Dissolved inorganic carbon and pCO2 in two small streams draining different soil types in Southwestern Amazonia, Brazil

Eliete Sousa

doi:10.4136/ambi-agua.51

Abstract

The objective of this study was to determine the dissolved inorganic carbon (DIC) and pCO2 concentrations in two third order streams in southwestern Amazonia, Brazil. From May 2004 to June 2005 water dissolved oxygen, pH, electrical conductivity, temperature were measured to stream water chemical and physical description. DIC and pCO2 measurements were made by headspace extraction and gas samples for pCO2 and DIC extractions were run on an infrared gas analyzer (IRGA, LI-COR Instruments model LI-820). Results indicate a relationship between soil type and water chemistry, where sandy soil stream presented lower pH than silty soil stream – consequently DIC and pCO2 concentrations also varied with soil type. Mean DIC concentration for sitly soil stream was 403±130 ?M month-1, while sandy soil stream DIC concentration was 170±59 ?M month-1. Free CO2 was the dominant form of DIC in both streams. Nevertheless, HCO3- contribuition to DIC was greater for the silty soil stream. DIC contentration also varied seasonally with greater values in the drier period. Absolute pCO2 values were greater for silty soil stream, mean 3067±1228 µatm month-1 and 2321±1020 µatm month-1 for sandy soil stream. Seasonality, pCO2 was higher in the dry season in both streams. Our findings have important implications on the role of soil type in water chemistry and carbon dynamics and also are used in other studies on carbon balance at the landscape level.

Highlights

The Amazon is the largest basin in the world, occupying an area of about 6 million km2.Its drainage area comprises rivers, lakes, and streams that together culminate in the great Amazon River, which alone accounts for 20% (6 x 1012 m3 yr-1) of the global freshwater inputs to the ocean via riverine transport and about 6% (1.2Gt yr-1) of the total sediment discharge (De Master and Aller, 2001)
In this paper we describe dissolved inorganic carbon concentrations and pCO2 in two streams located in southwestern Amazonia, focusing on the effects of soil type and seasonal variations
Igarapé Floresta, hereafter referred to as “sandy stream”, is a stream located in the Catuaba Experimental Farm, where its soil is dominated for sand (62%) and the pH is about 4.0

Summary

Introduction

The Amazon is the largest basin in the world, occupying an area of about 6 million km2.Its drainage area comprises rivers, lakes, and streams (igarapés) that together culminate in the great Amazon River, which alone accounts for 20% (6 x 1012 m3 yr-1) of the global freshwater inputs to the ocean via riverine transport and about 6% (1.2Gt yr-1) of the total sediment discharge (De Master and Aller, 2001). Amazon, scaling up to an evasion flux estimate of about 0.5 Gt C yr-1 from the whole basin, which is more than an order of magnitude higher than inorganic carbon riverine transport to the ocean from the Amazon (Richey et al, 2002). Harald Sioli was one of the first researchers to study and to classify the different water typologies in Amazonia, taking into consideration the regional geology and mineralogy (Esteves, 1998). According to his classification scheme, Amazonian rivers fall into three categories—white-water, black-water, and clear-water—each group with distinct characteristics with respect to dissolved and particulate organic and inorganic chemistry

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