Drilosphere dynamics in subsoil horizons : insights from an anthropo-chrono-sequence from Parisian basin

<p>Soil structure dynamics is considered to be 'rapid' with a characteristic response time to changing environmental conditions of between 10 and 100 years (Targulian & Krasilnikov, 2007). Although land-use or agricultural practices are known to induce soil structure changes, there is a lack of quantitative data about the associated dynamics especially at generational time scale (Brown et al., 2000) and in subsoil horizons as eartworms are supposed to be less active in subsoil horizons (Sauzet et al., 2023). Biological processes are widely known to influence soil structure and earthworms are recognised as major biological agents in temperate climates (Lee & Foster, 1991). The quantification of earthworm biostructures (burrows and casts) in soil profiles help to quantify the dynamics of the sphere of influence of earthworms (i.e. the “drilosphere”) and therefore their influence on soil functioning (Piron et al., 2012).</p> <p>This study aims thus at characterizing soil structure changes in subsoil horizons induced by earthworms after (i) a change in land use, and (ii) the introduction of a common agricultural practice, i.e., organic amendments input.</p> <p>In our investigation, we built an anthropo-chrono-sequence of three Luvisols near Paris (France). The first one is under a deciduous forest (FOR) and the second one is under conventional agricultural management (CULT) both with no land use change for the past two centuries. The last one is a cultivated Luvisol with regular manure application (FYM) since 15 years. Thanks to a new and validated image analysis procedure on large thin sections (Sauzet et al., 2017), macroporosity and papules were quantified in subsoil horizons (from 30 to 160 cm depth) to assess drilosphere dynamics.</p> <p>The volume of bioturbated soil varies between 70% and 20% of the total soil volume whatever the modalities. This result suggests that soils present a juxtaposition of a structure under the control of earthworm activity with a structure largely independent of it due to re-bioturbation, i.e, the tendency of earthworms to remobilise the same soil masses over the long term (Wilkinson et al., 2009). The differentiation of the CULT and FYM profiles shows that 150 years of cultivation without the addition of organic matter resulted in an increase of eartworm macroporosity in the Bt horizon. As this porosity concerns the first meter of soil, the average increase of macroporosity was at least 0,16% per year. The differentiation of the CULT and FYM profiles suggests that about 15 years of exogenous organic matter input was sufficient to induce the disappearance of part of the porosity associated with earthworms on the first meter of soil. The dynamics of macroporosity disappearance seems thus to be faster than the dynamics of creation.</p> <p>This anthropo-chrono-sequence based study points out the fact that the drilosphere is more reactive to agricultural driving forces than commonly thought in subsoil horizons. Furthermore, this evolution implies several feedbacks on other soil processes such as a potential increase of water infiltration (Rounsevell et al., 1999) that may explain the highest intensity of illuviation quantified in the CULT profile in Sauzet et al. (2016).</p>