Chemical, physical and microbial properties and microbial diversity in manufactured soils produced from co-composting green waste and biosolids

Abstract

The effects of adding biosolids to a green waste feedstock (100% green waste, 25% v/v biosolids or 50% biosolids) on the properties of composted products were investigated. Following initial composting, 20% soil or 20% fly ash/river sand mix was added to the composts as would be carried out commercially to produce manufactured soil. Temperatures during composting reached 50 °C, or above, for 23 days when biosolids were included as a composting feedstock but temperatures barely reached 40 °C when green waste alone was composted. Addition of biosolids to the feedstock increased total N, EC, extractable NH₄, NO₃ and P but lowered pH, macroporosity, water holding capacity, microbial biomass C and basal respiration in composts. Additions of soil or ash/sand to the composts greatly increased the available water holding capacity of the materials. Principal component analysis (PCA) of PCR-DGGE 16S rDNA amplicons separated bacterial communities according to addition of soil to the compost. For fungal ITS-RNA amplicons, PCA separated communities based on the addition of biosolids. Bacterial species richness and Shannon’s diversity index were greatest for composts where soil had been added but for fungal communities these parameters were greatest in the treatments where 50% biosolids had been included. These results were interpreted in relation to soil having an inoculation effect and biosolids having an acidifying effect thereby favouring a fungal community.

Introduction

In most cities in the developed world, green waste (also known as yard waste), which is composed of tree wood and bark, prunings from young trees and shrubs, dead and green leaves and grass clippings, is collected separately from other wastes. It is normally mechanically shredded and then composted, either alone or with other organic wastes. It is then used in products such as garden mulches and composts, organic soil amendments, soilless potting media, and particularly in Australia, manufactured “topsoils” for landscaping purposes. For the latter product, small quantities (e.g. 10–20% v/v) of inorganic additives (e.g. fly ash, sand, soil) are often blended with the compost and it is then used as a topsoil substitute. Little is known regarding the properties of these manufactured soil materials.

Decomposition of green waste during composting is characteristically slow and in order to initiate intense microbial activity, the addition of a more readily decomposable material (e.g. animal manure, grease trap waste) is required (Francou et al., 2008, Belyaeva and Haynes, 2010). Biosolids, which are also produced by municipalities in large quantities, may well be a suitable organic material since there is, characteristically, intense microbial activity during its composting (Epstein, 2003). Indeed, in order to stabilise and sanitise biosolids, they are often composted before use. This normally involves blending dewatered biosolids with a bulking agent (added to give adequate aeration), and green waste would act as such an agent.

The composting process includes four phases: (i) an initial decomposition phase, (ii) a thermophilic phase of intense microbial decomposition, (iii) a second thermophilic phase, and (iv) a maturation phase. Rapidly multiplying thermophilic bacterial species dominate during the thermophilic phase but once the bulk of the easily decomposable substrate is exhausted, the majority of the remaining material is woody, lignin-dominated material plus stabilised humic material and fungi tend to dominate (Ryckeboer et al., 2003). Although succession of bacterial (Adams and Frostick, 2009, Partanen et al., 2010), fungal (Hansgate et al., 2005, Bonito et al., 2010) and archaeal (Yamamoto et al., 2011) communities during the composting phases for specific wastes has been investigated by a number of workers using molecular techniques (e.g. PCR-DGGE) very little is known about the comparative nature of the microbial communities in matured composts derived from different materials. Klammer et al. (2008) did, however, show that there were clear distinctions between bacterial communities present in matured composts made from biowastes versus sewage sludge. The addition of other materials such as fly ash and soil to the matured compost presumably has an inoculation-effect and this is likely to influence the microbial community present.

The aim of this study was to investigate composting intensity and the chemical, physical and microbial properties and bacterial and fungal diversity, in composts made from green waste alone of green waste plus 25% or 50% v/v biosolids. From a practical viewpoint, the 25% biosolids treatment represents greenwaste compost where biosolids have been added to increase composting intensity while 50% biosolids represents composted biosolids where green waste was the bulking agent. Following initial compost production, the products were amended with 20% topsoil, or 10% coal fly ash plus 10% river sand (to produce manufactured soil material) and allowed to mature.