Ammonia sources and sinks in an intensively managed grassland canopy

• Grasslands represent canopies with a complex structure where sources and sinks of ammonia (NH3) 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-sink relationship at the canopy level as well as the interaction with the atmosphere. There is therefore a need to understand the exchange of ammonia between grasslands and the atmosphere better, especially regarding the location and magnitude of sources and sinks. Fluxes of atmospheric 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 (NH+4 ) concentration and the pH of a given part of the plant or soil, leading to an estimated ammo- nia compensation point (Cp ). The combination of the cuvette and the extraction techniques was used to identify the poten- tial sources and sinks of NH3 within the different compart- ments of the grassland: the soil, the litter or senescent “litter leaves”, and the functioning “green leaves”. A set of six field experiments and six laboratory experiments were performed in which the different compartments were either added or removed from the cuvettes.The results show that the cuvette measurements agree with the extraction technique in ranking 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 and   an NH3 flux of 6 to 7 ng m−2 s−1. 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 Cp of up to 1000μgm−3 NH3 andanNH3 fluxupto90ngm−2 s−1. The litter was found to be a much smaller NH3 source when dried (Cp =160 μg m−3 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 and FNH3 =60 ng m−2 s−1 ), which was nevertheless always smaller than the litter source. The soil NH3 emissions lasted, however, for less than one day, and were not observed with sieved soil. They could not be solely explained by xylem sap flow extruding NH+4 . 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.

Ämnesord

NATURVETENSKAP  -- Biologi -- Ekologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Ecology (hsv//eng)

Nyckelord

COMPENSATION POINT

ATMOSPHERIC AMMONIA

OILSEED RAPE

EXCHANGE

PLANT

SOIL

PARAMETERS

FLUXES

LEAVES

WHEAT

Terrestrial ecology

Terrestrisk ekologi

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