Effect of soil moisture conditions on CO2 exchange and carbon budget of managed grassland in central Europe

Managed agricultural ecosystems form a significant part of the European land cover. Greenhouse gas mitigation options like the conversion from arable land to permanent grassland and an optimized management could help to sequester carbon in agricultural soils. However, the annual carbon budget is also influenced by the regional climate conditions, which might change considerably in the future. Especially the soil moisture conditions are critical for the productivity and the carbon cycling of temperate grassland.

We have monitored the carbon cycle of a managed grassland site on the Swiss Central Plateau since 2002 as part of the FLUXNET and CarboEurope-IP network. The grassland was established in 2001 by conversion from arable land and is used for producing hay and silage. The experimental field is divided in two plots, the one undergoing intensive management (high nitrogen input), the other extensive management (no fertilization). Continuous eddy covariance (EC) measurements and the quantification of carbon export and import by harvest and manure application allow for the assessment of the complete carbon budget of both plots. In addition, partitioning of EC fluxes and static chamber measurements were used to analyze the dependence of CO2 respiration and assimilation on environmental parameters including soil temperature and moisture conditions.

The most dramatic event during the observation period occurred in connection to the summer heat wave in 2003, when the soil water content dropped below the wilting point for more than two months, which is highly unusual for the region. It led to a significant reduction in the productivity of the ecosystems. The low soil moisture had also a negative influence on soil respiration, which was, however, partly compensated by the positive effect of the high temperatures. During the drought period, minor rain events were only able to wet the uppermost few centimetres of the soil. They led to almost immediate increase of soil respiration while having no positive effect on CO2 assimilation by plants. As a result of the 2003 summer drought, the net ecosystem exchange of CO2 was strongly reduced compared to the other years and the annual carbon budget of the grassland fields was shifted towards a net loss.