Soil biopore estimation: effects of tillage, nitrogen, and photographic resolution

Networks of biopores created by plant and animal activity might accumulate in untilled cropping systems. These would be relatively well connected to the soil surface. The objective of this study was to count biopores after long-term no-till in comparison to recently tilled soil. Biopores were counted and measured to 80 cm depth at 10 cm increments in plots either under no-till wheat production for 1 year or for 17 years, and receiving zero or 130 kg ha⁻¹ N. The measurements were repeated using different photographic methods with increased resolution. The only significant difference between the long and short term no-till was in biopore sizes over 1 mm diameter, where long-term no-till produced from 30 to 100% more biopores, probably caused by increased earthworm activity. Over 99% of biopores measured were less than 1 mm diameter. There was no difference between tillage or N treatments in the number of these smaller biopores at any depth. This means small biopores did not accumulate either above or below the plow layer in an untilled cropping system. Improved resolution in the second set of measurements produced a 100-fold increase in detection of biopores in the 0.3–0.5 mm range. This provides evidence that a substantial portion of biopores are very small and were missed in the first year of this study and perhaps in other studies of this type. It is hypothesized that biopores of 0.05–0.5 mm diameter make up over half of total biopore volume and might have a significant role in movement of water and gases.