Modelling the daily soil respiration of an Irish cropland incorporating aboveground biomass as a variable

Many cropland agroecosystems are characterized by phases where there is little vegetation cover (fallow period) interspersed with the main crop growing periods and soil respiration can be strongly influenced by this change in land cover. For fallow soil, soil respiration is driven by heterotrophic metabolism, which is mainly driven by soil temperature and soil moisture, whilst in the presence of vegetation, the additional contribution of autotrophic respiration by plant roots has to be taken into account.

To examine the influence of vegetation cover measurements of soil respiration, soil temperature, soil moisture and aboveground biomass were taken from two barley crops grown in the Irish Midlands that differed in their management: one was subjected to conventional tillage (CT), the other to non-inversion tillage, with mustard as a winter cover crop (NIT+CC).Based on these measurements, we modelled daily soil respiration using three independent valuables: soil temperature, soil moisture and aboveground biomass. The relationship between soil respiration and soil temperature was described by an exponential function, but the relationship to soil moisture was determined as linear. This resulted in a model fit of R²=0.27 for the CT and R²=0.20 for the NIT+CC treatments, respectively.

To include the effect of aboveground biomass a quadratic function was used; this increased the model fit to R²=0.51 for CT and to R²=0.42 for NIT+CC, respectively. A quadratic function was chosen in order to take account of changes in the development of the plants. After sowing soil respiration increases due to a high relative growth rate and increasing root biomass. Later, when the plants are more mature relative growth ratio and root respiration decreases. This, and the manner in which root development occurs, was found to be specific for species examined. The model fit for NIT+CC significantly increased to R²=0.58 if the barley and the mustard growing seasons were modelled separately. These results highlight the importance of considering a biotic factor, e.g. aboveground biomass, for modelling daily soil respiration and its use should improve the accuracy of estimates of the annual C budgets of croplands.