Relating soil microbial activity to water content and tillage-induced differences in soil structure

Several studies have identified optima in soil water content for aerobic microbial activity, and this has been ascribed to a balance between gas and solute diffusivity as limiting processes. We investigated the role of soil structure, as created by different tillage practices (moldboard ploughing, MP, or shallow tillage, ST), in regulating net nitrification, applied here as an index of aerobic microbial activity. Intact soil cores were collected at 0–4 and 14–18 cm depth from a fine sandy (SAND) and a loamy (LOAM) soil. The cores were drained to one of seven matric potentials ranging from − 15 to − 1500 hPa and subjected to measurements of gas diffusivity prior to incubation at 20 °C for 31 days. Net nitrification was calculated from nitrate accumulation during incubation. The upper layer of ST and MP soil had similar physical properties in terms of bulk density, pore size distribution, and relative gas diffusivity (RD). The soil from the 14–18 cm layer under ST had reduced air-filled pore space and RD at any given matric potential, and MP soil had a significantly higher volume of pores not active in gas diffusion (blocked pores). Net nitrification in the 0–4 cm layer was significantly higher for ST than for MP soil, while the opposite was true for the 14–18 cm layer. The net tillage effect calculated per mass of soil across the same area of land was negligible (e.g. SAND: 20.8 and 21.9 kg NO3–N ha− 1 for the MP and ST soils, respectively). Net nitrification generally increased with water content to a maximum and then decreased. This relationship was modelled with a second order polynomium. Model parameters did not show any tillage effect on the optimum water content, but the optimum coincided with a lower matric potential in ST (SAND: − 140 to –197 hPa; LOAM: − 37 to − 65 hPa) than in MP soils (SAND: − 42 to − 67 hPa; LOAM: − 15 to − 22 hPa). Net nitrification for a given soil and depth peaked at about the same level of RD irrespective of tillage treatment (SAND, 0–4 cm: 0.040; SAND, 14–18 cm: 0.015; LOAM, 0–4 cm: 0.010; LOAM, 14–18 cm: 0.003). Our study suggests that gas diffusivity may become one indicator of conditions for aerobic microbial activity, but more studies are needed to reveal soil type dependent drivers in play.