Soil organic matter in podzol horizons of the Amazon region:
Humification, recalcitrance, and dating

by

Amanda M. Tadini, Gustavo Nicolodelli, Giorgio S. Senesi, Débora A. Ishida, Célia R. Montes, Yves Lucas, Stéphane Mounier, Francisco E.G. Guimarães, Débora M.B.P. Milori

Embrapa Instrumentação, São Carlos, SP, Brazil.

Corresponding author: Email address: debora.milori@embrapa.br

Abstract

Characteristics of soil organic matter (SOM) are important, especially in the Amazon region, which represents one of the world’s most relevant carbon reservoirs. In this work, the concentrations of carbon and differences in its composition (humification indexes) were evaluated and compared for several horizons (0 to 390 cm) of three typical Amazonian podzol profiles. Fluorescence spectroscopy was used to investigate the humic acid (HA) fractions of SOM isolated from the different samples. Simple and labile carbon structures appeared to be accumulated in surface horizons, while more complex humified compounds were leached and accumulated in intermediate and deeper Bh horizons. The results suggested that the humic acids originated from lignin and its derivatives, and that lignin could accumulate in some Bh horizons (Figure 1). The HA present in deeper Bh horizons appeared to originate from different formation pathways, since these horizons showed different compositions. There were significant compositional changes of HA with depth, with four types of organic matter: recalcitrant, humified, and old dating; labile and young dating; humified and young dating; and little humified and old dating. Therefore, the humification process had no direct relation with the age of the organic matter in the Amazonian podzols.

Figure 1: Illustration shows there were structural variation in the humic acids and lignin derived at the depth profile.

Reference:

Montes, C.R., Lucas, Y., Pereira, O.J.R., Achard, R., Grimaldi,M.,Melfi, A.J., 2011. Deep plantderived carbon storage in Amazonian podzols. Biogeosciences 8, 113–120.

Pereira, O.J.R., Monte, C.R., Lucas, Y., Melfi, A.J., 2013. Use of remote sense imagery for mapping deep plant-derived carbon storage in Amazonian podzols in regional scale. IEEE International Symposium on Geoscience and Remote Sensing, Bourne, Australia.

Senesi, G.S., Martin-Neto, L., Villas-boas, P.R., Nicolodelli, G., Milori, D.M.B.P., 2016. Laser based spectroscopic methods to evaluate the humification degree of soil organic matter in whole soils: a review. J. Soils Sediments http://dx.doi.org/10.1007/s11368-016-1539-6.