532 Dieckow et al. COMPARISON OF CARBON AND NITROGEN DETERMINATION METHODS FOR SAMPLES OF A PALEUDULT SUBJECTED TO NO-TILL CROPPING SYSTEMS Jeferson Dieckow1*; João Mielniczuk2; Heike Knicker3; Cimélio Bayer2; Deborah Pinheiro Dick4; Ingrid Kögel-Knabner3 1 UFPR - Depto. de Solos e Engenharia Agrícola, R. dos Funcionários 1540 - 80035-050 - Curitiba, PR - Brasil. 2 UFRGS - Depto. de Solos, C.P. 15100 - 91501-970 - Porto Alegre, RS - Brasil. 3 TUM - Lehrstuhl für Bodenkunde, D-85350 - Freising-Weihenstephan - Germany 4 UFRGS - Depto. de Físico-Química, 90001-970 - Porto Alegre, RS - Brasil. *Corresponding author <[email protected]> ABSTRACT: Organic carbon (C) concentration evaluated by the Walkley-Black method, and total nitrogen (N) concentration determined by Kjeldahl method, were compared with corresponding results of C and N concentrations obtained through the dry combustion method (Elementar Vario EL analyzer), using samples of ten soil layers down to the depth of 107.5 cm of a kaolinitic Typic Paleudult (220-418 g clay kg-1 along the profile) subjected to no-till cropping systems [fallow bare soil, pigeon pea (Cajanus cajan L. Millsp.) plus maize, and lablab (Lablab purpureum L. Sweet) plus maize]. A close correlation (R2 ≥ 0.96) was observed between the C results of the Walkley-Black and dry combustion methods, but a correction was suggested to be applied to C results of Walkley-Black (C = 1.05CW.Black + 0.47). A close correlation (R2 ≥ 0.96) was also found between results of Kjeldahl-N and dry combustion-N, but no correction was necessary to be applied. The relationships between results of Walkley-Black and dry combustion and between results of Kjeldahl and dry combustion did not change among soil samples from different management systems, in disagreement to findings of a previous study where results of analytical C recovery were influenced by samples from different managements, supposedly because changes in lability or recalcitrance of organic matter might have affected the wet combustion reactions. A poor correlation (R2 = 0.42) was found between the C:N ratios based on Walkley-Black and Kjeldahl analysis and the C:N ratios based on dry combustion analysis. The methods for C and N determination did not affect the interpretation of the effects of management systems on soil C and N stocks. Key words: Walkley-Black, Kjeldahl, wet combustion, dry combustion COMPARAÇÃO DE MÉTODOS DE DETERMINAÇÃO DE CARBONO E NITROGÊNIO EM AMOSTRAS DE ARGISSOLO SOB PLANTIO DIRETO RESUMO: Resultados de concentração de carbono (C), obtidos pelo método Walkley-Black, e resultados de concentração de nitrogênio (N) total, determinados pelo método Kjeldahl, foram comparados com os correspondentes resultados de C e N obtidos pelo método de combustão seca (analisador Elementar Vario EL), utilizando-se amostras de dez camadas de um Argissolo Vermelho até a profundidade de 107.5 cm (220-418 g argila kg-1 ao longo do perifl), manejado em sistemas de culturas de plantio direto [solo descoberto, guandu (Cajanus cajan L. Millsp.) mais milho, e lab lab (Lablab purpureum L. Sweet) mais milho]. Alta correlação (R2 ≥ 0.96) foi observada entre os resultados de C-Walkley-Black e C-combustão seca. Uma correção foi sugerida para os resultados de C-Walkley-Black (C = 1.05CW.Black + 0.47). Elevada correlação (R2 ≥ 0.96) também foi observada entre os resultados de N-Kjeldahl e N- combustão seca, não sendo necessária a aplicação de uma correção aos resultados de N-Kjeldahl. A relação entre os resultados de Walkley-Black e combustão seca e entre os resultados de Kjeldahl e combustão seca não mudaram entre amostras de solo de diferentes sistemas de manejo, diferindo dos resultados de um trabalho anterior onde a recuperação analítica de C foi influenciada por amostras oriundas de diferentes manejos, supostamente por alterações na labilidade ou recalcitrância da matéria orgânica terem afetado as reações de combustão úmida. Uma baixa correlação (R2 = 0.42) foi observada entre a relação C:N baseada nas análises Walkley-Black e Kjeldahl e a relação C:N baseada na combustão seca. Os métodos de determinação de C e N não afetaram as interpretações do efeito de sistemas de manejo sobre os estoques de C e N no solo. Palavras-chave: Walkley-Black, Kjeldahl, combustão úmida, combustão seca Sci. Agric. (Piracicaba, Braz.), v.64, n.5, p.532-540, September/October 2007 C and N determination in a Paleudult 533 INTRODUCTION ference among samples of distinct management sys- tems was attributed to the greater amounts of chemi- The reliability of methods for carbon (C) and cal stable organic compounds in pasture than in crop nitrogen (N) measurements is of fundamental concern residues. This influence of samples from different in soil organic matter studies because C and N analy- management on C results could not be however con- ses constitute the first analytical step towards soil or- firmed in a study conducted in a Russian Chernozem ganic matter quantification and characterization. Au- (Mikhailova et al., 2003). tomated dry combustion methods, although being of The hypothesis of this study is that different easy and simple operation, not requiring excessive analytical methods for soil C and N determinations (dry amounts of reagents, presenting high accuracy and not and wet combustion) provide different C and N con- having the problem related to chemical waste disposal, centration results, and thus corrections to a standard are not always feasible for small laboratories due to method should be necessary. Therefore, this study acquisition costs of the equipment. Thus, in many soil compares the results of soil organic C concentration laboratories the conventional Walkley-Black and and stock obtained by Walkley-Black method and the Kjeldahl methods for C and N analysis, respectively, results of total N concentration and stock determined are still routinely being employed. by Kjeldahl method with the respective C and N con- Although the C concentration results given by centrations and stocks obtained through the automated the Walkley-Black rapid dichromate oxidation method dry combustion method (Elementar Vario EL analyzer), are generally lower than those obtained through the ref- using samples of a subtropical kaolinitic Typic Paleudult erence dry combustion methods (McGeehan & Naylor, subjected to different no-till cropping systems. 1988; Mikhailova et al., 2003) a high correlation is commonly found between these two methods MATERIAL AND METHODS (Mikhailova et al., 2003; Pérez et al., 2001). To over- come the problem of incomplete C recovery which Soil samples leads to the underestimation of C concentrations by Soil samples were collected from a long-term Walkley-Black method, the analytical results of this experiment (18 years) located in Eldorado do Sul, State technique are corrected by a factor, commonly reported of Rio Grande do Sul, Brazil (30°51’ S and 51°38’ W). as 1.30 when no external heat is applied (Nelson & The soil was a kaolinitic Typic Paleudult, and the cli- Sommers, 1996). A correction factor of 1.63 was pro- mate as humid-subtropical (Cfa, Köppen). Ten soil lay- posed for a Russian Chernozem (Mikhailova et al., ers were sampled down to the 107.5-cm depth, in three 2003), while a factor of 1.12 was suggested for sub- replicates of three no-till cropping systems: bare soil; tropical soils of the State of Rio Grande do Sul, Brazil pigeon pea (Cajanus cajan L. Millsp.) plus maize (Zea (Tedesco et al., 1995), but with application of exter- mays L.) [P+M]; and lablab (Lablab purpureum L. nal heat. Sweet) plus maize [L+M]. The P+M and L+M sys- The Kjeldahl method of soil N analysis is be- tems were conducted under two N levels (0 and 180 ing commonly employed in laboratories not equipped kg N ha-1 applied to the maize crops). Soil samples with a dry combustion automated analyzer, although were air dried and ground in a mortar to pass through more personal labor and reagents are required. In or- a 0.50-mm mesh sieve. ganic soils or soils with high organic matter contents, The clay fraction of the 0-20 cm layer is con- the results of N concentration obtained through the stituted mainly by kaolinite (720 g kg-1), but contains Kjeldahl method are generally lower than results of dry also iron oxides (109 g kg-1) and other unidentified min- combustion (Bremner & Shaw, 1958; Stewart et al., erals (Bayer, 1996). An increment in clay content from 1963). For mineral soils or soils with low organic mat- 221 g kg-1 in the top layer to 418 g kg-1 in the deepest ter contents, results have been comparable between layer was found, due to the presence of the argillic these two methods (Bremner & Shaw, 1958; Stewart horizon (Table 1). Soil texture analyses were performed et al., 1963). using the hydrometer with a Bouyoucos’ scale, (Gee Regarding the influence that soil samples origi- & Bauder, 1986). More details about the experiment nated from different management systems might have and sampling are given elsewhere (Diekow et al., on analytical results of wet combustion or digestion, 2005). Diaz-Zorita (1999) showed that C recovery by Walkley- Black method was 15% lower compared to dry com- Carbon analyses bustion method in samples of a pasture land in Argen- The Walkley-Black procedure of C analysis tina Pampa, while for agricultural land samples, results was based on the description of Nelson & Sommers were comparable among the two methods. This dif- (1996), with adaptation to the employment of exter- Sci. Agric. (Piracicaba, Braz.), v.64, n.5, p.532-540, September/October 2007 534 Dieckow et al. nal heat (Tedesco et al., 1995). The heating was pro- according to the core method (Blake & Hartage, 1986). vided by swirling the suspension over a Bunsen For N-fertilized treatments, considered the same bulk burner-flame after the addition of H2SO4 and was con- density of no-N treatments was considered.