Ammonia sources and sinks in an intensively managed grassland using dynamic chambers

Abstract

Abstract. Grassland is a canopy with a complex structure where sources and sinks of ammonia may coexist at the plant level. Moreover, management practices such as mowing, hay production and grazing, may change the composition of the sward and hence the source-sinks relationship at the canopy level as well as the interaction with the atmosphere. There is therefore a need to better understand the exchanges of ammonia between grasslands and the atmosphere, especially regarding the locations of sources and sinks and their magnitudes. Fluxes of atmospheric ammonia (NH3) within a grassland canopy were assessed in the field and under controlled conditions using a dynamic chamber technique (cuvette). These cuvette measurements were combined with extraction techniques to estimate the ammonium (NH4+) concentration and the pH of a given part of the plant or soil, leading to an estimated ammonia compensation point (Cp). The combination of the cuvette and the extraction techniques was used to identify the potential sources and sinks of NH3 within the different compartments of the grassland: the soil, the litter or "litter leaves", and the functioning "green leaves". A set of 6 field experiments and 6 laboratory experiments were performed in which the different compartments were either added or removed from the cuvettes. This study shows that the cuvette measurements agree with the extraction technique in ordering the strength of compartment sources. It suggests that in the studied grassland the green leaves were mostly a sink for NH3 with a compensation point around 0.1–0.4 μg m−3 NH3 and an NH3 flux of 6 to 7 ng m−2 s−1 NH3. Cutting of the grass did not increase the NH3 fluxes of the green leaves. The litter was found to be the largest source of NH3 in the canopy, with a compensation point up to 1000 μg m−3 NH3 and an NH3 flux up to 90 ng m−2 s−1 NH3. The litter was found to be a much smaller NH3 source when dried (Cp=160 μg m−3 NH3 and FNH3=35 ng m−2 s−1 NH3). Moreover emissions from the litter were found to vary with the relative humidity of the air. The soil was a strong source of NH3 in the period immediately after cutting (Cp=320 μg m−3 NH3 and FNH3=60 ng m−2 s−1 NH3), although always relatively smaller than the litter source. The soil NH3 emissions were, however, not lasting more than one day, and were not observed with sieved soil. Soil emissions not be solely explained by xylem sap flow extruding NH4+. These results indicate that future research on grassland-ammonia relationships should focus on the post-mowing period and the role of litter in interaction with meteorological conditions.