Net ecosystem exchange of grassland in contrasting wet and dry years

Temperate grasslands represent about 32% of the earth's land area and cover approximately 56% of the area of Ireland; yet their role as sources/sinks of atmospheric CO2 is not well quantified. We used an eddy covariance (EC) system to measure the net ecosystem exchange (NEE) at a managed grassland site in southern Ireland for 2 years. Rainfall in 2002 and 2003 was 1785 and 1185 mm, respectively, compared to an annual average of 1470 mm. The EC measured NEE was less in the wet year (−193 ± 50 g C m−2, uptake) than in the dry year (−258 ± 50 g C m−2, uptake). Combining NEE measurements with estimates of the components of the farm scale carbon (C) balance we estimated the amount of C fixed to the soil as −24 ± 62 g C m−2 for 2002 and −89 ± 62 g C m−2 for 2003, indicating that this ecosystem was a small sink for carbon. For the same months in different years, we found that the NEE was similar, although their soil moisture status was very different. This was due to the fact that the soil moisture status in this region, even in dry periods, was always well above the wilting point which resulted in no moisture stress on the vegetation at any time over the 2 years. We concluded that the NEE for this humid grassland ecosystem was not very sensitive to the variation in precipitation over the 2 years. We found that herbage harvesting had a direct effect of reducing the NEE in the month of harvest. We conclude that the interannual variation in NEE of 65 g C m−2 is of the order of uncertainty of the EC measurements.