Use of compost as an alternative to conventional inorganic fertilizers in intensive lettuce (Lactuca sativa L.) crops—Effects on soil and plant

Abstract

Intensive crop over-fertilization is causing the diffuse contamination of superficial and ground-waters by nitrates in many Mediterranean areas. At the same time this practice is also damaging the soil in the long-term. In this work the feasibility of using composts as total or partial substitutes for conventional inorganic fertilization (CIF) was evaluated in two successive crops of lettuce (Lactuca sativa L.), one of the most consumed vegetables in Europe. The aim was to offer alternative agricultural practices that are more environmentally friendly. Treatments consisting of various rates of inorganic and organic fertilizers (composts) were assayed in quadruplicate on a sandy loam soil classified as Haplic Calcisol in Southern Spain. Macro- and micronutrients and heavy metal concentrations were analysed in soil and plant leaves. Parameters such as soil basal respiration (BR), microbial biomass C (Cmic) and dehydrogenase (DHA) and hydrolase activities, indicators of soil microbial abundance and activity, were also determined after each crop. After the second lettuce crop, certain physical soil properties such as aggregate stability and soil water holding capacity were also determined. In the first crop lettuce yields in the compost-treated soils did not significantly (p≤ 0.05) differ from the yields of CIF-treated soils. In the second crop, however, lettuce yields were higher in organically treated soils than in soils receiving only inorganic fertilization. Organic fertilizers were able to supply sufficient nutrients to achieve similar yields as those obtained with additional inorganic fertilizer. After the second crop, organically treated soils also showed higher C, N, P concentrations and higher Cmic, BR and DHA than soils receiving conventional inorganic fertilization as well as improved physical conditions (a higher percentage of stable soil aggregates and a greater water holding capacity). Also after the second lettuce crop, compost-treated soils showed significantly higher β-glucosidase and phosphatase activity than CIF-treated soils. Furthermore, lettuces grown in compost-treated soils showed significantly lower leaf nitrate concentration than lettuces grown in soil receiving CIF, thus reducing the potential toxicological risks associated with consuming nitrate-rich foods. Moreover, compost fertilization did not increase the concentration of heavy metals in lettuce leaves. Results confirm that manure and sewage sludge composts can be used as an alternative to inorganic fertilization in lettuce crop cultivation, leading to similar or even higher yields and reducing nutrient-leaching risks, while improving soil chemical, physical and microbiological properties.