Comparison of the bacterial composition of two commercial composts with different physicochemical, stability and maturity properties

Highlights

• Composts with different physicochemical properties also had different bacterial communities.

• EC, Cu, OM contents, GI, and fungi counts were related with the bacterial communities.

• The mature compost had a more diverse bacterial community than the immature.

• Pseudonocardiaceae, Streptomycetaceae and Bacillaceae were abundant in the mature compost.

• Aerococcaceae and Corynebacteriaceae prevailed in the immature compost.

Abstract

Previously, two municipal solid waste commercial composts (MSW1 and MSW2) were characterized. Although sharing the same type of raw material, most of their physicochemical, stability and maturity properties differed. The present study aimed to characterize them at a microbiological level, and to infer on possible relationships between the composts properties and the structure of their bacterial communities. Both the 16S rRNA gene-based PCR-DGGE profiling and 454-pyrosequencing technology showed that the structure of the bacterial communities of these composts was distinct. The bacterial community of MSW1 was more diverse than that of MSW2. Multivariate analyses revealed that the high electrical conductivity, Cu content as well as the low phytotoxity of compost MSW1, when compared to MSW2, contributed most to shape its bacterial community structure. Indeed, high abundance of halophilic (Halomonadaceae and Brevibacteriaceae) and metal resistant organisms (Brevibacteriaceae and Bacillaceae) were found in MSW1. In addition, Pseudonocardiaceae, Streptomycetaceae, Bacillaceae, and Brevibacteriaceae may have contributed to the high humic-like acids content and low phytotoxicity of MSW1. In contrast, the high organic matter content and the high density of the cultivable fungi population were the parameters most correlated with the structure of the bacterial community of compost MSW2, dominated by Corynebacteriaceae and mainly Aerococcaceae, taxonomic groups not commonly found in composts.

Introduction

Composting is an ancient strategy to recycle a variety of organic wastes. It can be considered a biotechnological aerobic process where during the first phase (active phase) microbial communities transforme and partially mineralize the most easily biodegradable materials of organic matter (OM) present in the raw materials. The organic matter becomes stabilised as a consequence of the intense microbial activity occurring in this phase (Adani et al., 1997), which is also responsible for the rise of the compost temperature, inducing a transition from a mesophilic to a thermophilic stage (Tang et al., 2006). Then, a gradual decrease in the microbial activity leads to the cooling and the curing or maturation phase. In this phase occurs the conversion of part of the stabilised organic matter into humic-like substances (Chen and Inbar, 1993). Thus, while compost stability can be regarded as the result of high-rate microbial reactions occurring throughout the active phase of the process, compost maturity appears, on the contrary, as the effect of the curing phase. Given its high content in humic-like substances, compost is considered an added value product, allowing the recovery of degraded soil and the sustainable management of agricultural land when used as a soil amendment (Albrecht et al., 2011, Cayuela et al., 2009). However, only stable and mature composts should be used as a soil amendment (Castaldi et al., 2008, ECC, 2001, Gómez-Brandón et al., 2008). The application of unstable composts to soil may promote competition for oxygen between microorganisms and plant roots and/or seeds. In addition, it may promote nitrogen starvation of plants as microorganisms scavenge soil N as a consequence of the high C/N ratio of unstable composts. Unstabe composts may be also phytotoxic due to the emission of ammonia and the presence of other phytotoxic substances like phenolic compounds and ethylene oxide added to soil when the readily metabolizable organic matter of the compost is not fully degraded (Gómez-Brandón et al., 2008). Additionally, as referred to by Danon et al. (2008), some studies have emphasized the importance of achieving compost maturity to ensure balanced plant nutrition and the biological control of soil-borne plant disease.

Green waste, municipal solid waste, sewage sludge and manure are the raw materials commonly used in composting (e.g., Cahyani et al., 2003, de Gannes et al., 2013, Storey et al., 2015, Takaku et al., 2006), which is carried out in windrow or reactor systems. Independently of the raw material or the system, to obtain stable and mature composts, the operating conditions must favor the development of the microbial communities present at the beginning of the process. In fact, Partanen et al. (2010) have demonstrated that low initial pH and insufficient aeration limits microbial activity and delays the increase in temperature. Other abiotic parameter known to influence the composting process is the C/N ratio (Cayuela et al., 2009, Chroni et al., 2009, Guo et al., 2012, Tiquia, 2005), since without an adequate proportion of C and N, microorganisms are unable of growing (Maier, 2009). Indeed, the composition and function of the microbial communities depends on these and on other environmental factors, such as the temperature and the available nutrients (e.g., Blanc et al., 1999, Nakasaki et al., 2009, Ryckeboer et al., 2003, Tiquia, 2005). Hence, being the product of microbial transformations, the properties of the final compost are expected to depend on the operating conditions, including the type of the raw material and time of composting, which modullate the microbiological populations prevailing in each of the composting stages (Cayuela et al., 2009, Fracchia et al., 2006, Partanen et al., 2010, Storey et al., 2015).

Table 1 summarizes the bacteria described as the most predominant in composts. This revision was based on studies published after 1999 in journals indexed to the ISI – Web of Knowledge, in which the composting system was windrow or reactor. In addition, only studies where the bacterial identification was based on 16S rRNA gene sequence analysis were included, independent of using culture-dependent and/or independent methods. Based on Table 1 it is possible to conclude that, in general, composts harbor a wide diversity of bacteria. Indeed, organisms belonging to ten different phyla have been shown to be abundant in composts. As expected, some groups are only abundant when mesophilic (e.g. Enterococcaceae) or thermophilic (e.g. Rhodothermaceae) conditions prevail, while others occur during the cooling or maturation phases (e.g. Cytophagaceae). However, several bacterial groups have been detected in composts, independent of the raw material, composting system or phase. Examples are bacteria affiliated to phylum Chloroflexi, and to the families Flavobacteriaceae and Sphingobacteriaceae (Bacteroidetes), Bacillaceae and Paenibacillaceae (Firmicutes), Alcaligenaceae (Betaproteobacteria) and Xanthomonadaceae (Gammaproteobacteria).

Although several studies revealed the composition and dynamics of the microorganisms during the composting (Table 1), little is known about the correlation between the microbial composition and the compost quality. In a previous study, based on an exhaustive physicochemical, stability and maturity characterization, it was shown that two commercial composts (MSW1 and MSW2) differed greatly (Silva et al., 2013). This variation was mainly due to differences on the content of organic matter, moisture, electrical conductivity, heavy metals and humic-like acids, degree of stability and germination index. Regarding these differences, in the present study, we intended to verify if this dissimilarity was also observed at the bacterial community level and if there was a relationship between these parameters and the bacterial community structure. We also intended to identify the common and unique populations and to evaluate if the physiological characteristics of the most abundant groups were compatible with the physicochemical, stability and maturity properties of the studied composts.