Effects of biochar on the dynamics of aggregate stability in clay and sandy loam soils

Recent advances suggest that organic substances of different origins might have different aggregate stability dynamics. We investigated the extent to which contrasting soil types affect the dynamics of aggregation after the addition of crop residues (R) and of biochar at two doses (BC20, 20Mgha(-1); BC40, 40Mgha(-1)) in a 2-year experiment. To evaluate disaggregation, we measured a set of physical-chemical and structure-related properties of clay and sandy loam aggregates sieved to 1-2mm, including wet aggregate stability after different pretreatments combined with laser diffraction analysis. The electrochemical properties of the colloidal suspension were also analysed to identify changes in soil chemistry affected by organic inputs. Different amounts of added biochar and soil types produced contrasting effects on wet aggregate stability. In sandy loam, the increased soil surface area from added biochar (at either dose) offset the initial small soil organic carbon (SOC) content and subsequently promoted SOC-controlled aggregation. Conversely in clay soil, the larger biochar dose (BC40) strengthened the repulsive forces between particles with the same charge and monovalent cations, which led to chemical perturbation and some aggregate breakdown not found with BC20. Pore structure also changed in clay aggregates. A shift towards more micropores (30-5m, +29% more than in the control) and ultramicropores (5-0.1m, +22% more than in the control), which contributed to aggregate stabilization, resulted when biochar was added, but not for residue. Our results suggest that biochar promotes aggregate stability, which, in turn, improves the physical fertility of soil, especially if it has a coarse texture and small organic carbon content. Further study is needed of the physical-chemical interactions between added biochar and surface-charged clay-rich soils.