Soil Carbon Inventory by Wet Oxidation and Dry Combustion Methods

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Soil Carbon Inventory by Wet Oxidation and Dry Combustion Methods: Effects of Land Use, Soil Texture Gradients, and Sampling Depth on the Linear Model of C-Equivalent Correction Factor Florent Tivet Centre de Coopération Internationale en Recherche Agronomique pour le Développement CIRAD, UR SIA F-34398 Montpellier, France

João Carlos de Moraes Sá* State Univ. of Ponta Grossa Dep. of Soil Science and Agricultural Engineering Av. Carlos Cavalcanti 4748 Campus de Uvaranas 84030-900, Ponta Grossa-PR, Brazil

Paulo Rogério Borszowskei State Univ. of Ponta Grossa Graduate Program in Agronomy Av. Carlos Cavalcanti 4748 Campus de Uvaranas 84030-900, Ponta Grossa-PR, Brazil

Philippe Letourmy Centre de Coopération Internationale en Recherche Agronomique pour le Développement CIRAD, UR SCA F-34398 Montpellier, France

Monitoring C content is essential for carrying out surveys and inventories on C storage in soils under different land uses (LUs). The objectives of the present study, which was conducted in five agro-ecoregions in Brazil with contrasting climates, LU managements, soil texture gradients, and soil depths, were to: (i) develop a C-equivalent correction factor (CF) between total organic carbon determined by dry combustion (TOCDC) and organic carbon by wet oxidation recorded by the Walkley–Black (OCWB) method, and (ii) assess the influence of LU, soil texture gradients, and sampling depth on a C-equivalent CF based on a linear model adjusted for each experimental location. The results indicated an effect of LU at almost all sites, except for one subtropical location where clay content and sampling depth were the most important factors. Additionally, there were LU × clay and LU × sampling depth interactions on the C-equivalent CF. The linear models computed to estimate the CF from this set of qualitative (LU) and quantitative variables (soil texture gradient and soil depth) differed among and within the agro-ecoregions and LU managements. The C-equivalent CF in subtropical sites ranged from 1.37 to 1.55, while for tropical sites ranged from 1.13 to 1.60. These models are site specific, and the results demonstrate the need to develop models calibrated for each site before extrapolating to other agroecoregions to recalculate past soil C inventories and to evaluate temporal changes of SOC stocks. Abbreviations: CF, correction factor; CRB, Carambeí; CSOC, chemically stabilized organic carbon; LEM, Luiz Eduardo Magalhães; LRV, Lucas do Rio Verde; LU, land use; MAOC, mineral-associated organic carbon; MIR, mid-infrared; NIR, near-infrared; OCWB, organic carbon by wet oxidation; OM, organic matter; PG, Ponta Grossa; SOC, soil organic carbon; TBG, Tibagi; TOC, total organic carbon; TOCDC, total organic carbon determined by dry combustion.

Clever Briedis State Univ. of Ponta Grossa Graduate Program in Agronomy Av. Carlos Cavalcanti 4748 Campus de Uvaranas 84030-900, Ponta Grossa-PR, Brazil

Ademir Oliveira Ferreira Federal Univ. of Santa Maria Dep. of Soil Management and Conservation Av. Roraima 1000 Camobi CEP 97105900 Santa Maria-RS, Brazil

Josiane Burkner dos Santos Thiago Massao Inagaki State Univ. of Ponta Grossa Graduate Program in Agronomy Av. Carlos Cavalcanti 4748 Campus de Uvaranas 84030-900, Ponta Grossa-PR, Brazil

A

n accurate quantification of soil organic carbon (SOC) stocks is of fundamental interest to develop reliable inventories related to emissions and sinks of carbon dioxide concentrations influenced by LU change and agricultural management. Several studies, including those by Ogle et al. (2003) and Goidts et al. (2009), have emphasized the large uncertainties involved in estimating SOC stocks, which hamper reliable assessments, comparisons among LU management types and changes over time. Soil organic carbon content determination is one of the main sources of these uncertainties, in addition to bulk density measurement, sampling depth and rock fragment content. Several analytical methods and tools are currently available to quantify SOC content, and several studies (Bisutti et al., 2004; Jankauskas et al., 2006) have ex-

Soil Sci. Soc. Am. J. 76:1048–1059 doi:10.2136/sssaj2011.0328 Received 21 Sept. 2011. *Corresponding author ([email protected]). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

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amined their efficiency. More recently, mid- and near-infrared (MIR, NIR) spectroscopies have been found to provide a rapid fingerprint of the composition of organic matter (OM) in soil (Kaiser et al., 2007) and have gained considerable importance in monitoring soil properties and OC pools (Fontan et al., 2010; Shepherd and Walsh, 2002). However, some practical issues still need to be addressed to increase the accuracy, low-cost, and high throughput analysis of MIR and NIR spectroscopies (BellonMaurel and McBratney, 2011). Conducting C analysis of field-collected samples using a dry combustion method is regarded as the standard method due to its high precision and the accuracy of its results (Matejovic, 1997; Soon and Abboud, 1991; Yeomans and Bremner, 1988) compared to traditional wet oxidation methods. The use of automatic elemental analyzers has brought considerable advances to this field, allowing quick and reliable analyses. However, carrying out C oxidation using potassium dichromate in sulfuric acid (the Walkley–Black method) is still widely used in many countries that are still far behind with respect to the use of modern drycombustion analyzers because it is simple, rapid, and presents minimal equipment needs. Meanwhile, disposal of the waste produced by Walkley–Black method is problematic because this procedure uses significant amounts of contaminating reagents. Most laboratories throughout the world have established past inventories of SOC stocks based on the oxidation of C using dichromate in sulfuric acid. The major limitation of the Walkley–Black method is that only the most active OC is oxidized (Nelson and Sommers, 1996), leading to incomplete oxidation of organic compounds. Thus, the C content obtained by the Walkley–Black method generally underestimates the values obtained through dry combustion methods (Mikhailova et al., 2003), although a high correlation is commonly found between these two methods regardless of agro-ecoregion conditions (Dieckow et al., 2007; Jankauskas et al., 2006; Lettens et al., 2007; Meersmans et al., 2009). Furthermore, the Walkley–Black procedure is not sufficiently sensitive to identify small changes over time in response to LU changes and management practices. To avoid methodological biases when SOC contents obtained by wet oxidation are compared with that obtained by dry combustion, in the absence of a site-specific C-equivalent CF, analytical results are commonly adjusted with a C-equivalent CF of 1.33 (Walkley and Black, 1934). However, Mikhailova et al. (2003) reported a C-equivalent CF of 1.63 for a Russian Chernozemic soil; Nelson and Sommers (1996) obtained C-equivalent CFs ranging from 1.16 to 1.59 for a range of soils; and Dieckow et al. (2007) proposed a value of 1.05 for a subtropical soil in southern Brazil under different tillage and cropping systems. Contradictory results can be found regarding the effects of land use, textural gradients, and sampling depth with regard to the relationship between the results of the Walkley–Black method compared with dry combustion. Dieckow et al. (2007) reported no significant differences among the analytical methods for soil samples from different no-tillage systems. In contrast, several authors (De Vos et al., 2007; Diaz-Zorita, 1999; Gatto et al.,

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2009; Lettens et al., 2007; Meersmans et al., 2009; Wang et al., 1996) have shown large influences of different land uses, sampling depths and soil types on the interpretation of their results based on a wide range of conditions (native vegetation, cropland, textural gradients). Past inventories and current C stock inventories involve different analytical methods, and methodological biases and uncertainties should be reduced to develop reliable estimates of the effects of LU changes on SOC stocks. In this context, the aims of this study were as follows: (i) to assess the C-equivalent based on a correction factor between total organic carbon (TOC) obtained by TOCDC and OC recorded by OCWB method for a wide range of Brazilian agro-ecoregions, LU management types, textural gradients, and soil depths; and (ii) to assess the influence of LU management, soil textural gradients, sampling depth, and the interactions between LU and quantitative variables on the C-equivalent CF by using linear models adjusted for each experimental location.

MATERIALS AND METHODS Location of Experimental Sites and Land Use Description Five experimental sites (Fig. 1) located in contrasting agroecoregions in Brazil (subtropical and tropical environments) characterized by different LU and management types (natural vegetation, conventional tillage [CT], and no-tillage [NT]), soil texture gradients, and soil depths were considered in this study. In a subtropical environment, sites associated with three long-term tillage experiments located in Paraná state, southern Brazil were selected: (i) the experimental station of the Instituto Agronômico do Paraná- IAPAR in Ponta Grossa city (PG site), (ii) the experimental station of the ABC Foundation in Carambeí city (CRB site), and (iii) Santa Branca Farm, in Tibagi city (TBG site). In tropical environments, the sites of two tillage experiments located in central-western Brazil were selected: (iv) the experimental station of the Lucas do Rio Verde (LRV site) Foundation in Mato Grosso State, and (v) the experimental station of the Bahia Foundation, Luiz Eduardo Magalhães (LEM site) city in Bahia State. A detailed description of the locations, climates, soil types, LUs, sampling depths, and chemical and particle size analyses for these sites is presented in Tables 1 and 2.

Soil Sampling Undisturbed soil samples were collected to measure the soil bulk density (ρb) for each soil layer described in Table 1 using the core method (Blake and Hartge, 1986), employing steel cylinders of 2.5 by 5 cm at CRB and TBG sites, and 5 by 5 cm at PG, LRV, and LEM sites. The cores from 10- to 20- to 80- to 100cm soil layers were taken in the middle part of the corresponding layer using a core sampler with steel cylinders of 5 by 5 cm. Disturbed soil samples were collected to obtain a bulk soil sample by digging small pits with dimensions of 20 cm (width) by 20 cm (length) by surface layers (e.g., 0–5, 5–10, and 10–20 cm layers). Pits with these dimensions were dug at each replicate sampling location. Samples for the 20- to 40-, 40- to 60-, 60- to 80- and 80-

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Fig. 1. Locations of the experimental sites in the Brazilian agro-ecoregions.

to 100-cm layers were collected with an auger. A total of 480 soil samples were taken, 302 from subtropical sites (PG, CRB, and TBG) and 178 from tropical sites (LRV and LEM). The bulk samples were oven-dried at 40°C, gently ground, sieved through a 2-mm sieve, and homogenized. Texture analyses were performed for each replicate using Bouyoucos’ scale (Gee and Bauder, 1986).

Carbon Analyses by Wet Oxidation and Dry Combustion The wet oxidation method used in this study was the Walkley–Black procedure (Nelson and Sommers, 1996), in which SOC is oxidized by potassium dichromate with sulfuric acid without external heating. A subsample of 1 g from a 2-mm sample of oven dried (40°C), sieved soil was placed in a 125 mL

Table 1. Location, soil type, parent material, climate, land use, and management, duration of experiment, sampling depth, replicates, and number of samples for the experimental sites. Sites Description

Ponta Grossa– PR, IAPAR Carambei–PR, ABC Tibagi–PR, Santa (PG) Foundation (CRB) Branca Farm (TBG)

Lucas do Rio Verde, MT (LRV)

Luiz Eduardo Magalhães–BH (LEM)

Location-coordinates

25°09′ S, 50°09′ W

25°00′ S, 50°09′ W 24°30′ S, 50°26′ W 13°00′ S, 55°58′ W 12°05′ S, 45°42′ W

Altitude

865 m

910 m

880 m

Soil type (FAO and Soil Red Latosol, Oxisol, Typic Red Latosol, Oxisol, Red Latosol, Taxonomy) Rhodic Hapludox Typic Hapludox Typic Hapludox Parent material

Shale

Climate, type Mean temperature Mean annual rainfall Land use† Years of experiment

Mesothermic, Summer and Winter wet, cold winter 18.5°C 18.7°C 20.7°C 1545 mm 1545 mm 1532 NF, CT, MT, and NT CT, MT, NTch, and NTNF, CT, and NT 29 18 22

Sampling depth

0–5, 5–10, 10–20, 20–40, 0–2.5, 2.5–5, 5–10, 40–60, 60–80, 80–100 10–20, 20–40

380 m

760 m

Red Yellow Latosol Arenosols, Quartzipsamments Typic Haplustox

Shale and sandstone, Shale and sandstone, Shale and reworked material reworked material sandstone

Sandstone

Humid tropic 25°C 1950 NF, CT, and NT1–4 8

Semi-humid tropic, Winter dry 22°C 1480 NF, CT, and NT 2

0–2.5, 2.5–5, 5–10, 0–5, 5–10, 10–20, 0–10, 10–20, 20–40, 40–100 10–20, 20–40 20–40, 40–60, 60–80, 80–100

Replicates 6 3 5 3 3 Number of samples 167 60 75 126 52 † Land use: PG and CRB sites: NF = forest, CT = conventional tillage such as a plow tillage with a 70-cm disking after summer harvest and one after winter harvest to 20-cm depth plus two 60-cm narrow disking to break the clods, MT = minimum tillage: one chisel plowing to 25-cm depth and one 60-cm narrow disking to break the clods, NT = no-tillage, NTch = no-tillage and one chisel plowing every 3-yr period. Cropping sequence such as winter and summer crop based on wheat–soybean/oat–soybean/oat–maize. TBG site: NF = prairie grassland dominated by C4 species, such as Andropogon spp., Aristida spp., Paspalum spp., Panicum spp.; LRV and LEM sites: NF = Cerrado or savannah woodland with an almost closed canopy. 1050

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Erlenmeyer flask, and 10 mL of 0.2 M potassium dichromate solution was added. Then, 10 mL of concentrated sulfuric acid was slowly added to this solution. When the SOC content was >4% (e.g., PG, TBG, and CRB for surface layers), larger volumes of potassium dichromate and sulfuric acid were used. Oxidation was performed at room temperature, and after a 30-min interval, 50 mL of distilled water, 3 mL of concentrated H3PO4 and four drops of the diphenylamine indicator were added. The SOC content after oxidation was determined by titration of the excess potassium dichromate using a 0.1 M solution of Mohr’s salt. For each set of soil samples, three blank reagents were used to record the exact molarity of the Mohr’s salt solution. Carbon content was determined using the following equation:

g C kg -1 =

(Vb - Vs )× CFe2+ × 0.003 ×1000 wt. of sample (g)

[1]

where Vb and Vs are the volumes of Mohr’s salt solution used for the titration of the blank and the soil sample, respectively; CFe2+ is the molarity of the Mohr’s salt solution; 0.003 g mmol–1 represents the ratio [(0.012)/4], where 0.012 is the molecular mass of C (g mmol–1), and 4 refers to the number of electrons involved in the oxidation of OC; and wt. refers to the mass of the soil sample (g). No CF was used to compensate for partial oxidation of SOM. The dry combustion method was performed using an elemental analyzer (LECO TruSpec CN, St. Joseph, MI). Quantification of TOC was performed in soil samples in which C was oxidized at a 950°C for 2 min in a combustion tube. A subsample from the 2-mm samples of oven dried (40°C) sieved soil was finely ground (