Changes in litter chemistry and soil lignin signature during decomposition and stabilisation of C-13 labelled wheat roots in three subsoil horizons

Despite their importance for C sequestration, especially in the subsoil, little is known about decomposition and stabilisation processes affecting root litter in soil horizons at different depths. In particular the influence of specific conditions at depth on molecular alterations of degrading root litter is unknown. We took advantage of a decomposition experiment, which was carried out at different soil depths under field conditions and sampled litterbags with C-13-labelled wheat roots, incubated in subsoil horizons at 30, 60 and 90 cm depth for up to 36 months. Changes of bulk root chemistry were studied by solid-state C-13 NMR spectroscopy, and lignin content and composition was assessed after CuO oxidation. Compound-specific isotope analysis allowed assessment of the role of root lignin for soil C storage at the different soil depths. Results indicated that decomposition proceeded in a similar way at all three depths, but at a different rate. The alkyl/O-alkyl C ratio was a meaningful indicator to assess the degree of root litter degradation within the mineral soil. After three years, the greatest increase of this ratio, corresponding to the most advanced degradation degree, occurred at 30 cm compared to the lower depths despite a similar carbon loss. The greater proportion of O-alkyl C persisting in deeper subsoil horizons was consistent with their higher clay content. Root derived lignin-C concentration decreased at all soil depths and soil lignin content reached a similar level after 12 months, suggesting that microbial communities in all subsoil depths had capability to degrade lignin. However, the intensity of degradation appeared to be different at different soil depths, with lignin being less transformed at 60 and 90 cm depth. We conclude that chemistry of subsoil organic matter is determined by horizon-specific conditions, which have to be fully understood in order to explain the long residence times of subsoil C. In our study physico-chemical parameters only partly explained the observations.