Chemical and microbiological aspects of soil amended with citrus pulp

Citrus pulp is the pulp and peel waste generated by citrus fruit processing. It can be a pollutant when it is disposed of by burning or dumping, but it is rich in organic carbon (monosaccharides, polysaccharides, pectins, and organic acids), suggesting that it could also be a valuable soil amendment. For the purpose of studying the effects of citrus pulp amendant on soil fertility, a study was carried out using lysimeters where the soil was amended with different quantities of dried citrus pulp. Soil samples were taken periodically over a period of 2 years to evaluate the evolution of the organic matter and the activity of the microbial biomass. The former was evaluated by calculating the TOC and by extracting and characterizing the humified component, while the latter was evaluated through the calculation of the basal respiration, the C microbial content and some enzymatic activities (alkaline phosphatases, β-glucosidase, and arylsulphatase). The introduction of citrus pulp brought about an improvement in soil quality which continued for 20 months after the application of the amendant. In particular, there was an increase in the soil organic matter, despite the notable mineralization process which took place in the short-term due to the citrus fruit industry residues. In the medium term, the chemical-spectroscopic characterization of the organic compounds of the soil highlighted a great similarity to the natural humic substances. The improvement in soil quality was also demonstrated by the activity of the microbial biomass, with a large and long-lasting increase in microbial and respired C and the stimulation of enzymatic activity.