Monitoring the 3-D Evolution of Soil Macropore Networks Under Natural Boundary Conditions.

Soil pore-networks are always evolving with time. The topsoil macro-pore network in conventionally managed arable soils is for example heavily modified due to annual tillage operations. Along with the changes of the soil pore-network, the soil physical properties are changing. Nevertheless, they are still modelled as temporally stable in state of the art transport models for water, gas or solutes through soil. One reason for why temporal variations in soil physical properties are commonly not included into soil system models is the lack of quantitative knowledge on the evolution of soil structure under natural boundary conditions. I am demonstrating in this study that 3-D time-lapse X-ray imaging provides abundant data to fill this knowledge gap.

An undisturbed soil sample (7 cm diameter and 10 cm height) was taken from a loamy allotment-soil in Uppsala (Sweden) in June 2013. The soil had been hand-plowed and seeded with rocket (eruca vesicaria) prior to the sampling. Directly after sampling, a 3-D X-ray image was taken and the soil column was installed back in the allotment, with its bottom open to the soil below. It remained there until October of the same year when it was once more removed from the allotment for the next 3-D X-ray scan. Again the sample was put back into the field after the image acquisition. The whole procedure was repeated another four times, March 2014, May 2014, September 2014, and March 2015. The six reconstructed 3-D X-ray images were registered and their brightness and contrast levels were normalized. Soil phases associated with air and fresh organic matter were extracted from the normalized images.

The results demonstrate that the column was visited by earthworms, which had dug several burrows during the first half year of the experiment. After this time no new burrows were dug but there are signs that old burrows had been reused. Furthermore, there is evidence that also other macrofauna had been visiting the soil column, i.e. in form of a nest of eggs (from snails?), a spider burrow and some larvae feeding on roots. Rocket roots and the roots of a dandelion that had established itself on the column during summer 2013 had grown out of the soil column into the soil below. The macroporosity increased from 9% directly after plowing to over 14% and only receded back to approximately 8% due to the growth of the dandelion’s tap root from autumn 2014 onwards. The average macropore diameter rose from 1 mm to over 2.5 mm during the first 4 months of the experiment, obviously due to the earthworm burrowing. It only slightly decreased to approximately 2 mm during the rest of the experiment. Increasing Euler numbers of the largest macropore cluster show that the macropore network evolved from a well-connected structure immediately after plowing to a more preferentially connected one.