Using Sentinel-2 Images for Soil Organic Carbon Content Mapping in Croplands of Southwestern France. The Usefulness of Sentinel-1/2 Derived Moisture Maps and Mismatches between Sentinel Images and Sampling Dates

Diego Urbina-Salazar,Anne C Richer-De-Forges,Nicolas Baghdadi,Dominique Arrouays,Sébastien Lehmann,Emmanuelle Vaudour,Eric Ceschia

doi:10.3390/rs13245115

Abstract

In agronomy, soil organic carbon (SOC) content is important for the development and growth of crops. From an environmental monitoring viewpoint, SOC sequestration is essential for mitigating the emission of greenhouse gases into the atmosphere. SOC dynamics in cropland soils should be further studied through various approaches including remote sensing. In order to predict SOC content over croplands in southwestern France (area of 22,177 km²), this study addresses (i) the influence of the dates on which Sentinel-2 (S2) images were acquired in the springs of 2017–2018 as well as the influence of the soil sampling period of a set of samples collected between 2005 and 2018, (ii) the use of soil moisture products (SMPs) derived from Sentinel-1/2 satellites to analyze the influence of surface soil moisture on model performance when included as a covariate, and (iii) whether the spatial distribution of SOC as mapped using S2 is related to terrain-derived attributes. The influences of S2 image dates and soil sampling periods were analyzed for bare topsoil. The dates of the S2 images with the best performance (RPD ≥ 1.7) were 6 April and 26 May 2017, using soil samples collected between 2016 and 2018. The soil sampling dates were also analyzed using SMP values. Soil moisture values were extracted for each sample and integrated into partial least squares regression (PLSR) models. The use of soil moisture as a covariate had no effect on the prediction performance of the models; however, SMP values were used to select the driest dates, effectively mapping topsoil organic carbon. S2 was able to predict high SOC contents in the specific soil types located on the old terraces (mesas) shaped by rivers flowing from the southwestern Pyrénées.

Highlights

The Soil organic carbon (SOC) content prediction performance was analyzed for the six S2 image dates and soil sampling periods using either S2 data only or S2 data together with surface soil moisture (SM)
The performance of the SOC content predictions varied according to the soil sampling periods and the dates of the S2 images, reaching RPD and RPIQ values ≥ 1.7 on two different dates (6 April and 26 May 2017) determined using the soil samples collected between 2016 and 2018
This study confirms the previous statement according to which dates of acquisition of S2 images are crucial [21]

Summary

Introduction

Soil organic carbon (SOC) is fundamental to the global carbon cycle. Croplands represent approximately 143.4 Pg of SOC stocks worldwide at a depth of 30 cm, i.e., 20.7% of all land cover types followed by forests and grasslands with 43.1% and 25.9%, respectively [1]. Agricultural production and land use change account for 24% of global greenhouse gas emissions [2]. When converted from forest or native vegetation, cultivated soils lose between 20 and 50% of the carbon content in their top layers (0–30 cm), with the 4.0/).

Methods

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Discussion

Conclusion