Pore-size distribution and compressibility of coarse sandy subsoil with added biochar

Sustainable agricultural production on coarse sandy soil is constrained by the restricted growth of roots, and poor water and nutrient retention. Amending the soil with biochar can reduce these problems, but the processes involved are not known in detail. We investigated in the laboratory the effects of two fine-grained gasification biochars made of straw (LTST) and other materials (LTSN) and of one fast pyrolysis straw biochar (FPST) on pore-size distribution and soil compressibility when added to coarse sandy subsoil. Water retention and therefore pore-size distribution were affected systematically. All biochars converted drainable pore space with pore diameters in the range 60–300 µm into water-retaining pores of size 0.2–60 µm, which was taken as an estimate of available water capacity (AWC). Effects were linear over the whole range of biochar (0–4% by mass). The effect of LTST and LTSN on AWC (3.6% by volume per % biochar) was about 70% larger than the effect of the somewhat coarser FPST biochar (2.1% by volume per % biochar). The compression index increased linearly with biochar content without any significant effects from the type of biochar. The common least squares estimate of the slope was 21.2 kg m−3 %−1 by weight. The results reflect a strong interaction at the microscopic scale between biochar and soil rather than intrinsic properties of the added biochar.