Wavelet analysis of the correlations between soil properties and potential nitrous oxide emission at farm and landscape scales

We analysed data on nitrous oxide emission rate and some other properties of soil cores collected on a 7.5-km transect across contrasting land uses and parent materials in eastern England. The variation on this landscape-scale transect was compared with that of a comparable set of measurements made in a previous study on within-farm spatial scales in the same region. We used the wavelet transform to analyse the scale dependence and spatial uniformity of the correlations between soil properties and emission rates from farm to landscape scale. The analysis revealed a complex pattern of scale dependence. Soil organic carbon was correlated with emission rates at within-field scales only. There was a pronounced correlation between emission rates and a process-specific function of the water-filled pore space, seen only at landscape scales. Emission rates were strongly correlated with soil nitrate content at intermediate and coarsest scales (and significantly, although weakly, at the finest within-field scales), and with pH at the intermediate scales. The wavelet analysis showed that these correlations were not spatially uniform. The correlation between nitrate concentration and emission rates at the finest landscape scale (between approximately 60 and 120 m) was not significant in the northern part of the transect corresponding primarily to soils over the Lower Greensand, but these variables were significantly correlated at this scale over other parent materials. These findings have implications for modelling and inventory of nitrous oxide emissions from soil. They indicate that, at the landscape scale, nitrate content and water-filled pore space are key soil properties for predicting nitrous oxide emissions and should therefore be incorporated into process models and emission factors for inventory calculations.