2010a); moreover, Defra’s headline indicator for agricultural soil quality was to “maintain and enhance soil organic matter levels”, with the maintenance and enhancement of soil organic matter a key requirement of cross compliance.
In order to fully exploit/maintain a sustainable agricultural landbank, there is a need to provide robust experimental evidence to demonstrate the long-term effects of biosolids on soil quality and fertility. A previous UK study measured some changes in soil bio-physical and physico-chemical properties (e.g. plant available water capacity), but not others (e.g. soil microbial activity, structural stability, soil strength) (Chambers et al. 2001; Gibbs et al. 2006a). The lack of measured effects was no doubt (in part) due to the relatively low amounts of organic matter applied by the biosolids applications (organic matter loading rates in the range c.5-9 t/ha) at the sites. In this study, we used experimental plots that had received 20 years of repeated biosolids additions (Gibbs et al. 2006b) to better assess the effects of repeated biosolids organic matter additions on soil bio-physical and physico-chemical properties, as they had received much greater organic matter loadings (c.12-33 t/ha).
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