Are variations in soil aggregate sizes and soil depth key drivers of soil biological properties?

Soil aggregates are considered the fundamental building blocks of soil structure (Reeves et al., 2019), and their formation is closely tied to the content of soil organic carbon (Bronick and Lal, 2005). As key indicators of soil quality, soil aggregates play a critical role in determining soil health and functionality.

Previous studies have shown that macro-aggregates tend to exhibit higher rates of organic carbon mineralization and microbial respiration compared to micro-aggregates (Rabbi et al., 2014). However, conflicting results have emerged from other research, suggesting that micro-aggregates may harbor higher levels of microbial activity (Zhang et al., 2013). These inconsistencies highlight the need for further investigation into the role of different aggregate fractions in soil microbial dynamics and activity.

The major objectives of the present study were as follows (i) Identification of bacterial composition by analyzing, both taxonomically and functionally, in two distinct soil aggregate sizes (2-4 mm and 1-2 mm), (ii) evaluate the aggregate stability analysis in the 1-2 mm fraction, and (iii) investigate the impact of land-use practices such as tree growth and animal grazing in permanent pastures on soil aggregate properties. Additionally, these characteristics were assessed at two different soil depths: 0-15 cm and 15-30 cm

The analysis was conducted in a silvopastoral system located in Boimorto (A Coruña, Galicia, NW Spain), an area with a mean annual temperature of 12.6 °C and 1898 mm of mean annual precipitation. The study focused on Juglans regia plantations owned by the private company Bosques Naturales S.A., including hybrid walnut trees (Juglans major MJ 209 x Juglans regia) planted at densities of 6 m x 6 m (277 trees ha⁻¹) and 4m x 4m (and 625 trees ha^-1). For controlling weeds grow the owner included extensive sheep grazing. Composite soil samples were collected along the tree line, in the open pasture area, and within grazing exclusion chambers across three replicate plots to assess their contribution to reducing soil degradation.

Results on soil aggregate stability, soil enzymatic activities (involved N, C and P cycles) and taxonomic composition of bacterial communities in the different locations sampled and aggregates sizes will be presented.

Acknowledgments: This study was funded by the “Ramón y Cajal” fellowship (ref: RyC 2021-615 033370-I) financed by the “Ministerio de Ciencia Innovacion y Universidades” (Spain) and the Regional Goverment of Galicia (Programa de consolidación de unidades de investigación competitivas del SUG. Proyectos de excelencia ED431F 2024/024 Xunta de Galicia)

References

Bronick, C.J., Lal, R., 2005. Soil structure and management: a review. Geoderma. 124 (1–2), 3–22.

Rabbi, S.F., Wilson, B.R., Lockwood, P.V., Daniel, H., Young, I.M., 2014. Soil organic carbon mineralization rates in aggregates under contrasting land uses. Geoderma. 216, 10–18. 

Reeves, S.H., Somasundaram, J., Wang, W.J., Heenan, M.A., Finn, D., Dalal, R.C., 2019. Effect of soil aggregate size and long-term contrasting tillage, stubble and nitrogen management regimes on CO 2 f luxes from a Vertisol. Geoderma. 337, 1086–1096. 

Zhang, S., Li, Q., Lü, Y., Zhang, X., Liang, W., 2013. Contributions of soil biota to C sequestration varied with aggregate fractions under different tillage systems. Soil Biol. Biochem. 62, 147–156.