Elsevier

Geoderma

Volumes 226–227, August 2014, Pages 94-102
Geoderma

Soil organic carbon assessment by field and airborne spectrometry in bare croplands: accounting for soil surface roughness

https://doi.org/10.1016/j.geoderma.2014.02.015Get rights and content

Highlights

  • Soil roughness effect on SOCa by VNSWIR spectroscopy is studied.

  • Soil roughness is estimated by its shadowing effect on field vertical photographs.

  • Outdoor field and airborne and laboratory spectroscopy are used for SOCa.

  • SOCa by spectroscopy was improved with a shadow correction factor.

  • The shadow disturbing effect for SOCa was found wavelength dependent.

Abstract

Visible, Near and Short Wave Infrared (VNSWIR) diffuse reflectance spectroscopy (350 nm to 2500 nm) has been proven to be an efficient tool to determine the Soil Organic Carbon (SOC) content. SOC assessment (SOCa) is usually done by using calibration samples and multivariate models. However one of the major constraints of this technique, when used in field conditions is the spatial variation in surface soil properties (soil water content, roughness, vegetation residue) which induces a spectral variability not directly related to SOC and hence reduces the SOCa accuracy. This study focuses on the impact of soil roughness on SOCa by outdoor VIS-NIR-SWIR spectroscopy and is based on the assumption that soil roughness effect can be approximated by its related shadowing effect.

A new method for identifying and correcting the effect of soil shadow on reflectance spectra measured with an Analytical Spectral Devices (ASD) spectroradiometer and an Airborne Hyperspectral Sensor (AHS-160) on freshly tilled fields in the Grand Duchy of Luxembourg was elaborated and tested. This method is based on the shooting of soil vertical photographs in the visible spectrum and the derivation of a shadow correction factor resulting from the comparison of “reflectance” of shadowed and illuminated soil areas.

Moreover, the study of laboratory ASD reflectance of shadowed soil samples showed that the influence of shadow on reflectance varies according to wavelength. Consequently a correction factor in the entire [350–2500 nm] spectral range was computed to translate this differential influence.

Our results showed that SOCa was improved by 27% for field spectral data and by 25% for airborne spectral data by correcting the effect of soil relative shadow. However, compared to simple mathematical treatment of the spectra (first derivative, etc.) able to remove variation in soil albedo due to roughness, the proposed method, leads only to slightly more accurate SOCa.

Introduction

The monitoring of soil attributes and their evolution over time as well as the development of pedological models rely on the availability of accurate and extensive soil data. The high spatial variability of soils arising from both local and global factors of soil formation requires generally collecting soil information from a very dense network of sites. Diffuse reflectance spectroscopic techniques, and in particular Visible, Near and Short Wave Infra Red (VNSWIR) spectroscopy (350 nm to 2500 nm), allows rapid sampling and instantaneous determination of many soil properties, at field and regional levels (in remote sensing mode). This technique can provide in a cost effective way the large quantity of spatial data required in soil monitoring or modelling studies like the monitoring of decline of soil organic matter in the topsoil.

Spectral libraries and multivariate modelling are often used to predict soil attributes of unknown samples. In the laboratory, such approach has proven to provide accurate determinations of SOC (Viscarra Rossel et al., 2006). When using the same approach, field spectroscopy and hyperspectral remote sensing, however, may fail to produce reliable and robust determinations due to uncontrolled measuring conditions and spatial variation in surface soil properties. According to Atzberger (2000), the main factors affecting the soil reflectance are soil water content, vegetation residues and surface roughness.

For the purpose of this study, field spectroscopic measurements were taken over bare and dry soils. Hence soil roughness remained the main disturbing factor and other influences have not been taken into consideration.

The effect of roughness on soil reflectance has been addressed in several studies. Arnfield (1975) showed that, for a relatively rough soil surface, soil albedo is generally lower than for a corresponding smooth surface due to self shadows. Atzberger (2000) simulated the influence of soil roughness on reflectance by using the SOILSPEC model. He found that, when the soil becomes smoother, due to decreasing micro-shadow effects, soil reflectance increases throughout the visible range.

Geometrical models have been developed to simulate bidirectional reflectance of light from rough soil surface based on the assumption that reflection is strongly correlated with the area of shadowed soil as well as on illumination and viewing geometry. Even though these models have been validated (Cierniewski, 1987, Cierniewski and Verbrugghe, 1997) their application in field conditions is not trivial, since many input parameters have to be considered which are quite difficult to assess in practical cases.

Several geometrical models (Cierniewski and Verbrugghe, 1994, Cooper and Smith, 1985, Irons et al., 1992, Norman et al., 1985) predict soil reflectance based on the assumption that shadowing of soil aggregates or clods has a greater influence than the scattering properties of a soil (Cierniewski and Verbrugghe, 1997). This study is also based on this principle and therefore the influence of roughness on soil reflectance is estimated by assessing its shadowing effects. This approach has been recently validated by (García Moreno et al., 2008).

The purpose of this study is to propose a new method to identify and reduce the effect of soil Relative Shadow (RS, the percentage of shadowed soil of the studied surface) on the assessment of SOC content from VNSWIR hyperspectral (350–2500 nm) field and airborne spectroscopic data.

First a method to measure RS and to correct its impact on field reflectance, measured with an Analytical Spectral Devices (ASD) spectroradiometer and the Airborne Hyperspectral Sensor (AHS), is proposed. Secondly the impact of RS on reflectance and SOCa accuracy is studied under laboratory conditions. Then SOC content is predicted by using uncorrected and corrected field reflectance values to evaluate the improvement in SOCa accuracy achieved with this method. The proposed method is finally compared with well-known mathematical pre-processings that intend to enhance SOCa accuracy.

Section snippets

Study area

The study area consisted of a north-south transect of ~ 7 km width and ~ 60 km length (NW corner: 50°03′N 6°03′E; SE corner: 49°33′N 6°12′E), crossing 4 of the 5 agro-geological regions of the Grand Duchy of Luxembourg.

The Grand Duchy of Luxembourg is characterised by a large variability of soils on a relatively small area (2586 km2). The four agro-geological regions in the study area are:

  • The North, called Ardennes or Oesling, is a relatively homogeneous area consisting of plateaus and dissected

Method

The logic of the method aiming at enhancing SOCa by reducing RS effect is described below and consists of four steps:

  • 1.

    Computing shadow correction factors “K” for field reflectance from collected data

  • 2.

    Correcting field reflectance (ASD and AHS) with the “K” produced

  • 3.

    Estimating whether corrected reflectance improve SOCa

  • 4.

    Comparing the performance of the proposed correction method with existing methods

The global workflow of this study is presented in the Fig. 2 (steps 1–3).

Impact of soil Relative Shadow (RS) on reflectance and Soil Organic Carbon assessment (SOCa) accuracy based on laboratory data

Fig. 5 shows the reflectance decrease with increasing RS on one soil sample studied under laboratory conditions. Table 1 presents the impact of RS on SOCa accuracy. Generally increasing soil RS decreases SOCa accuracy. In particular the range of RS measured during the field campaign, as modelled in laboratory, decreases SOCa accuracy significantly. An increase of 20% of RMSECV is observed for the SOCa realised under the field RS range (RMSECV of 3.75) compared to the one realised under full

Discussion and conclusion

This study is based on the idea that the soil shadow, which is due to soil roughness in field conditions, decreases the accuracy of the SOCa from soil reflectance. This has been confirmed by laboratory experiments that showed that the accuracy of SOCa from prepared soil samples decreases when the RS level applied on these soil samples increases. As a result the purpose of this study was to find a method that reduces the RS effect on soil reflectance in order to enhance SOCa.

A new method for

Acknowledgements

The research in this paper is funded by the Belgian Science Policy Office and the National Research Fund of Luxembourg in the framework of the STEREO II program – Project “Monitoring soil organic carbon in croplands using imaging spectroscopy” (SR/00/110). We thank also the Vlaamse Instelling voor Technologische Onderzoek (VITO) at Mol (Belgium) for the organization of the flight campaign and geometric/atmospheric corrections of the images and Damien ROSILLON who elaborated part of the method

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