Nitrogen Isotope Fractionation during Composting of Sewage and Agri-Food Sludge with Pruning Waste

José Antonio Sáez,Pilar Flores,María Dolores Pérez-Murcia,Juan C Sanchez-Hernandez,María Ángeles Bustamante,Raúl Moral

doi:10.3390/agronomy10121954

Abstract

This work studies the changes in δ15N signature during the composting of sewage sludge (MS) and agri-food sludge (AS) with different bulking agents and the potential relationships between the changes in this parameter with both the source of the organic matter used as a raw material and the stability of the end-materials obtained. For this, eleven mixtures were prepared in commercial composting conditions using sewage sludge from municipal wastewater in a range of 60–85% (on a fresh weight basis) or agri-food sludge in a range of 65–75%, mixed with seven different pruning wastes as bulking agents. The thermal profile was monitored throughout the composting processes, and the main physico-chemical and chemical parameters were determined. The results obtained confirmed a correct development of the composting processes, observing slight differences in process evolution depending on the type of sludge used. The composts obtained showed adequate contents of nitrogen, phosphorus and potassium (NPK) and reached a good degree of maturity. Significant differences in the specific nitrogen isotopic composition were found in the initial materials. Moreover, the results suggest that the type of sludge had a main contribution in the δ15N value of the initial composting mixtures. The use of δ15N is recommended as an indicator of the composting process, especially to evaluate N dynamics, and the quality of the resultant compost.

Highlights

Composting constitutes one of the main treatments for the management and valorisation of a wide typology of organic wastes
The main raw materials used in the elaboration of these composting mixtures were sewage sludge (MS) and agri-food sludge (AS), respectively
The results obtained show higher values of δ15 N in the sewage sludge samples compared to agri-food sludge samples, while the bulking agents showed a wide variation in the δ15 N values, even between wastes within the same plant species

Summary

Introduction

Composting constitutes one of the main treatments for the management and valorisation of a wide typology of organic wastes. There is extensive literature on the benefits of the use of compost as soil amendment, maintaining soil organic matter contents, sustaining crop productivity and improving soil quality, especially in the long-term, due to the slow release of nutrients [12,13]. Currently there are no standard analytical techniques to identify the type of fertilisers used, which makes it difficult to detect the use of prohibited synthetic fertilisers in organic farming systems [14,15]. In this context, in the last decade, the use of the 15 N natural

Objectives

Methods

Conclusion