S07 - Session O3 - Shading effect of trees reduces CO2 but not CH4 and N2O fluxes in urban lawns

Authors: Tom Kunnemann *, Patrice Cannavo, René Guénon, Vincent Guérin

Soils in urban green spaces (UGS) can store significant quantities of organic carbon (C), particularly in the case of lawns largely represented in cities. Lawns are often contrasted in terms of vegetation cover since they are usually associated with heterogeneously distributed trees. Trees create a local microclimate (shade, rainfall, …), modifying biogeochemical cycles (input of litters, symbiotic fungus, …) and can therefore have an important influence on greenhouse gas (GHG) emissions and consequently on C storage. The objective of this study was to understand how the physicochemical properties of soils and microclimatic (temperature and humidity) effects of trees can influence GHG emissions in urban lawns. Soil atmosphere exchange of CO 2 , CH 4 and N 2 O was measured monthly from March to November 2021, with infrared analyzers (CFLUX-1, PP Systems, USA and a DX4040, Gasmet, FINLAND) in 15 UGS composed of open and treed lawns in Angers city (France). Air temperature and soil moisture content were measured simultaneously with GHG fluxes and soil properties (bulk density, pH, conductivity, C and N content) were quantified. We observed that CO 2 fluxes and soil surface temperature were significantly higher in open lawns compared to treed lawns (1216 and 791 mg CO 2 .m - ².h - ¹ respectively with 2,4°C higher in mean during growing season). Soil moisture did not explain the difference in soil respiration. The strong correlation found between CO 2 flux and temperature whatever the soil type, clearly indicates that soil respiration was driven by temperature. However, during this experiment, water availability was not a limiting factor and could change the latter relationship. CH 4 and N 2 O fluxes showed no significant difference between treed and open lawns. These results highlight that treed lawn systems presented less C loss as CO 2 compared to open lawns and could therefore enhance the C sink capacity of urban lawns but night efflux need also to be quantified. 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