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Effects of applying exogenous organic matter onto soil on carbon storage, greenhouse gases emissions and nitrate leaching in periurban agricultural soils (PROSTOCK)

Effects of applying exogenous organic matter onto soil on carbon storage, greenhouse gases emissions and nitrate leaching in periurban agricultural soils (PROSTOCK)

Summary
The PROSTOCK project develops within the context of peri-urban agriculture and the aim of recycling organic waste of either agricultural or urban origin (or exogenous organic matter, EOM) in keeping with the perspective of restoring soil organic carbon stocks (SCS) in agricultural areas. Its main objective was to spatially assess the potential of EOM recycling for improving SCS over these areas. The secondary objective was to assess the resulting possible impacts of EOM use on greenhouse gas emissions (CO2, N2O in this project) and nitrate leaching. Moreover, PROSTOCK included a methodological aim oriented towards the feasibility assessment of monitoring SCS variations using spectroscopy and remote sensing in the visible near and shortwave infrared range (NIRS).
The spatial level considered was that of small peri-urban agricultural regions, where waste management and its resulting effects on water and soil quality operate. The Versailles Plain and the Alluets Plateau, a peri-urban region covering 221 km² (of which 100 km² are croplands), unique in terms of available data, spatial and thematic representativeness and references on EOMs, was chosen as study region.
A database of field and lab measurements (reflectance, roughness, bulk density, moisture, physico-chemical analysis…) and observations (soil surface conditions, amendment practices and cultural operations) with unprecedented size was collected and structured from more than 250 point locations spread over the cropped areas. Some fifteen remote sensing images from varied satellite and airborne sensors were acquired over the course of 3 field campaigns of field measurements synchronous with sensor acquisitions in 2011, 2012, 2013.
This project enabled considerable progress in using imaging data related to field spectral measurements, for the purpose of estimating topsoil organic carbon content (SOC) from soil reflectance spectra. The accuracy that is expectable from such method not only depends on sensor type, but also on atmospherical and angular conditions of imaging, and cultural operations, which influence the performance of image atmospheric correction into reflectance units. Conversely to predictions obtained from lab or even field reflectance spectra, multispectral satellite images with medium spatial resolution do not enable to discriminate changes in SOC resulting from EOM applications over a large 15 years-duration, but they result in a rough mapping of SOC over large extents. This project also enabled to demonstrate the potential of optical/radar synergy for detecting cultural operations, which otherwise can hardly be spatially surveyed from interviews to farmers. In the same prospect of facilitating surveys, this project favoured ongoing studies about very high resolution Pleiades images showing their potential for mapping crop types and phenological stages, as well as for locating those fields having recently received EOM application.
More, PROSTOCK enabled to parameterize the CERES-EGC agro-environmental model which was chosen for the purpose of simulating SCS dynamics according to various EOM applications. This parameterized model succeeds in retrospectively predicting temporal trends of SCS as observed over the course of 10 years for the QualiAgro Experiment. The inventory of actually of potentially available EOMs over the study region reveals multiple agronomical scenarios of amendment practices with EOM, depending on either their stocking or their fertilizing properties, through their proportion of easily available N: nitrogen mineralization and CO2, N2O emissions dynamics were characterized in this project for the main EOMs identified. The main N2O-emitter soils are now identified according to the EOMs applied. All required elements are therefore ready for the spatial modeling of simulations, to anticipate future evolutions.
The PROSTOCK project paves the way for numerous related questions addressing the validity domain of agri-environmental models when soil types and cultural operations vary at the scale of a small agricultural region. It relied on both support and commitment from the study area farmers and appears to be rich in further developments for their common benefit, that the large size of collected data permits to anticipate. It could be continued and add value to its results with the prospect of spatially assessing soil ecosystem services according to the effects of EOM application.

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Contact
AgroParisTech (Emmanuelle Vaudour)

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