Soil Organic Carbon (SOC) is the primary means of directly measuring carbon sequestration in agricultural soils. SOC is a measurable component of Soil Organic Matter (SOM), which is a commonly measured soil property. SOC can be measured in most scientific and commercial soil labs and is considered a standard soil test. Although SOC measurements in soil samples are straightforward, the development of representative soil sampling methodologies for SOC at the field and landscape scale poses challenges that are reflective of biophysical factors that drive changes in SOC.
At the field and landscape scales, SOC tends to have significant temporal and spatial variability. Temporally, SOC levels fluctuate seasonally as a result of annual fluctuations in temperature and precipitation. Over the longer term (five or ten year intervals, for example), SOC levels can be slow to change, even with shifts to climate-friendly BMPs. SOC measurements must be timed in a way that permit accurate sampling over seasonal and multi-year timescales. Spatially, SOC is tied to biophysical factors such as historical and present-day land management practices, soil type distribution, landscape position, and climate (Abram 2020; Department of Agriculture and Food, Western Australia 2013; World Bank 2021)
In order to accurately reflect temporal and spatial SOC variability at the field and landscape scales, sampling methodologies must rely upon geospatial approaches. But in the grey literature and scientific publications on SOC measurement, geospatial approaches are being overlooked. For example, a recent World Bank carbon sequestration guidebook provides detailed information on soil sampling and testing, but glosses over the geospatial work required to ensure representative soil sampling (World Bank 2021). Similarly, a recent study on SOC in Ontario agricultural landscapes utilizes a geospatial approach, but the geospatial methodologies used are not made explicit and do not align with contemporary land use and land cover research (Mazzorato 2022). Geospatial methodologies for accurate temporal and spatial SOC sampling can largely rely upon geospatial data collected in the field or public geospatial datasets. The below table lists geospatial methodologies that should be considered mandatory for building accurate SOC sampling methodologies. These methodologies are fundamental to the fields of remote sensing and GIS and can be developed using standard textbooks and scientific articles in these fields (Campbell and Wynne 2011; Islam, et al. 2019; Bassett and Zuéli 2000)
|Geospatial Methodology||Data Type(s)||Description|
|site boundary delimitation with GPS + GIS||GPS field boundaries; georeferenced vertical aerial photographs||Enables accurate field boundaries to be drawn using GIS software to process and combine field GPS boundary data and aerial photographs|
|site stratification via aerial photographs||field GPS boundary data; georeferenced vertical aerial photographs||Enables the accurate delimitation of different land use and land cover types at the field scale, such as separating areas of tree or perennial grass cover from cropping areas|
|soil sampling with GPS||GPS soil sampling site coordinates||Enables accurate GPS coordinates to be taken at each soil sampling site. Essential for repeated sampling over multi-year periods|
|land use and land cover surveys and interviews||quantitative and qualitative data; supplements site stratification process (above)||Land use and land cover history is often poorly documented. Robust methodologies which pair land manager interviews and surveys with historical air photos and vegetation maps can be used to gain insight into historical patterns of land use and land cover. It is necessary for surveys and interviews to be conducted in a manner which aligns with the site stratification process (above)|