Paysages virtuels et analyse de scénarios pour évaluer les impacts ...

Abstract and key words
The environmental assessment of agrosystems is a critical issue for the design of sustainable agricultural
practices and for the preservation or recovery of the ecological functions of agrosystems. However, this
assessment is difficult to achieve through expert opinion and experimentation alone, due to the diversity of
transformation and transfer processes of organic matter within agricultural landscapes, their interactions, and
their dynamics. This project aimed at proposing a spatialized and dynamic environmental assessment method
to integrate the effects of the physical environment, climate, and agricultural practices.
Our approach was structured by the development of a landscape modeling platform (Qualscape), which
provided the conceptual framework for the different steps of this study:
– analysis of observed field crop sequences to identify the spatial and temporal drivers of crop
transitions, because landcover plays a major role in the location of agricultural practices;
– the construction of a set of virtual agricultural landscapes based on the intensively farmed Naizin
catchment (12 km 2 , Morbihan department, France) that differed in terms of climate, soil-type
patterns, and crop management;
– the multi-target and multi-criteria assessment of these agrosystems by coupling and integrating the
outputs of the nitrogen (N) fate model TNT2 and a soil phosphorus (P) budget model.
In addition, Stochastree, a crop-transition model based on stochastic decision trees, was developed. It
maintained the spatial distribution of crops around the farmsteads while accounting for other constraints such
as soil waterlogging and farm-production objectives. Environmental assessments of the virtual landscapes
showed the relative influence of climate, soil-type patterns, and cropping systems on a set of agricultural,
soil, and hydrological indicators. Although a moderate, less-intensive cropping system reduced soil P
surpluses and improved N- and P-use efficiencies, climate series and soil-type patterns influenced the stream
N concentration more heavily than cropping systems. Moreover, the magnitude of soil-climate interactions
on stream N concentrations was equivalent under certain scenarios to a 15% reduction in the amount of total
N applied. This shows that remediation practices aimed at achieving a specific environmental goal (such as
not exceeding the 50 mg NO3.l -1 threshold in streams) must take local landscape characteristics into account,
and that transplanting them to another landscape may not attain the expected goal.
Key words: virtual landscape, environmental assessment, agrosystem, soil, climate, agricultural practice,
fertilization, nutrient, nitrogen, nitrate, phosphorus, hydrology, TNT2, Qualscape, Stochastree, Rotomatrix
Abstract – p. 5