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Effects of organic residue application on the biogeochemical cycle of nitrogen in sugarcane agroecosystems on Reunion Island

Poultney D., Versini A., Feder F., Thuriès L.. 2018. Saint Gilles : ISSCT, p. 39-39. ISSCT Agricultural Engineering, Agronomy and Extension Workshop : ¿"Farming for the future: improving productivity and ecological resilience in sugarcane production systems". 3, 2018-09-23/2018-09-28, Saint Gilles (Réunion).

Reunion Island is a densely populated Overseas Territory of France in the Indian Ocean. There is an increasing need for waste management, given the rising population density and the associated increase in waste production. This waste can take the form of human waste (sewage sludge) and livestock waste (animal manure). The primary means of adding value to this waste would be to use it as organic fertiliser in agroecosystems. Sugarcane is the most abundant crop on the island - 54 % of the 43 000 hectares allocated as agricultural land, across 9272 farms. Recycling agricultural waste can be a means of improving the economic sustainability of sugarcane plantations, as local products are recycled and the quantity of fertiliser imported is reduced. This in turn promotes a circular economy on the island. Nitrogen (N) is an essential nutrient for crop growth and development, and is frequently a limiting factor in sugarcane agroecosystems. Despite the clear benefits of N fertilisers in contributing to adequate crop nutrition, excessive amounts accumulated in terrestrial and aquatic ecosystems can lead to a significant impact on environmental quality, ecosystems, biodiversity and human health. Environmental impacts linked to the N cycle include the loss of N to the atmosphere in the form of NH3 during the process of volatilization and in the form of the greenhouse gas N2O during the process of nitrification and denitrification. Excess N is lost via leaching in the form of nitrates, which can lead to negative impacts on freshwater ecosystems. Here we present the findings of one-complete-year monitoring of nitrogen fluxes (N uptake, N2O and NO3) in sugarcane plots fertilized with liquid pig manure and sewage sludge as compared with the use of urea fertiliser. The study is being conducted on Reunion Island in a long-term monitoring field trial site belonging to the SOERE-PRO network, where the objective is to observe the effects of organic residue application on the different 'compartments' of a sugarcane agroecosystem. The contribution of organic residues to N sugarcane (nitrogen recovery efficiency) was assessed on a monthly basis using 15N labelling, dilution curves, cane inventories (height and basal diameter) and allometric relationships. The annual flux of N2O was estimated for the different treatments using 12 automatic chambers linked to a gas analyser. These semi- continuous measures allowed the temporal variability of N2O emissions to be studied, in the original context of organic residue application in tropical conditions. Dissolved organic nitrogen (DON), NH4 and NO3 leaching in the soil was studied using a mixed system, consisting of porous cups under tension at depths of 10 and 40 cm; and lysimetric plates at a depth of 1m.

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