63 Greenhouse Lettuce: Assessing Nutrient Use Efficiency of Digested Livestock Manure as Organic N-Fertiliser A. Trinchera, V. Baratella, S. Rinaldi, M. Renzaglia, A. Marcucci and E. Rea Consiglio per la Ricerca e la Sperimentazione in Agricoltura Centro di Ricerca per lo Studio delle Relazioni tra Pianta e Suolo Rome Italy Keywords: bovine slurry, corn silage, co-digestion, horticulture Abstract Given the short cultivation cycle of many greenhouse crops, the need to provide them with adequate nutrition is critical, especially when organic fertilisation is applied: an organic N-fertiliser should be able to supply nitrogen at the proper time, without incurring luxury consumption or detrimental effects on the crop or environmental quality. Bovine livestock manure is characterised by high organic matter and N content, and its digestion or co-digestion under anaerobic conditions produces N-fertilisers applicable in organic greenhouse production. In a greenhouse experiment, plantlets of Lactuca sativa (‘Romana’) were transplanted into pots containing one of two soils with different textural, physical and chemical characteristics. Bovine livestock manure was digested alone or co- digested with corn silage, and the resulting solid portions of the digestates were used as organic fertilisers, applied at 200 or 400 kg N ha -1 , and compared with urea applied at the same rates and an unfertilised control. After six weeks, lettuce fresh and dry weight, plant water content, dry matter content, total leaf area, number of leaves per plant, specific leaf fresh and dry weight, leaf N-content and related N-use efficiency were determined. Soil characteristics partially influenced the agronomic performance of organic fertilisers on lettuce production. Digested and co-digested livestock manure gave higher biomass dry weight, dry matter content and in a particular case, higher specific leaf weight compared with urea fertilisation. Lettuce N-use efficiency of digestate and co-digestate was slightly lower than that recorded with urea, although it was strongly increased by digestate application at the lower rate in the loamy soil. INTRODUCTION Given the brevity of the cultivation cycle of many greenhouse crops, adequate nutrition is one of the most critical production factors, especially under an organically managed system. An organic N-fertiliser should provide nitrogen at the proper time, without resulting in luxury consumption and detrimental effects on vegetable and environmental quality (Tittarelli et al., 2009; Trinchera et al., 2012). A good source of organic N fertiliser for use in organic farming is the digestate from animal or vegetable residues, particularly as these residues are a common on-farm by-product. One such by- product, bovine livestock manure, has high organic matter and N content, and its digestion or co-digestion with vegetable residues under anaerobic conditions produces N- fertilisers acceptable for use in organic greenhouse production (Trinchera et al., 2013). When defining a fertilisation plan, amendment properties of composted or digested organic biomasses should be taken into account (Hammermeister et al., 2006), and the stability of the final organic matter content of a digestate (Trinchera et al., 2013) should be considered a key point. A proper digestion process guarantees the final product and does not cause dangerous phytotoxic effects on crops, as this risk is connected to a high volatile organic compound content that results from an incomplete fermentation process (Schievano et al., 2012; Tambone et al., 2009). The nutrient benefit of a given digestate ultimately depends upon the overall manure management of the producing unit, including the nature of the slurry (low solids and NH 4 -N contents), the post-digestion Proc. II nd IS on Organic Greenhouse Horticulture Eds.: M. Dorais and S.D. Bishop Acta Hort. 1041, ISHS 2014