Research project SR/10/143 (Research action SR)
Understanding the dynamics of soil organic carbon (SOC) in response to environmental change is posing significant scientific challenges: SOC stocks in croplands are large compared to their annual increase or decrease, and their spatial and vertical variability, even within a single field, is large. As a result, large numbers of samples need to be analysed to obtain a correct estimate of the quantity and the vertical distribution of SOC within a field. Visible and Near Infrared Reflection spectrometry (VNIRS) techniques and in particular field and image spectroscopy have the advantage of producing soil property measurements in situ and can therefore quickly cover large areas.
Objective
The aims of the project are: i) develop a methodology to represent the spatial variation of SOC within the first meter of cropland soils integrating surface data from image spectroscopy and vertical SOC gradients from VNIRS profiling; ii) evaluate the potential of the methodology based on the estimation of the SOC stocks for an entire field, and vertical SOC profiles at any point within the field.
Method
For surface SOC content, an area of ~350 km2 will be over flown by an aircraft carrying the Airborne Prism Experiment (APEX) sensor. More than 400 soil samples will be taken for SOC analyses in 50+ fields across the study area for calibration and validation of the images. For SOC content in the first meter of the soil, an adapted head attached to an ASD spectrometer will be lowered in the soil in c. 10 recently ploughed fields. A correlation will be established between the SOC content and the spectral information (corrected spectral data from the APEX in the case of surface SOC and the ASD recordings for the profiling) using calibration samples.The continuous surface SOC data (imaging spectroscopy) with the point data on SOC at different depth (spectral profiler) will be integrated in a spatial model. Bulk density, required to convert SOC contents in SOC stocks, will be determined from sections of the cores needed to lower the profiler in the soil using the traditional volume/weight method.
Result
Proximal sensing techniques, such as those based on spectroscopy, can provide an efficient way for the data density required both at the surface and within the first meter of the soil. As remote sensing only records the characteristics of the first millimeters of the soil, this requires a combination of image spectroscopy and in situ spectral measurements of soil profiles. A calibration model will be established between the SOC content and the spectral information (at-surface reflectance data from the APEX in the case of surface SOC and the ASD recordings for the profiling). Once validated, the SOC contents will be used as input for spatial models producing a 3D SOC inventory. The advantage of such models is that they give an insight in the factors determining the spatial patterns and that they can be used to predict the vertical SOC distribution at un-sampled points. The main outcome of the project will be a methodology that allows remote sensing techniques to be combined with spectral profiling and thus determine the spatial and vertical patterns of soil properties. The proposed method will be able to map the SOC content for soil depth intervals up to one meter, produce SOC profiles for any point selected and when integrated both laterally and vertically produce a SOC stock for the field.
