Deep plant-derived carbon storage in Amazonian podzols

Introduction Conclusions References


Introduction
In spite of the great areas the tropical podzols cover worldwide -more than 140 000 km 2 in Amazonia (Bernoux et al., 2002;Batjes and Dijkshoorn, 1999) -and the fact that they have been recognized as early as 1941 (Richards, 1941), these sys- 20 tems are still incompletely known. Most of the knowledge about their genesis and dynamics comes from studies conducted in French Guyana (Boulet et al., 1982;Lucas et al., 1987;Veillon, 1991), Brasil (Lucas et al., 1984(Lucas et al., ,1988Chauvel et al., 1987;Volkoff, 1988, 1998;Righi, 1989, 1990;Volkoff et al., 1990;Righi et al., 1990;Dubroeucq et al., 1991;Horbe et al., 2004;Nascimento et al., 2004 (Veillon and Soria-Solano, 1988). Similar systems were also briefly described in Africa (Brammer, 1973;Schwartz, 1988) and in Borneo (Brabant, 1987). These systems have been reported on both crystalline or sedimentary rocks. They can develop when clay minerals, Al-hydroxides and Fe-oxides or Fe-oxyhydroxides are 5 no longer in equilibrium with regard to the soil chemistry. Such conditions can be achieved when soil solutions turn hyper-acidic, due to very low alkali or alkaline-earth cations in the soil solution, turning negative the alkaline reserve (Grimaldi and Pedro, 1996) or when the soil material is sandy enough to allow the leaching of Al-and Feorganic matter complexes, removing these metals from the solution-minerals equilibria 10 (Lucas, 2001). When initiated, the development of the sandy eluviated horizons (white sand horizons) is favoured by a positive feed-back and the podzols laterally develop at the expense of the ferralsols (Lucas et al., 1984). Where the time of evolution of such systems was sufficient, large areas of the landscape are covered by podzols (Dubroeucq and Volkoff, 1998). Aquifers in the white sands frequently reach the top- 15 soil and are rich in dissolved organic matter (DOM). Measured values of dissolved organic carbon (DOC) in the literature range from 25 to 38 mg L −1 (Patel-Sorrentino et al., 2007), when the mean value at the mouth of the Rio Negro, which drains extensive podzol areas in Amazonia, is estimated around 8.5 mg L −1 (Tardy et al., 2009).
The DOM moves freely through sandy materials, but adsorbs on surfaces of fine 20 minerals as clays, oxides or (oxi)-hydroxides (Davis, 1982;Kaiser and Zech, 2000). As a consequence, when the DOM-rich groundwater flowing in the white sands comes in a more clayey material, as the horizons immediately underlying the white sands, it leaves much of its DOM which adsorbs at the transition, forming an horizon rich in organic matter (Bh) . The aquifers in the more clayey materials 25 have a low DOC content, typically lower than 2 mg L −1 . As a result, podzols can play an important role in global carbon dynamics. Considering the Amazonian Basin, they provide approximately a tenth of the 0.13 GT of carbon annually exported to the Atlantic ocean (Tardy et al., 2009 by Batjes et Dijskhoorn (1999) and 6-7 kg C m −2 by Bernoux et al. (2002). The former 5 authors distinguished the histosols from the podzols, evaluating the carbon stored in the 0-1 m upper horizon as 9 kg C m −2 for the podzols and 72.4 kg C m −2 for the histosols. These values, however, do not take in account possible deep Bh and are based on a small collection of studied profiles, less than 30 podzol profiles for the whole Amazonia. 10 Four detailed studies have considered the deep Bh horizons of Amazonian podzols (Lucas et al., 1984;Veillon and Soria-Solano, 1988;Veillon, 1991;Nascimento et al., 2004). As their objectives were to study the genesis and dynamics of ferralsol-podzol systems, they were conducted in relatively well-drained areas of transition between ferralsols and podzols. They showed that the deep podzol Bh's had a thickness increasing 15 when going from the transition towards the podzolic, marshy areas. Their maximum C content calculated from the data given in the studies were 8, 15, 13 and 25 kg C m −2 , respectively, which suggests that marshy areas can store large quantities of carbon in depth. Existing studies in such areas, however, are scarce and limited to the upper horizons, due to the difficulties for observing deep horizons because of the collapse of 20 the sandy material when digging or trading. The purpose of this paper is to quantify the C content of both surficial and deep horizons of a typical podzol system located in the high Rio Negro Basin and to extrapolate the obtained data to the whole Amazonian Basin by using existing digitalized soil maps, in order to evaluate the importance of the podzol carbon reservoir at global scale. Introduction

Study site and methods
The studied area is located near the São Gabriel da Cachoeira city, Amazonia State, Brazil, at 0 • 6 35 S, 66 • 54 10 W (Fig. 1); it was described in a previous publication (Montes et al., 2007). The giant podzols area has developed from a plateau centre at the expense of reddish yellow, low activity clay ferralsols; processes and dynamics of 5 such systems are described elsewhere Cornu et al., 1998;Lucas, 2001). The soil system is typical of those observed in the Rio Negro Basin (Dubroeucq and Volkoff, 1998). The climate is typically equatorial, with an annual rainfall around 3000 mm and without a marked dry season. The geological substratum is composed of crystalline rocks having composition varying between monzogranitic, sienogranitic 10 and quartzomonzonitic (Dall'Agnol and Macambira, 1992).
In the bad-drained, hydromorphic podzol area the water-table is usually shallow, reaching the topsoil after heavy rains. The vegetation is a specific evergreen forest called campinarana and characterized by a high density of 20-30 m height trees (Anderson, 1981). The transition between hydromorphic podzols and well-drained ferral-15 sols is characterized by better-drained podzols, forming a 100 to 200 m large halo which surrounds the poorly drained area. The vegetation over both ferralsols and well drained podzols is a typical lowland Amazonian evergreen forest where dominant trees height is between 30 and 55 m. The difference of vegetation between well-and bad-drained areas is easily recognizable on remote sensing imagery (Fig. 2). 20 The soils were studied using a structural analysis approach (Boulet et al., 1982;Fritsch and Fitzpatrick, 1994;Delarue et al., 2009). Observations and sampling were done by hand auger drilling following a 50 m square grid and along various footpaths. A total of 164 profiles were studied, giving more than 500 samples. Due to the collapse of the sandy material when digging or trading, we have developed a light method for Introduction

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | a Ø 6.3 cm Eijkelkamp valve bailer. The auger hole was cased with a Ø 7 cm PVC pipe bevelled at its end, progressively pushed down to the cemented Bh where it was pressed down by force, providing sufficient sealing to prevent fast water infiltration during drilling. The cemented part of the Bh was broken using a chisel welded to the lower end of a drill string. Soil organic C content was determined with a Shimadzu 5 TOC-5000 apparatus. The soil densities were calculated from relationships previously established (du Gardin et al., 2002) and checked by direct measurements by the ring method. The carbon content for each identified group of horizons was extrapolated between the observation points by linear interpolation. For a given group of horizons, the given error was calculated from the propagation of the 5% error on the C measure-10 ment. For extrapolation calculation, the podzol areas perimeters was calculated with a threshold-based pixel counting algorithm and the width of the better-drained halo has been set at 150 m. Mineralogy was determined on selected samples by X ray diffraction on powder samples, diffuse reflectance spectroscopy and thermogravimetric analysis. Soil water was sampled for DOC (dissolved organic carbon) analysis. Zero tension 15 lysimeters were installed inside drilling holes at different depths, 2 points in the white sand at 20 and 150 cm in depth, respectively, (W-20 and W-150), one point in the Bh horizon at 230 cm in depth (W-240) and two points in the carbon-poor kaolin horizon beneath the Bh at 500 cm in depth (W-510). A spring gushing out from the center part of the podzolic area was also sampled (spring sample). After installing the lysimeters, 20 each drilling hole was tamped by filling with the previously extracted soil material at the corresponding depth. Each lysimeter was made of a 50 mL polypropylene bottle bored with 5 mm diameter holes all around. A 2 mm diameter capillary PTFE tube was inserted through the bottle cap in order to permit extracting water from the topsoil with a manual vacuum pump. applying a continuous suction of 25 mmHg during a 3 to 15 mn period, depending on the outflow rate. The first 10 to 50 mL of each sampling, depending on the outflow rate, was discarded in order to avoid dead volume and to rinse the sampling equipment. Sampling was done until 250 mL was reached or air was entering the system. Each sample was filtered using a 0.7 µm fiber glass filter, poisoned with sodium azide, stored 5 in 10 mL vacuum glass flasks (Vacutainer) then kept at low temperature (around 4 • C) for laboratory DOC analysis performed with a Shimadzu TOC-5000 apparatus.

Soil features and carbon content
Soils in the whole area are typical of old, highly weathered soil mantle. In all horizons located over the kaolinitic saprolite, residual primary minerals are quartz, heavy minerals and quite small amounts of muscovite; secondary minerals are kaolinite, gibbsite, goethite and hematite. A typical soil sequence from the edge to the centre of a podzol system is given in Fig. 2, the corresponding soil organic carbon content and horizons thickness, texture and type are given in supplementary Fig. S1 and Table S1. Bh hori- 15 zons were systematically observed at the transition between the white sand E horizons and the kaolinitic underlying horizons; both Bh thickness and carbon content are increasing when going from better-drained to poorly drained areas. At the transition between ferralsol and podzol, the Bh horizon has a thickness around 30 to 40 cm and overlies kaolinitic horizons (more than 83% kaolinite). When going towards the poorly 20 drained area, the Bh progressively turns thicker, up to more than 3 m. We divided the poorly drained podzol profiles in four groups have a thickness ranging from 10 to 40 cm, which highly depends on the local soil surface topography; the thickness is higher in slightly depressed areas where topsoil is waterlogged most of the time. Their C contents range from 16.8 to 178.0 kg C m −3 , giving a total carbon content ranging from 2.5 to 41.0 kg C m −2 with an average of 17.0 ± 4.0 kg C m −2 .

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The middle sandy horizons (white sand E horizons) are grey to light grey (10YR6/1 to 7/2). Their mineral fraction mainly consists of quartz sand (>96%). They have a thickness ranging from 115 to 200 cm and present thin (<4 mm thick) wavy black micro-horizons (thickness < 4 mm). Their C contents range from 1.9 to 3.3 kg C m −3 , giving a total carbon content ranging from 2.1 to 3.1 kg C m −2 with an average of 10 3.1 ± 0.9 kg C m −2 . The deep Bh horizons have a thickness ranging from 220 to 340 cm and a texture varying from sandy to clayey. Their upper part, at the transition between sandy and more clayey material, is hardened over a thickness of some 20 to 40 cm. Their colour varies from black (7.5YR2/1) to brown (7.5YR5/4), the upper part is usually darker and 15 the colour turns lighter in depth. Their mineral fraction is composed of quartz sand and kaolinite in variable proportions depending on the texture, with smaller amounts of gibbsite and Fe (oxy)hydroxides. The material, however, is frequently heterogeneous, with darker volumes of decimetrical size. Their C content ranges from 3.6 to 104.1 kg C m −3 , giving a total carbon content ranging from 50.1 to 80.9 kg C m −2 with 20 an average of 66.7 ± 5.8 kg C m −2 . The C content values are the highest in the top 60 cm of the deep Bh horizons and decrease in depth.
The underlying kaolinitic, saprolitic material is pale yellow to light olive gray (2.5Y8/3 to 5Y6/2). The mineral fraction is mainly kaolinitic, with fine quartz sand and muscovite and some completely kaolinized feldspar phenocrysts. The C content values of the 25 saprolitic material are lower than 2.1 kg C m −3 .
Without considering the carbon stored in the saprolitic material, the whole podzol profiles store an average of 86.8 ± 7.1 kg C m −2 for the poorly drained area and 27.9 ± 2.5 kg C m −2 for the better drained halo.

Soil water characteristics
Statistics of DOC concentration in the soil and spring waters are given in Table 1. The waters from the spring and the white sand horizons (spring, W-20 and W-150) had high DOC concentrations throughout all sampling periods, ranging from 24 to 55 mg L −1 with an overall average equal to 37 mg L −1 . In the Bh horizon (W-240), the DOC concentra-5 tion varied from 12 to 19 mg L −1 , 15 mg L −1 on average. In the deep clayey horizons (point W-510), the DOC concentrations were lower but not negligible, varying from 1.5 to 3.5 mg L −1 , 2.3 mg L −1 on average. 10 The Bh features and the DOC values of the percolating waters confirmed the podzolic processes acting in the studied soil cover. Bh horizons underlying podzolic E horizons are also characteristics of boreal and temperate podzols. What is new here is the thickness and the overall C content of the Bh, which indicates that unusually large quantities of DOC were transferred from the topsoil to the depth. This can be related 15 to four main factors: (i) higher DOC concentration in the groundwater transferred from topsoil to depth, (ii) higher volumes of water transferred in depth, (iii) quality of the organic matter, with a higher resistance to mineralization, and (iv) long time of evolution. The DOC concentration in the groundwater circulating in E horizons is of the same order of magnitude than in boreal or temperate podzols (Mossin et al., 2002;Lindroos 20 et al., 2008).

Bh genesis
The DOC annually produced at the topsoil and transferred in the E horizons groundwater (DOC y ) is given by Eq. (1), where P is the rainfall (mm y −1 ), ET the evapotranspiration (mm y −1 ) and DOC E the DOC concentration in the white sand E horizons The present-day values of the parameters of the second member of this equation can be estimated from the literature and the present study: 3000 mm y −1 for P (Silva et al., 1977), 1500 mm y −1 for ET (Leopoldo et al., 1987) and 37 mg L −1 for DOC E (this study).

5
Using these values, DOC y is 55.5 g m −2 y −1 , which is more than 10 time higher than the DOC transferred at 40 cm in depth in boreal podzols (Lindroos et al., 2008). This higher value is clearly linked to higher annual rainfall. The DOC annually transferred and retained in the Bh (OM y , in g C m −2 y −1 ) is given by Eq. (2), where DOC S is the DOC concentration in the saprolite (mg L −1 ) and r dp the 10 ratio of non-evapotranspired water entering the saprolite: From this study, DOC S can be estimated as 2.3 mg L −1 . The value of r dp is more difficult to evaluate. Tardy et al. (2009)

Extrapolation to the Amazonian Basin
The deep carbon, as well as most of the carbon stored in the organic-rich upper hori-5 zons, was not considered in studies dealing with the quantification of Amazonian soil carbon (Batjes and Dijkshoorn, 1999;Bernoux et al., 2002;Cerri et al., 2007). Nevertheless, wherever a detailed study of Amazonian podzols has been achieved, the presence of deep Bh has been attested, so we may assume deep carbon storage in every podzol area. The quantification we made from a single area must obviously be refined throughout Amazonia and moreover throughout other equatorial podzol areas, particularly in the Kalimantan (Brabant, 1987), in Congo (Schwartz, 1988) and in Zambia (Brammer, 1973). Extrapolation of our data across Amazonia can, however, give an order of magnitude of the carbon stored in Amazonian podzols. Existing IBGE digitalized soil maps of Amazonia (IBGE, 2009) were made mainly 15 from radar surveys (RADAMBRASIL, 1978) quite sensitive to the physical characteristics of roughness, slope and moisture on the soil surface, so that the large flat, poorlydrained podzols areas have been identified with certainty. The poorly drained podzol area we calculated from IBGE maps is 1 554 105 km 2 (lower map of Fig. 1). Extrapolating the data of our detailed study, we calculated the carbon stored in the Amazonian 20 podzols to be 13.6 ± 1.1 Pg C, more than 12.3 Pg C higher than previous estimates (1 Pg for the topsoil 0-30 cm after Batjes and Dijkshoorn, 1999 or 1.3 Pg for the topsoil 0-1 m after Bernoux et al., 2002). This calculated value is certainly an underestimate, given that soil systems that include podzols, but where podzols are fragmented into small areas that do not appear in digitalized soil maps, cover areas around 8105 km 2 25 as calculated from RADAM maps (unit 1a in Fig. 1). In comparison, the total amount of carbon stored in the aerial biomass of the Amazonian forest was estimated as 93 ± 23 Pg C (Malhi et al., 2006).

Lability of the podzol organic matter
Is the carbon stored in the deep Bh likely to quickly return to the atmosphere if the climate changes? Most podzol areas are situated in the high Rio Negro Basin. For this area, 18 different global warming climate models give projections on both precipitation and soil moisture (Meehl et al., 2007). All of those predict an increase of the average 5 air temperature, seven a diminution of the annual precipitation, ten a diminution of the average soil moisture and eight an increase of contrast between the wet and dry seasons. Consecutive changes of temperature and dynamics of the water-table could destabilize the organic matter of the upper horizons as well as of the deep, resulting in a complete or partial mineralization. In temperate podzols, organic horizons with 10 relatively short turnover times could be particularly vulnerable to changes in climate or other disturbances (Schulze and al., 2009). Deep Bh from the Manaus area gave two apparent 14 C ages, 490 ± 90 and 2840 ± 90 y BP , which is young when considering the potential age of the podzols. Some tropical podzols from Congo that were formed in the past and which are nowadays under a drier climate keep 15 up thick deep Bh apparently rich in organic matter (Schwartz, 1988), but there is no comparison available between still hydromorphic and currently well-drained podzols.
As it is difficult to conclude about the sensibility of the deep Bh organic matter to climate change, some complementary studies similar to those realized for northern peatland organic matter (Dorrepaal et al., 2009) are needed to evaluate the feedback 20 between the podzols carbon cycle and climate.

Conclusions
The studied hydromorphic podzols from the high Rio Negro exhibited thick Bh horizons at depth. The carbon stored in the podzols was on average 66.7 ± 5.8 kg C m Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | as a sufficient time of evolution and a sufficient stability of the adsorbed OM. As the OM saturates the minerals exchange sites, the dissolved organic matter is able to percolate more in depth, resulting in an increase of the Bh thickness. The dynamics of Bh formation, however, need to be better assessed. Extrapolation of the obtained results to the whole Amazonia gave 13.6 ± 1.1 Pg C 5 for the carbon stored in hydromorphic podzols, which is at least 12.3 Pg C higher than previous estimates. This value is likely an underestimate, because soil systems where podzols are fragmented into small areas were not taken in account in the quantification. This quantification has thus to be refined by additional investigations, not only in Amazonia but in all equatorial areas where podzols have been identified.
Considering the volume of carbon stored in the podzol Bh, the stability of the Bh organic matter in a context of climate change needs to be assessed. Because of the lack of knowledge on the quality and behaviour of the podzol organic matter, the question of the feedback between the climate and the equatorial podzol carbon cycle is open. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Bardy, M., Fritsch, E., Derenne, S., Allard, T., do Nascimento, N. R., and Bueno, G. Fig. 3. A typical soil sequence from well-drained to bad-drained podzol area. 1: Upper horizons, C > 50 kg m −3 ; 2: upper horizons, 5 < C < 50 kg m −3 ; 3: white sand horizons, 1.9 < C < 3.3 kg m −3 ; 4 to 9: Bh horizons, 3.6 < C < 104.1 kg m −3 (pink numbers give C isolines in kg m −3 ); 10: kaolinitic horizons, C < 1.7 kg m −3 ; 11: kaolinitic saprolite, C < 2.1 kg m −3 ; 12: ferralsolic horizons, C < 1.8 kg m −3 . Black circles indicate soil water sampling points.