Regulation of soil surface respiration in a grazed pasture in New Zealand

This study investigated the regulation of soil surface respiration (R-s) in a grazed pasture, in New Zealand. The interactive responses of R-s to soil temperature (T-s) and root zone volumetric water content (theta) were quantified and modelled using soil cores in controlled conditions. Chamber measurements of R-s, made at a field site approximately twice a month throughout a growing season, were used to validate the model. A closed gas-exchange system was used in the field to measure ecosystem respiration (R-e) and R-s in order to partition the contribution of above- and below-ground respiration. This result was then applied to partition nighttime measurements of R-e using eddy covariance into R-s and above-ground respiration (R-a). Eddy covariance soil surface respiration (R-sEC) estimates were compared to modelled estimates of R-s. The response of R-s to T-s could be described using an Arrhenius-type function, while the response of R-s to normalised theta(theta(n)) was described by linear relationship. The model showed that when theta(n) was >0.9, R-s remained constant. In the field, R-s, was strongly influenced by T-s, which ranged from a minimum of 12.2 degrees C to a maximum of 20.1 degrees C throughout the measurement period. During the year, theta was rarely low enough to limit R-s. The base value of respiration at 10 degrees C (R-10) at the field site was 0.21 mg CO2 m(2) s(-1). The model was able to explain half of the variability observed in the field measurements of R-s. Total R-e was comprised of 84% R-s and 16% R-a. For nights with valid eddy covariance data, the estimate of R-sEC from measurements of nighttime R-e multiplied by the fraction of below-ground to total respiration was 48% less than the estimate from the model. Total growing season R-s estimated using the model was 1.94 kg (C) m(2). (C) 2008 Elsevier B.V. All rights reserved.