Iron oxides as proxies for characterizing anisotropy in soil CO2 emission in sugarcane areas under green harvest

Soil CO2 emission (FCO2) is a main contributor of atmospheric carbon transfer and is the subject of research aimed at developing effective methods for characterizing and mitigating CO2 emissions. The FCO2 is related to various soil properties including porosity, density and moisture, which are in turn related to gas transfer, O-2 uptake and CO2 release, as well as mineralogical components (particularly iron oxides, which are closely associated with aggregation and protection of soil organic matter). As estimated by diffuse reflectance spectroscopy (DRS), soil iron oxides such as hematite (Hm) and goethite (Gt) can be useful in determining FCO2. The main objective of this experiment was to assess the usefulness of the mineralogical properties Hm, Gt, and iron oxides extracted by dithionite-citrate-bicarbonate (Fe-d) to estimate the FCO2 in a sugarcane area under green harvest in southeastern Brazil. The experiment was conducted using an irregular 50 m x 50 m grid containing 89 sampling points 0.50-10 m apart to assess the soil properties. The FCO2 at each sampling point was measured at the beginning of crop growth and 54 days after planting with the use of two portable LI-COR LI-8100 Soil CO2 Flux Systems. The soil properties studied were found to be spatially dependent and exhibited well-defined anisotropy (particularly the mineralogical properties Hm, Gt and Fe-d). The first two components of a principal component analysis (PC1 and PC2) jointly accounted for 73.4% of the overall result variability with PC1 essentially related to the physical and mineralogical properties of the soil. Based on a multiple linear regression analysis, free water porosity (FWP) and Hm accounted for 71% of the FCO2 variability. Our results indicate that soil preparation and management practices in mechanically harvested sugarcane affect some factors inherent in the soil forming processes, including physical and mineralogical properties, which in turn affect FCO2. These results affirm the potential of DRS as an auxiliary tool for determination of properties that are typically associated with FCO2. In addition, the ensuing method allows for large-area FCO2 mapping to developing greenhouse gas emission inventories for agricultural soils. (C) 2014 Elsevier B.V. All rights reserved.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq