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The Effect of organic fertiliser application on the nitrogen biogeochemical cycle of sugarcane crops in Réunion Island

Abstract : Nitrogen fertilisers have contributed substantially to global food security and nutrition. However, when accumulated in excessive amounts in ecosystems, and the atmosphere, they lead to significant negative environmental impacts. There is frequently a large disparity between what is supplied by fertilisation and what is used by crops, leading to low nitrogen use efficiencies (NUE) of fertilisers. The recycling of organic residues in agroecosystems could be a promising alternative or complement to synthetic fertilisers, and a means to promote circular economic and agricultural sustainability. The overall aim of this PhD thesis was to evaluate and provide a detailed quantitative and temporal account of N inputs and outputs into and from a highly monitored experimental site cultivated with sugarcane. Secondly, it was to determine the fate of N from two types of organic fertilisers (pig slurry and methanised, limed, pelleted sewage sludge) as compared to mineral fertiliser, in the soil-sugarcane system. The evolution of sugarcane biomass and total N mass accumulation was measured monthly over 24 months for the four distinct fertiliser types: unfertilised, urea, pig slurry, and sewage sludge. The results revealed that the proportion of plant N contained in the roots could be considerable, representing up to 65 and 104 % of aboveground N mass in the urea and unfertilised treatments respectively.A combination of minimally destructive methods is proposed to determine the NUE throughout the sugarcane growth-cycle. Microplots containing 15N labelled urea or mulch were assessed over the two experimental years to study the respective contributions of different fertiliser sources to sugarcane N content. The mulch and previous fertiliser applications provided a constant but low contribution of less than 5 %, with mineral and organic fertilisers contributing 9.6-17.8 % and 4.4-7.1 % of the sugarcane N respectively. The soil was by far the largest source of N, providing a minimum of 74 % of the sugarcane N content. Calculations of fertiliser NUE were evaluated on a monthly basis with the difference and isotopic dilution methods, highlighting 1/ a difference between the values calculated using the two methods, which is reduced when the root component is considered, 2/ a pronounced decrease in the NUE over the last 6 months of the sugarcane growth-cycle when using the isotopic method suggesting a deficit in 15N which should be further interrogated, 3/ a particularly low NUE of 9.2 – 16.1 % for the reference fertiliser urea, partly as a result of a particularly high level of N loss via volatilisation. The N output from the sugarcane-soil system studied in detail was the leaching of fertiliser N. The N content in soil solutions and the corresponding water flux was studied using TDR probes and porous cups at three soil depths for the four fertiliser treatment types. The estimated losses at a depth of 100 cm did not exceed 18.3 kgNha-1 for the different fertiliser treatments. This result is probably in part as a result of the soil’s capacity to retain nitrates, as well as importantly due to the effective N uptake of the sugarcane after fertiliser N application, A complete budget of N flux at the scale of the agroecosystem was established for each fertiliser treatment, and revealed high levels of N loss for the urea and pig slurry fertilisers (36 % and 63 % respectively), as well as a particularly high level of immobilisation for the sewage sludge treatment (70 % of applied N). In conclusion, this thesis highlights 1/ the central role of soil as a major source of N, and 2/emphasised the important role of the root component of sugarcane, and 3/ the need to improve the efficiency of fertiliser use by lowering the level of volatilisation in Réunion. The use of organic fertilisers as a partial substitute for mineral fertilisers appears to be a good means to supply additional N to sugarcane and to the soil, while limiting environmental pollution.
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Daniel Mika-Nsimbi Poultney. The Effect of organic fertiliser application on the nitrogen biogeochemical cycle of sugarcane crops in Réunion Island. Agricultural sciences. Université de la Réunion, 2021. English. ⟨NNT : 2021LARE0007⟩. ⟨tel-03342497⟩

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