Abstract:
Soil organic carbon (SOC) is an important component of soils for the various goods and
services that soils perform. But SOC stocks have declined significantly in soils around
the world over many years due to poor land management. To enable land managers and
policy makers to manage SOC better, simple guideline values and measures of SOC
concentration are needed. An index based on the SOC to clay concentration ratio as
related to soil structural conditions was tested for soils across England and Wales using
data from the National Soil Inventory (NSI). Threshold values of SOC/clay equal to 1/8,
1/10 and 1/13 indicated Very Good, Good, Moderate and Degraded levels of SOC.
Land use was a driver of SOC/clay ratio, with 38% of arable soils classed as Degraded
compared with < 7% of permanent grass or woodland soils. To examine how SOC/clay
ratios have been changing over time, I analysed data from resampled sites in the NSI
(mean interval of 15 years). The Very Good class was particularly vulnerable to losses
compared with other classes. This finding agrees with SOC protection being limited by
soil clay concentration. Long-term experiments on soils of contrasting clay
concentration showed that the index was sensitive to management activities. In further
work I explored the use of dry soil spectral analysis to measure SOC and clay
concentrations. I compared dry spectral and conventional wet laboratory analyses of
soils in the NSI and in the US National Soil Survey Center-Kellogg Soil Survey
Laboratory spectral library (NSSC-KSSL). The NSSC-KSSL results, and to a lesser
extent the NSI results (which used older, less-accurate wet laboratory analyses), showed
that the technique is suitable for assigning soils to Very Good, Degraded, or
Good/Moderate ranges. The index provides quantitative guideline concentrations for
SOC with a functional basis and scope for rapid assessment.