Submitted to: Soil & Tillage Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/2001
Publication Date: N/A
Interpretive Summary: Our understanding of how fluids move through soil is insufficient to help solve many important agronomic and environmental problems. We know certain soil management practices result in increases in water infiltration rates, but we do not understand what changes in soil physical properties are responsible. This research project measured soil pores formed by roots and earthworms (biopores) in tilled and untilled cropping systems known to produce different water infiltration rates. Surprisingly, it was discovered that the biopore sizes and numbers differed little, except that there were more of the very rare larger pores in the no-till plots. It was also found that a large portion of biopores are too small to detect using methods common among researchers. This research makes two major contributions to our understanding of the flow of water and fluids in soils. First, tilled and untilled cropping systems can have similar biopore arrangements, but still differ in water infiltration rates, indicating that other characteristics or very few large pores are responsible for most of the infiltration differences. Second, biopore estimates are dependent upon the resolution of the methods used, and biopores are a significant, often unmeasured proportion of soil macropores. Soil physicists are concerned with the characterization of soil pores, especially well connected pores which have been found to spoil the predictive ability of most current water flow models. Characterizing biopores by use of existing characterization of root geometry may prove to be a giant step toward successful mechanistic fluid flow models.
Technical Abstract: 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 hundred-fold increase in detection of biopores in the 0.3 to 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 to 0.5 mm diameter make up over half of total biopore volume and might have a significant role in movement of water and gases.