Elsevier

Bioresource Technology

Volume 102, Issue 21, November 2011, Pages 9997-10005
Bioresource Technology

Carbon mineralization and distribution of nutrients within different particle-size fractions of commercially produced olive mill pomace

https://doi.org/10.1016/j.biortech.2011.08.009Get rights and content

Abstract

Composting is a realistic option for disposal of olive mill pomace (OMP) by making it suitable as a soil amendment for organic farming. The chemical and physical characteristics and contribution of particle-size fractions to total nutrients and carbon mineralization of seven commercial composts of OMP (COMP) were investigated. Higher proportions of manure, co-composted with OMP, reduced the organic matter (OM), total carbon and C:N ratio of the product, but increased the content of nutrients and fine particles. The fine particles had higher nutrient contents, but less OM and carbon and, unlike larger particles, did not exhibit any phytotoxicity. Less than 1.5% of added carbon was mineralized in whole compost, but a lower rate was found with larger particles. Separation of COMP by particle size fractionation and application as a soil conditioner is recommended for better optimization of COMP with the <1 mm fraction providing the higher quality compost.

Highlights

► Nutrient and carbon mineralization of composted olive mill pomace were analyzed. ► Quality and particle distribution differed according to the material co-composted. ► Higher content of nutrients and lower carbon contents were found in finer particles. ► Larger particles had higher contents of organic matter and phytotoxic properties. ► Particle size separation facilitates better management of this compost.

Introduction

Olive mill pomace (OMP) is the main by-product of the two-stage olive oil extraction process. Currently, between 2.5 and 4.0 million tonnes of OMP are produced annually in the Andalusia region of southern Spain. OMP is a potentially harmful by-product for the environment arising from the phytotoxic and antimicrobial properties, mainly due to the phenolic and lipid constituents (Paredes et al., 1987, Perez et al., 1992), and therefore, direct application to rivers or soil is not permitted under regional environmental regulations. Thus in recent years, efforts have been intensified to provide better solutions for the safer disposal of OMP in the environment.

One of the main options involves recycling OMP organic matter and nutrients through composting and subsequent land application back onto organic olive tree farms. Composting OMP (COMP) enables it to be sanitized; the mass and volume of the product are reduced and stabilized prior to land spreading. This approach helps to improve soil fertility, reduces the use of inorganic fertilizers in olive oil farming and for a commercial enterprise, can provide an additional source of revenue for the olive mill economy. Despite this, the current amount of COMP produced annually in Andalusia is relatively small (at around 70,000 t), but has increased exponentially during the last 5 years (Álvarez de la Puente et al., 2010).

There are few studies on the agrochemical characterization of the final COMP product (e.g. Alburquerque et al., 2006), however, these studies have been undertaken using only a limited number of types of COMP, which were produced in small quantities and on a small experimental scale. Typically before composting, OMP is mixed with olive tree leaves, which are gathered along with the olive fruit, twigs and small branches, as well as straw and manure, or any other locally available animal or plant material which may be added to increase the nutrient content. The relatively high diversity of the bulking agents and their mixing proportions will therefore, be responsible for some inherent heterogeneity in the COMP, not only in carbon (C), nitrogen (N), phosphorus (P) and potassium (K) contents, but also in the particle size distribution of the final product.

The relationship between particle-size distribution of an exogenous source of organic matter (OM), and the C and N dynamics in soils after OM application has been demonstrated for sludge compost by Doublet et al. (2010) among others. Similarly, C mineralization and turnover was seen to differ according to the particle-size fraction in a cattle slurry compost (Grilo et al., 2011, Fangueiro et al., 2007), also from a sludge-straw mixture (Robin et al., 2008) and in an aerobically digested sewage sludge composted along with screened green waste compost, stored yard trimmings and crushed wood pallets (Doublet et al., 2010).

Nitrogen availability in compost has also been shown to be related to particle size: increasing as particle size decreased in sludge compost (Tester et al., 1979). Generally, N mineralization is greater in the fine and water-soluble fractions than in coarser fractions, at least for manure and city refuse composts (Aoyama, 1985) and in composted sewage sludge (Doublet et al., 2010). These results, which can most likely be explained by differing OM characteristics, suggest that the size of fractions in compost contributes substantially to the total C and N dynamics of mineralization after the compost is incorporated into soil. This is of particular interest, because depending on the range in particle sizes, composts could therefore provide a means for storage of C in soil (i.e. from the larger sized compost fractions), or a source of available N (i.e. from the finer compost fractions).

As far as we know, there have been no studies investigating the particle size distribution of COMP which is produced commercially. The characterization of the particle size distribution of composts is of importance, because the relatively high volume of COMP produced in olive mills could make it economically feasible to undertake an industrially-based fractionation of composts and thereby generate different composts with the desired qualities through particular combinations of different particle-sizes.

In the present study, the characterization of particle size fractions including C and N distribution within those fractions and the potential contribution to C mineralization were investigated for seven out of the eleven commonly occurring COMP products found in southern Spain.

Section snippets

Collection and particle size fractionation

Seven ready-to-apply olive mill pomace composts were collected from several olive mills located in different provinces of Andalusia (Almeria, Cádiz, Córdoba, Jaén and Málaga). All the composting methods used aerated heaps and the duration of the composting period was similar at between 7 and 9 months. However, composts from these olive mills differed in their composition and in the proportions of primary materials such as olive mill pomace (OMP), olive leafy material (OMP), manure and straw.

Characterization of whole (non-fractionated) COMP

The main characteristics of the seven COMPs, which are currently produced in Andalusia, differ markedly according to the raw material co-composted with OMP.

Compost organic matter content (measured as LOI) ranged from 27.2% to 87.9% for COMP3 and COMP6, respectively, and averaged 60.6% (Table 2). These values were similar or slightly higher than the experimentally-produced COMP of Alburquerque et al. (2006), which ranged between 34.4% and 60.5%, but were higher than the LOI content typically

Conclusions

The separation and application of different COMP particle sizes could be advantageous for better optimization of COMP management considering that the particles 1.0–6.0 mm have higher values for GI, total N, P and K and potentially mineralizable N, providing a higher quality compost. Alternatively, the larger particles had higher contents of organic matter and carbon and tended to have the lowest C mineralization and thus their application to olive groves as a soil conditioner could improve the

Acknowledgements

This research was carried out in the framework of the “Olive grove project” of the General Secretary for Rural Development and organic production of the Junta de Andalucía, and with the economic help of the Minister of Science and Technology of Spain. The authors would like to thank José Álvarez for assistance in searching for composted olive mill pomace and to the staff of the North Wyke for their technical assistance.

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