Partnerships aimed at adding value to organic residues on Réunion Island

The rational management of organic matter produced locally by agricultural, agro-industrial and urban units is a real challenge for Reunion Island. However, this organic matter represents an important opportunity for sugarcane farmers who are heavily dependent on the importation of mineral fertilisers. Four projects have been initiated to optimize the use of these resources from agronomic, economic and environmental perspectives. These projects propose to answer four major questions about the optimal use of organic matter resources to replace mineral fertilisers either totally or partially: 1) what are the environmental impacts; 2) what are their fertilising properties; 3) how to optimize fertilisation advice to farmers; and 4) how to manage the available materials? In response, CIRAD through the SOERE-PRO project, is assessing the environmental impacts of organic matter applications by means of a highly instrumented experimental system. This includes time domain reflectometry and use of tensiometers, lysimeters and an automated soil gas flux measurement system for N2O and CO2 determinations. This mechanism allows the impacts of organic matter to be followed within the soil, air, water and plant 'compartments'. At the same time, eRcane and CTICS, within the TERO project, are utilizing four test sites, corresponding to different soil and climate conditions, to characterize the composition of organic matter and evaluate their nitrogen (N) fertilising power. This will be done by determining apparent use coefficients and equivalences to fertiliser N. Closer to the farmers, the Ferti-Sol project of La Chambre d'Agriculture proposes to use soil analyses to develop balanced fertilisation trials with organic residues and mineral fertilisers. The use of these manures will be compared to the farmers' current fertilisation practices. The results will provide fertilisation advices that will allow for the farmers to improve their practices. Finally, in order to optimize the management of organic matter, the multi-partnership Gabir project, led by CIRAD, will analyze the biomass transfers between stakeholders. They will also simulate various valuation scenarios based on the principles of a circular economy. These four projects are conducted in partnership and complementarity by the following research and development organizations: CIRAD, eRcane, CTICS and Chambre d'Agriculture. This initiative reflects Reunion Island's intention for sustainable management of the territory resources over the long term, especially through agriculture

Non-destructive sampling procedures for studying nitrogen use efficiency throughout the sugarcane crop growth cycle

Nitrogen (N) is a fundamental nutrient in agroecosystems, but has considerable negative environmental impact when used in excess. Meeting N demand while reducing excess N can be achieved by improvements in N use efficiency (NUE) of a crop agroecosystem. This is generally determined destructively (i.e. by cutting the cane) at the end of the crop cycle during harvest, where NUE is typically higher than during initial phases of crop development. Here we present the findings of a methodological study that aimed at developing non- destructive estimations of NUE throughout the crop cycle. We investigated the ability of allometric relationships to estimate sugarcane biomass by testing different functions and measurable traits (cane height and basal diameter) on six sampling dates. A dilution curve was constructed to predict N content of the sugarcane biomass by harvesting 20 stalks of cane on five separate dates. The number of harvested stalks was then rationalised to reduce the amount of harvested cane without affecting the ability of the curve to predict N content. As N derived from fertiliser (Ndff) can be determined from a representative leaf using an 15N-labelling approach, 15N content of each leaf on three stalks of cane was determined on four sampling dates. This enabled identification of the most representative leaf for determining 15N content of the biomass. The percentage difference and the associated error between sugarcane biomass predicted by allometric models and 2.5m harvested plots varied depending on the combinations of measurable traits used for the allometric relationships. The values for cane: 1) height on each date; 2) height x diameter on each date; and 3) height x diameter using a global relationship across sampling dates was 2±3 %, 2±3 % and 5±3 % respectively. In order to construct an adequate dilution curve, we determined statistically that a minimum of 5 sugarcanes needed to be sampled. The leaf with an 15N content most representative of the entire cane aboveground biomass was L+1 at 3, 8 and 11 months and L+3 at 5 months of growth (L+1 is the first leaf from the top with a visible dewlap). Allometric relationships appear to predict sugarcane biomass effectively at a plot scale. Models developed for each month may provide slightly better predictions, but a single global model across dates using cane height combined with basal diameter is best practically. The results indicated that sugarcane N concentration can be determined by non-destructive means using dilution curves based on sampling of five sugarcane stalks once a month during the cane cycle. In order to determine the Ndff, only a single leaf from the cane plant needs to be harvested rather than the entire plant. These methods make it possible to study NUE by using mostly non-destructive techniques throughout the crop cycle

Assessment of the susceptibility of roads to flooding based on geographical information – test in a flash flood prone area (the Gard region, France)

International audienceIn flash flood prone areas, roads are often the first assets affected by inundations which make rescue operations difficult and represent a major threat to lives: almost half of the victims are car passengers trapped by floods. In the past years, the Gard region (France) road management services have realized an extensive inventory of the known road sub- mersions that occurred during the last 40 years. This inven- tory provided an unique opportunity to analyse the causes of road flooding in an area frequently affected by severe flash floods. It will be used to develop a road submersion suscep- tibility rating method, representing the first element of a road warning system.This paper presents the results of the analysis of this data set. A companion paper will show how the proposed road susceptibility rating method can be combined with dis- tributed rainfall-runoff simulations to provide accurate road submersion risk maps.The very low correlation between the various possible ex- planatory factors and the susceptibility to flooding measured by the number of past observed submersions implied the use of particular statistical analysis methods based on the general principals of the discriminant analysis.The analysis led to the definition of four susceptibility classes for river crossing road sections. Validation tests con- firmed that this classification is robust, at least in the con- sidered area. One major outcome of the analysis is that the susceptibility to flooding is rather linked to the location of the road sections than to the size of the river crossing structure (bridge or culvert)

Green-roof as a solution to solve stormwater management issues? Assessment on a long time period at the parcel scale

International audienceExperimental green-roof rainfall–runoff observations have shown a positive impact on stormwater management at the building scale; with a decrease in the peak discharge and a decrease in runoff volume. This efficiency of green-roofs varies from one rainfall event to another depending on precipitation characteristics and substrate antecedent conditions. Due to this variability, currently, green-roofs are rarely officially used as a regulation tool to manage stormwater. Indeed, regulation rules governing the connection to the stormwater network are usually based on absolute threshold values that always have to be respected: maximum areal flow-rate or minimum retention volume for example. In this context, the aim of this study is to illustrate how a green-roof could represent an alternative to solve stormwater management issues, if the regulation rules were further based on statistics. For this purpose, a modelling scheme has been established at the parcel scale to simulate the hydrological response of several roof configurations: impervious, strictly regulated (in terms of areal flow-rate or retention volume), and covered by different types of green-roof matter. Simulations were carried out on a long precipitation time period (23 years) that included a large and heterogeneous set of hydrometeorological conditions. Results obtained for the different roof configurations were compared. Based on the return period of the rainfall event, the probability to respect some regulation rules (defined from real situations) was assessed. They illustrate that green-roofs reduce stormwater runoff compared to an impervious roof surface and can guarantee the respect of the regulation rules in most of the cases. Moreover, their implementation can appear more realistic than that of other infrastructures strictly complying with regulations and demanding significant storage capacity

Connaître les fertilisants et les outils disponibles pour un pilotage raisonné

La valorisation de la fertilité de son sol passe par une connaissance et une utilisation raisonnée des fertilisants (amendements et engrais) à sa disposition. Pour cela, nous disposons d'outils d'aide à la décision qui gagnent à être mieux connus et mieux utilisés

Variability of nitrogen use efficiency of urea in Réunion

In Reunion, sugarcane covers more than half of the cultivated area. Its production is highly dependent on the importation of mineral fertilisers such as nitrogen (N), a major driver for yield. Fertiliser materials from agro-industrial, urban and farming waste products (MAFOR) are locally available. Although these are important sources of nutrients, their fertiliser value, particularly N, remains unknown. The TERO project was launched in 2014 for this purpose. The behaviour of eight “MAFOR” products are being compared with urea (the most commonly used mineral fertiliser) via response curves. TERO has implemented four trials in sites in high-contrast production areas - an altitude gradient of more than 500 m, average minimum temperatures between 15.6 and 20.8 °C and maximum temperatures between 22.7 and 28.7 °C, rainfall variation of 3,500 mm and soils with variable physico-chemical properties. In each trial, the N requirements (X) were calculated from soil analysis results and based on the expected cane yield. Response curves were obtained from five treatments (0X, 0.5X, 0.75X, 0.9X, 1.5X) replicated three times. In the plant crop, half of the urea was buried at planting and the other half was spread on the soil surface four months later. Cane (including trash and tops) was harvested and weighed. The N content (NDumas) and N recovery was determined. Nitrogen use efficiency (NUE) values were calculated from the N response curves. Data from three trials were available for this poster. Nitrogen response curves and NUE were very different from site to site. The trial conducted in the north of the island, on a Nitisol at an altitude of 50 m, resulted in a response curve that was sigmoidal, with an NUE of 0.35. The trial in the west, on a brown Dystric Cambrisol (Humic) at an altitude of 570 m, resulted in a response curve that was also sigmoidal, but with an NUE of only 0.12. In contrast, the trial conducted on the eastern side of the island, on an Haplic Umbrisols at an altitude of 330 m, resulted in a linear response between 0X and 0.9X. The calculated NUE was 0.92, highlighting better recovery of urea. The variability in the responses to N, from one production area to another, justifies the diversity of trial sites in order to evaluate the NUE of the available MAFOR. The study will be conducted over two seven-year cane cycles to assess the direct and indirect effects of eight MAFOR products

Modeling sugarcane growth and yield using STICS model, parameterization and applications in complex agro-systems

Sustainable and resilient sugarcane production requires agro-systems to improve nutrient use efficiency through nutrient recycling, reduced herbicide applications and reduced system outflows by erosion and/or leaching. In that context, crop models are particularly useful tools to assess the agronomic and environmental performances of simple (monoculture) or complex (multi-species) agro-systems. Among others, the STICS model, developed and applied in temperate climate, could be used to assess technical management in sugarcane tropical plantations. In this study we present the STICS model parameterization and validation for two sugarcane cultivars (R570 and R579) using the CIRAD ECOFI database that includes about 100 monoculture trials on Reunion Island from 1995 to 2016. The model's ability to accurately simulate sugarcane yield and growth in more complex systems was assessed for different management scenarios using data from recent trials by contrasting: i) mineral fertilization regimes and organic residue applications, based on trials in the eRcane TERO project, and ii) multi-species cropping systems with cover crop, based on trials in the eRcane CanecoH project. Despite being only recently applied in tropical areas and in sugarcane cropping systems, the STICS crop model appeared to be an accurate tool to simulate both monoculture and complex sugarcane agro-systems. Its successful combination with large-scale vegetation models, previously done in temperate areas, opens opportunities for the community of modelers of tropical agriculture to assess agronomic and environmental performances of sugarcane at regional scale in tropical areas