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

This study investigated the regulation of soil surface respiration (Rs) in a grazed pasture, in New Zealand. The interactive responses of Rs to soil temperature (Ts) and root zone volumetric water content (θ) were quantified and modelled using soil cores in controlled conditions. Chamber measurements of Rs, 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 (Re) and Rs in order to partition the contribution of above- and below-ground respiration. This result was then applied to partition nighttime measurements of Re using eddy covariance into Rs and above-ground respiration (Ra). Eddy covariance soil surface respiration (RsEC) estimates were compared to modelled estimates of Rs. The response of Rs to Ts could be described using an Arrhenius-type function, while the response of Rs to normalised θ (θn) was described by linear relationship. The model showed that when θn was >0.9, Rs remained constant. In the field, Rs, was strongly influenced by Ts, which ranged from a minimum of 12.2 °C to a maximum of 20.1 °C throughout the measurement period. During the year, θ was rarely low enough to limit Rs. The base value of respiration at 10 °C (R10) 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 Rs. Total Re was comprised of 84% Rs and 16% Ra. For nights with valid eddy covariance data, the estimate of RsEC from measurements of nighttime Re multiplied by the fraction of below-ground to total respiration was 48% less than the estimate from the model. Total growing season Rs estimated using the model was 1.94 kg (C) m−2.