SOIL CARBON STOCKS UNDER PASTURES IN THE BRAZILIAN CERRADO REGION
THEIR ASSESSMENT BY ORBITAL REMOTE SENSING
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G.G.Szakäcs!*, V.Eschenbrenner?, C.C. Cerri , M. Bernoux?
! Laboratory of Environmental Biogeochemistry - Center of Nuclear Energy in Agriculture (CENA/USP)
CP 96, 13400-970 Piracicaba, Brazil — (*gabor@cena.usp.br)
? UR041 — SeqC, Institut de Recherche pour le Développement (IRD), Montpellier, France
Key Words: Climate, Agriculture, Soil, Estimation, Landsat, Spectral
ABSTRACT:
The article 3.4 of the Kyoto Protocol acknowledges agriculture soils as a potential sink for soil organic carbon (SOC). To estimate
current and potential SOC on a broad scale for these potential sinks in a fast and inexpensive manner, the applicability of orbital
remote sensing techniques was studied. Four pastures on sandy soils of the Brazilian Cerrado were chosen due to their SOC
sequestration potential. Thereupon, the four chosen pastures were characterized in terms of their SOC (0-50em). Subsequently was
investigated the correlation degree between SOC and Leaf Area Index (LAI) and SOC and pasture reflectance in 6 different spectra
in the range from 450nm — 2350nm, using the Enhanced Thematic Mapper (ETM+) sensor of satellite Landsat 7. The SOC showed a
good correlation with the LAI (r = 0,97). As exists a good correlation between LAI and Spectral Vegetation Indices (SVIs), the good
correlation between SOC and LAI permits an adequat estimation for current and potential SOC through the NDVI.
A regression analysis showed also particularly good correlations between SOC and pasture reflectance in the red (r = 0,96) and
shortwave infrared (SWIR) (r = 0,95) spectrum. Both methods, the orbital measured LAI and the direct reflectance approach, seem to
be a promising tool to estimate current and potential SOC by orbital remote sensing.
KURZFASSUNG:
Der Artikel 3.4 des Kyoto Protokolls hebt hervor, dass landwirtschaftlich genutzte Bóden potentielle Fixierer von Bodenkohlenstoff
darstellen. Um den aktuellen und potentiellen Bodenkohlenstoffgehalt grossfláchig, schnell und kostengünstig ermitteln zu kónnen,
wurde für diesen Zweck die Eignung der orbitalen Fernerkundung analysiert. Es wurden aus Gründen des
Kohlenstofffixierungspotentials vier Weidefláchen im Brasilianischen Cerrado untersucht. Es wurde die Korrelation zwischen
Bodenkohlenstoff /Blattoberflàchenindex und Bodenkohlenstoff/Weideflüchenreflexion untersucht. Zur Spektralanalyse wurden
sechs Spektralbánder des Satelliten Landsat 7 herangenommen, welche den Spektralbereich von 450nm - 2350nm umfassen. Der
Bodenkohlenstoff zeigte eine gute Korrelation mit dem Blattoberflächenindex. Da es eine gute Korrelation zwischen
Blattoberflächenindex und spektralen Vegetationsindexen (z.B. NDVI) gibt, kann daraus abgeleitet werden, dass aktuelle und
potentielle Bodenkohlenstoffgehalte mittels orbitaler Fernerkundung ermittelt werden kônnen.
Zudem zeigte eine lineare Regressionsanalyse zwischen Bodenkohlenstoffgehalten und Weidereflexion besonders gute Korrelationen
im roten (r = 0,96) und mittleren Infrarotbereich (r = 0,95) und qualifiziert auch den Ansatz der Bodenkohlenstoffbestimmung mittels
Weidereflexion als geeignet.
assimilation, which is partly stored in the soil. Hodgson
1. INTRODUCTION (1990) considers, that LAT is one of the two variables, which
represent a major consistence in relation to pasture NPP.
The signature of the *United Nations Framework Convention. Therefore this study explored the correlation between SOC
on Climate Change" (UNFCCC) of Brazil and 150 other and IAF. According to Asner (1998), LAI represents,
countries in Rio de Janeiro in June 1992 indicated the wide together with the LAD (Leaf Angle Distribution), the
acknowledgement that climate changes are representing a dominant control on canopy reflectance. Furthermore it is
major threat for the environment and the economic legitimist to interpret subsoil information (e.g. SOC.) by the
development in the world. Climate changes are caused by an use of upper soil surface characteristics (e.g. LAI) with
increasing greenhouse effect, above the desired level. CO» satellite images, if there is a strong correlation among them.
has the biggest impact of all greenhouse gases with a global Several studies point out a good relationship between the IAF
warming potential of 60% (IPCC, 2000). The main goal of and spectral vegetation indexes (SVIs) as the Normalized
the Kyoto Protocol is the slowing of human contribution in Difference Vegetation Index (NDVI) (e.g. Broge & Leblanc,
relation to the increasing atmospheric concentrations of CO; 2000). As the SVIs can be expressed in satellite images, SOC
(Walsh, 1999). Therefore, the Kyoto Protocol acknowledges estimation based on IAF determination by satellite images
the possibility of SOC sequestration in cultivated soils (art seems to be a promising approach, if a significant correlation
3.4), The protocol also promotes research for SOC estimation between these two parameters can be found. Several studies
in a quick, inexpensive, but precise manner, which led to this also explored the relationship between soil organic matter
study.. and soil reflectance (e.g. Henderson et al., 1989). By so far
Sandy pastures under Brazilian Cerrado . conditions with there exists very little research about the spectral relationship
Brachiaria forage were studied due to their high SOC between subsurface SOC combined with vegetation/soil
sequestration potential. A higher Net Primary Productivity reflectance at orbital remote sensing scale. The objective of
(NPP) increases SOC due to higher carbohydrate this study is to explore this relationship, taken into account,
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