Decomposition of olive-mill waste compost, goat manure and Medicago sativa in Lebanese soils as measured using the litterbag technique

Abstract

Organic amendments, green manure and plant residues are the main sources of nutrients under organic farming. The decomposition of compost from olive-mill waste, aged goat manure and lucerne (alfalfa, Medicago sativa) shoots was studied in subhumid Mediterranean conditions. The locations of Abdeh, Lebaa and Sour were distinguished by their respective clay (521, 315 and 260 g kg–1 soil) and calcium carbonate (42, 591 and 269 g kg–1 soil) contents. Nutrients release was evaluated over 1 year by using litter bags buried at 10 cm depth. Despite close initial total nitrogen (N) contents, shoots lost 50–60% of N, whereas the stable compost and manure mineralised 20–26% after 30 days of incubation. Calculated coefficients of decomposition for organic carbon (OC) were significantly higher for shoots (0.0061 day–1) than for manure (0.0020 day–1) and compost (0.0011 day–1). Coefficients for total N were very similar to those for OC: shoots (0.0050 day–1) > manure (0.0018 day–1) and compost (0.0017 day–1). OC was the parameter most discriminating between fresh residue and decomposed products, and potassium the least. Of the soil properties, the calcium carbonate rather than the clay content affected decomposition. Coefficients of decomposition of OC were significantly different with Sour (0.0036 day–1) > Abdeh (0.0030 day–1) > Lebaa (0.0026 day–1). Coefficients for total N were also different with Abdeh ≡ Sour > Lebaa. Fresh and stable products could be a source of nutrients even during winter in Mediterranean conditions.