Biophysical characterization of soil aggregate fractions in different agroecosystems

Aggregates contribute to determining the structure of the soil and form the physical space and the habitat in which microorganisms live and play their role by regulating soil functioning. Consequently, the study of the biophysical properties of aggregates can be an effective tool for assessing what influence soil management has on its functionality, and especially on carbon sequestration. Interdisciplinary research on the biophysical properties of aggregates therefore needs to be carried out in order to assess the effect of management on the biophysical properties of different aggregate size classes. In this study we investigated the biophysical parameters of macroaggregates (4-1 mm) mesoaggregates (1-0.25 mm) and microaggregates (<250 μm) in soils under an alfalfa crop and oak wood (representative of a mountain agroecosystem), and under three walnut sites (representative of plain agroecosystem) characterized by differing urea distribution (one site was fertirrigated with 90 kg liquid urea/ha-1, one site received 90 kg granular urea/ha-1, one site acted as the control without urea addition). We assumed that different aggregate classes (different microhabitats) have specific biophysical properties and the spatial relationship between organic matter and pores should be different in aggregate classes, regulating soil carbon sequestration function. Our biophysical characterization showed that the aggregate classes investigated were easily distinguishable microhabitats. The soil management effects depended on aggregate size. Soil organic matter input and N fertilization affected the soil organic matter availability for microorganisms in macroaggregates. The aggregation process, by contrast, seemed more relevant for the C dynamics in meso- and microaggregates, thus in aggregates <1 mm. Indeed, thin aggregate sections confirmed that mesoaggregates were microhabitats in which a great accumulation of organic matter occurred as stable and transformed amorphous forms, as a result of aggregate genesis.