Spatial variation of ammonia volatilization from soil and its scale-dependent correlation with soil properties

Quantitative predictions of ammonia volatilization from soil are useful to environmental managers and policy makers and empirical models have been used with some success. Spatial analysis of the soil properties and their relationship to the ammonia volatilization process is important as predictions will be required at disparate scales from the field to the catchment and beyond. These relationships are known to change across scales and this may affect the performance of an empirical model. This study is concerned with the variation of ammonia volatilization and some controlling soil properties: bulk density, volumetric water content, pH, CEC, soil pH buffer power, and urease activity, over distances of 2, 50, 500, and > 2000 m. We sampled a 16 km x 16 km region in eastern England and analyzed the results by a nested analysis of (co)variance, from which variance components and correlations for each scale were obtained. The overall correlations between ammonia volatilization and the soil properties were generally weak: -0.09 for bulk density, 0.04 for volumetric water content, -0.22 for CEC, -0.08 for urease activity, -0.22 for pH and 0.18 for the soil pH buffer power. Variation in ammonia volatilization was scale-dependent, with substantial variance components at the 2- and 500-m scales. The results from the analysis of covariance show that the relationships between ammonia volatilization and soil properties are complex. At the > 2000 m scale, ammonia volatilization was strongly correlated with pH (-0.82) and CEC (-0.55), which is probably the result of differences in parent material. We also observed weaker correlations at the 500-m scale with bulk density (-0.61), volumetric water content (0.48), urease activity (-0.42), pH (-0.55) and soil pH buffer power (0.38). Nested analysis showed that overall correlations may mask relationships at scales of interest and the effect of soil variables on these soil processes is scale-dependent.