Comparing pasture C sequestration estimates from eddy covariance and soil cores

Temperate pastures in the Northeast USA are highly productive and could act as significant sinks for soil organic carbon (SOC). However, soils under mature pastures are often considered to have reached equilibrium such that no further sequestration of SOC is expected. This study quantified changes in pasture SOC over nine years using micrometeorological (eddy covariance) measurements of net changes in ecosystem C and direct measurements of changes in SOC (soil cores). Eddy covariance estimates of C flux were made both with and without corrections for sensor self-heating. Change in SOC as measured by the soil core method was non-significant for the Low-N pasture (19±105gCm^-2yr^-1, P=0.89). However, a significant loss of -504±91gCm^-2yr^-1 (P=0.01) occurred for the High-N pasture with the amount lost increasing with depth in the soil profile. Eddy covariance measurements without the self-heating correction differed only slightly between pastures with a net flux of -9 and -20gCm^-2yr^-1 for the Low- and High-N pastures, respectively. Applying the sensor self-heating correction to the eddy covariance data increased the estimated loss of ecosystem C by 94gm^-2yr^-1 for the Low-N and 102gm^-2yr^-1 for the High-N pasture. Both soil cores and eddy covariance suggest that the Low-N pasture was C neutral over the nine years of the study. A large amount of C was lost from deep in the soil profile from the High-N pasture which could not be explained by fluxes measured with the eddy covariance system. Comparison of eddy covariance and soil core data was not useful for determining the appropriateness of using the sensor self-heating correction at this location.