The importance of subsoil when investigating soil organic persistence in perennial agroecosystems within Alpine valleys

E.R. FOLEY, R. TIZIANI, L. BORRUSO, E. TASKIN, S. CESCO Faculty of Agricultural, Environmental and Food Sciences, Free University of Bolzano, Bolzano, ITALY
T. MIMMO Faculty of Agricultural, Environmental and Food Sciences, Free University of Bolzano-Bozen, Bolzano- Bozen, ITALY, Competence Centre for Plant Health, Free University of Bolzano-Bozen, Bolzano-Bozen, ITALY

ABSTRACT

Studies on soil carbon (C) dynamics to date focus primarily on topsoil, despite an estimated ~50% of soil C being stored in subsurface soils. These deeper soil layers are thought to be dominated by C that has formed mineral-associations and is more persistent than its labile counterpart. Understanding the efficacy of this could be critical in developing practices aimed to mitigate climate change by increasing C storage in agricultural soils. The aim of this study is therefore to explore differences in soil C fractions in topsoil and subsoil in perennial agroecosystems, to gain an understanding of how C dynamics change along the soil horizon. For this purpose, soil samples at 0-30 cm and 30-60 cm were collected from four apple orchards in South Tyrol, Italy. Samples were analysed for labile C fractions (dissolved organic carbon (DOC), humic and fulvic acids (HA/FA), hot water extractable carbon (HWEC), permanganate oxidisable carbon (POXC), particulate organic matter (POM)) and the stable C fraction (mineral-associated organic matter (MAOM)).
Results indicate that soil samples taken from deeper layers contained significantly lower concentrations of
labile C fractions (HWEC, POXC and POM) across all sites, with HWEC measuring up to 2 times lower in
subsoil. However, when examining the MAOM, results show the proportion of stable (MAOM) to labile (POM) C was significantly higher in the subsoil, with MAOM constituting ~73% in subsoil, and just ~61% in topsoil. Furthermore, the concentration of C in the MAOM was ~2 times higher than in the POM. These
results highlight the importance of subsoil in the study of soil C storage, as indicated by the larger
proportion of MAOM. Furthermore, it underlines the importance of identifying and quantifying C fractions at different sampling depths and understanding their associated turnover rates. This will facilitate a deeper comprehension of C cycle dynamics within these environments and will in turn provide insight into the relevance of subsoil when studying C dynamics in agroecosystems.

Keywords: Carbon sequestration,Carbon storage,Subsoil,Carbon fractions,Apple orchards