New insights into the rates of soil formation and their contribution to our understanding of soil lifespans

Evans, Daniel and Quinton, John and Davies, Jessica and Tye, Andrew and Mudd, Simon (2020) New insights into the rates of soil formation and their contribution to our understanding of soil lifespans. PhD thesis, Lancaster University.

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Abstract

Sustaining the provision of services by soils for future generations has become a critical goal for soil scientists. Preventing soil thinning and the exposure of the underlying parent material is paramount for achieving this goal. Soil thinning occurs when rates of soil erosion exceed those of soil formation. Although measuring soil erosion has received widespread attention, there has not been a commensurate effort to obtain rates of soil formation, and this undermines our capacity to determine the long-term sustainability of our soils. This thesis responds to that sizeable knowledge gap in two ways: measuring soil formation rates at sites previously subject to soil erosion monitoring, and demonstrating how rates of soil formation and erosion can be used to estimate soil lifespans. Cosmogenic radionuclide analysis was used to measure the rates of soil formation across four UK hillslopes. These included the first measurements of their kind for arable soils, under some of the thickest soil profiles that have been subject to this technique. For the first time, the CoSOILcal model was used to account for the effect of variable soil bulk density on the attenuation of cosmic rays. A sensitivity analysis showed that accounting for these bulk density data for profiles thicker than 0.25 m brings about a significant difference in the calculated rates of soil formation. The rates obtained for the four hillslopes studied here fell within the range of those previously published for similar climates and lithologies. However, it was also found that rates were faster for lithologies with a greater porosity and a reduced matrix. At one of the sites, rates of soil formation and erosion were used to calculate the lifespans for both the A horizon and the profile to bedrock. The shortest lifespans were found on the backslope, with the loss of the A horizon and bedrock exposure occurring in 138 and 212 years, respectively, in a worst-case scenario. Longer lifespans were observed for less erosive zones such as the toeslope where the time until bedrock exposure was more than triple that of the backslope. Similar analyses of soil lifespans were undertaken at the global scale. Combining over 10,030 plot years of erosion data from 255 sites with rates of soil formation, 93% of conventionally managed soils were found to be thinning, and 16% reported lifespans of less than 100 years. However, adopting conservation land-use and management practices were found to extend these lifespans, with over a third exceeding 10,000 years.