Quantifying aggregation bias in marsh carbon flux estimates caused by rhizosphere oxygen heterogeneity

Abstract

Oxygen (O 2 ) in marsh rhizospheres varies strongly at millimetre scales due to radial oxygen loss (ROL), re-oxidation of reduced compounds, hydrology, and sediment structure. Yet many models estimate heterotrophic (microbial) respiration at a single mean O 2 , which can bias CO 2 fluxes because the O 2 response of respiration is curved (nonlinear). We calibrated an enzyme kinetic respiration function to marsh-soil incubations, then compared (I) respiration evaluated at a single mean O 2 value with (II) the mean of different respiration fluxes over a prescribed rhizosphere O 2 distribution. To isolate oxygen effects, temperature and the relation between substrate and soil moisture were held fixed; incubations were used only to calibrate carbon kinetics. For a gas-phase, air-equivalent O 2 proxy, the single mean approach overestimates respiration by ∼ 12 % on average. This bias increases with increasing variation of O 2 and with the ratio of mean O 2 to the O 2 half-saturation constant KM,O2. In the dissolved phase, mean O 2 ≪ KM,O2 and variation in O 2 is small, so the bias is negligible due to linear behaviour of the relation in these conditions; when physiology is unit-matched to a similar relative saturation, however, the dissolved case shows a negative bias comparable to the gas-phase one. Our findings suggest reporting the mean and variation of O 2 together with a calibrated KM,O2 allows modellers to anticipate or correct the bias associated with replacing heterogeneous rhizosphere conditions with a single mean.