532 Dieckow et al.

COMPARISON OF AND 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 - 4 UFRGS - Depto. de Físico-Química, 90001-970 - Porto Alegre, RS - Brasil. *Corresponding author

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

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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. trolled so that the initial temperature of 120°C could Nitrogen analyses rise to 150°C within one minute. Soil samples of about The Kjeldahl semimicro procedure was 0.50 g were used, but for some treatments with high based on acid digestion followed by steam distillation C concentration the analysis had to be repeated using of the NH formed after addition of alkali (Bremner, 0.25 g. 3 1965). The dry combustion was carried out Dry combustion method (975°C) was per- simultaneously with the C analysis, in the same Vario formed with a Vario EL II analyzer (Elementar EL analyzer, which has a detection limit of 1 μg for Analysensysteme, , Germany) detecting carbon N. The total N stocks in the 0-27.5 cm layer were as CO . The detection limit for carbon is 0.4 μg. Soil 2 calculated according to the same procedure for total samples of about 30 mg were used. Since no carbon- C. ate C is present in this Paleudult (Brasil, 1973), the total C is referred as the organic C. Total C stocks were Statistical analysis calculated for the 0-27.5 cm layer, after considering Data were submitted to analysis of variance to the soil bulk density of each layer (Table 2). Soil den- assess the significance of the effects of cropping sys- sity down to the 27.5 cm depth was determined as tems and N fertilization levels on the C and N con- suggested by Bayer (1996), and in no-N treatments, centrations and stocks measured through the differ- ent analytical methods. Analysis of variance (ANOVA) Table 1 - Average particle-size distribution in the sampled was carried out according to the employed split-plot soil layers. design, considering the cropping system factor as the main plot and N level as sub-plots. The soil layer fac- S.oil layer S.and ± s.d S.ilt ± s.d Clay ± s.d tor was not considered in ANOVA. Means were com- cgm ------g k -1 ------pared by using the Tukey test (P = 0.05), and linear 0 - 2.5 536 ± 20 243 ± 25 221 ± 5 regressions were made between results of Walkley- 24.5 - 7.5 5147 ± 1 2833 ± 2 220 ± Black and dry combustion, and between Kjeldahl and 7.5 - 17.5 525 ± 9 217 ± 12 258 ± 16 dry combustion methods. The confidence intervals of 137.5 - 27.5 4499 ± 2 2415 ± 1 287 ± 2 the angular and linear coefficients of these regressions 27.5 - 37.5 480 ± 34 198 ± 12 322 ± 45 were calculated at P < 0.05, across the three experi- mental replicates. 337.5 - 47.5 4860 ± 2 1881 ± 1 359 ± 1 47.5 - 57.5 425 ± 20 178 ± 15 398 ± 21 RESULTS AND DISCUSSION 577.5 - 67.5 4726 ± 2 1566 ± 1 408 ± 1 67.5 - 87.5 407 ± 24 178 ± 20 415 ± 35 Comparison of methods for C determination 837.5 - 107.5 4014 ± 2 1967 ± 2 418 ± 3 The results of C concentration obtained through the Walkley-Black method correlated well s.d. = standard deviation (R2 ≥ 0.96) with results of C concentration obtained through dry combustion technique, for soil samples of Table 2 - Soil bulk density within the 27.5 cm layer, all cropping systems (Figures 1a, 1b and 1c). The re- determined according to the core method, for gressions found between these two methods were treatments of bare soil, pigeon pea plus maize practically the same in the three cropping systems (Fig- (P+M) and lablab plus maize (L+M) no-till ures 1a, 1b and 1c), showing that results of C recov- systems, under two nitrogen levels (0 kg N ha-1 ery by the Walkley-Black method compared to the ref- – No N, and 180 kg N ha-1 – With N). Original erence dry combustion method were not influenced by data from Bayer (1996). samples originated from different managements. The NNo N With Walkley-Black C recovery was also not affected by land Soil layer BMare soil PM+ LM+ PM+ L+ use and soil management systems for a Russian Cher- cmm ------t -3 ------nozem (Mikhailova et al., 2003), but was for a Typic 0 - 2.5 1.42 1.44 1.55 1.44 1.55 Hapludoll in Argentina Pampa, where the chemical stable compounds of organic matter occurring after 28.5 - 7.5 15.5 14.6 15.6 14.6 1.6 the application of some managements has supposedly 7.5 - 17.5 1.60 1.65 1.64 1.65 1.64 reduced the Walkley-Black C recovery (Diaz-Zorita, 107.5 - 27.5 15.6 13.6 15.6 13.6 1.6 1999).

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Since the relationship between Walkley-Black and L+M or N fertilized, compared to the bare soil or and dry combustion remains the same across samples soils without N fertilization (Table 3). The same trend of different management systems, a general regression was observed for C concentrations as determined by was fitted considering all the treatments together (Fig- the Walkley-Black procedure, except of a lack of sig- ure 1d). In most of the samples, C concentration de- nificance for the difference between fertilized and non- termined through Walkley-Black method was lower fertilized treatments in the top layer of P+M and in the than that obtained through dry combustion (Figure 1), second layer of L+M systems (Table 3). possibly because of incomplete oxidation of the total The statistical trends for results of C concen- organic C in the Walkley-Black procedure (McGeehan tration between cropping systems and between N fer- & Naylor, 1988; Nelson & Sommers, 1996). By tak- tilization levels were comparable for the two meth- ing the inverse of the linear regression in Figure 1d, a ods (Table 3). This was also observed for results of correction equation is thus proposed to be applied on C stocks in the 0-27.5 cm layer, the layer which the C concentration results of Walkley-Black: C = nearly corresponds to the 0-30 cm depth used in soil

1.05CW.Black + 0.47. C inventory surveys (IPCC, 1996) (Table 4). There- The C concentrations of the two top layers as fore, the methods for soil C determinations did not determined through dry combustion were affected by affect the interpretation of the effects of management cropping systems and N fertilization levels, so that systems in this kaolinitic Paleudult. This differs from higher concentrations were found for soils under P+M the findings of Diaz-Zorita (1999) and Brye & Slaton

a) Bare soil b) P+M 20 50 C = 0.99(±0.15)C - 0.73(±0.86) CWB = 0.93(±0.08)CAN - 0.36(±0.65) WB AN 2 18 2 (r = 0.99) ) (r = 0.96) ) -1 16 -1 40 14 12 30 10 8 20 6 4 10 C-Walkley-Black (gkg 2 C-Walkley-Black (gkg 0 0 0 5 10 15 20 0 1020304050 C-analyzer (g kg-1) C-analyzer (g kg-1)

c) L+M d) All systems 50 50 C = 0.95(±0.04)C - 0.45(±0.14) CWB = 0.98(±0.04)CAN - 0.61(±0.38) WB AN 2 (r2 = 0.99) ) (r = 0.99) ) -1 40 -1 40

30 30

20 20

10 10 C-Walkley-Black (g kg C-Walkley-Black (g kg

0 0 0 1020304050 0 1020304050 C-analyzer (g kg-1) C-analyzer (g kg-1)

Figure 1 - Relationship between C concentrations obtained by the Walkley-Black method and C concentrations obtained by a dry combustion method (analyzer) in (a) bare soil (n = 30), (b) P+M system (n = 60), (c) L+M system (n = 60) and (d) all

systems (n = 150). For P+M and L+M, results of both N levels are included. “CWB” means C-Walkley Black, “CAN” means C-analyzer. The values in parenthesis following the ± signal refer to the confidence interval (P < 0.05) of the angular or linear coefficient of the regression.

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Table 3 - Carbon concentrations in bare soil, pigeon pea plus maize (P+M) and lablab plus maize (L+M) systems as measured by dry combustion and Walkley-Black methods. NNo N With Soil layer BMare soil PM+ LM+ PM+ L+ cm ------g C kg-1a ------Dry combustion 0B- 2.5 1b3.3 3b2.6 A 2a9.6 A 3a7.8 35.1 2.5 - 7.5 8.9 B 13.8 Ab 14.0 Ab 17.1 a 16.5 a 7A.5 - 17.5 8a.2 9a.2 A 9a.6 A 1a0.7 10.0 17.5 - 27.5 8.6 A 9.2 Aa 8.7 Ab 9.2 a 9.3 a 2B7.5 - 37.5 8a.4 9a.5 A 8a.8 AB 9a.1 9.1 37.5 - 47.5 7.8 B 9.2 Aa 8.4 ABa 8.6 a 8.8 a 4A7.5 - 57.5 7a.2 8a.2 A 7a.8 A 8a.7 8.2 57.5 - 67.5 6.3 A 7.8 Aa 7.5 Aa 8.0 a 7.5 a 6A7.5 - 87.5 5a.8 6a.6 A 5a.8 A 6a.0 6.2 87.5 - 107.5 5.0 A 5.0 Aa 4.9 Aa 5.1 a 5.1 a Walkley-Black 0 - 2.5 12.4 B 31.2 Aa 28.8 Ab 33.6 a 33.8 a 2B.5 - 7.5 8b.3 1a2.6 A 1a3.2 A 1a5.9 14.4 7.5 - 17.5 7.5 A 8.4 Aa 9.1 Aa 9.6 a 9.4 a 1A7.5 - 27.5 7a.7 8a.0 A 7a.9 A 8a.1 8.5 27.5 - 37.5 7.5 A 8.1 Aa 7.9 Aa 7.8 a 8.2 a 3A7.5 - 47.5 7a.1 8a.2 A 7a.6 A 7a.6 7.9 47.5 - 57.5 6.4 A 7.1 Aa 7.2 Aa 7.4 a 7.5 a 5B7.5 - 67.5 5a.8 6a.9 A 6a.5 AB 7a.1 6.8 67.5 - 87.5 4.9 A 5.7 Aa 5.2 Aa 5.2 a 5.5 a 8A7.5 - 107.5 3a.8 4a.4 A 4a.2 A 4a.3 4.7 aValues in rows followed by the same capital letter do not differ (Tukey test, P = 0.05). This is a comparison between cropping systems. Values in rows followed by the same small letter, in the same cropping system, do not differ (Tukey test, P = 0.05). This is a comparison between N levels.

Table 4 - Carbon stocks in the 0-27.5 cm layer of bare soil, Comparison of methods for N determination pigeon pea+maize (P+M) and lablab+maize (L+M) Nitrogen concentrations obtained by systems as measured by dry combustion and dry combustion were highly correlated (R2 ≥ 0.96) Walkley-Black methods. to those of the Kjeldahl method (Figure 2) and NNo N With the regressions between them were very similar Method for the three cropping systems (Figures 2a, 2b BMare soil PM+ LM+ PM+ L+ and 2c). There was, however, a tendency of the re- -1 a ------t C ha ------sults from Kjeldahl to become closer to those ob- Dry combustion 39 B 54 Aa 53 Aa 61 a 59 a tained by dry combustion, for the L+M system (Fig- WBalkley-Black 3a5 4a9 A 5a0 A 5a4 54 ure 2c). aValues in rows followed by the same capital letter do not differ Total N concentration by the Kjeldahl method (Tukey test, P = 0.05). This is a comparison between cropping tended to be lower than those obtained via dry systems. Values in rows followed by the same small letter, in the combustion, considering the results of all systems same cropping system, do not differ (Tukey test, P = 0.05). together (Figure 2d), but this cannot be considered This is a comparison between N levels. significant according to the confidence interval, which showed that the angular coefficient did not differ (2003), which showed influence of the analytical from 1 and the linear coefficient did not differ from methodology (Walkley-Black and dry combustion) on zero (Figure 2d). Therefore, no correction was nec- the interpretations of their results in different land use essary for the Kjeldahl-N results. The tendency, how- systems and crop rotations. ever, for the higher concentrations obtained by dry

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a) Bare soil b) P+M 1.5 4 N = 0.96(±0.16)N - 0.02(±0.12) KJ AN NKJ = 0.92(±0.06)NAN - 0.02(±0.06) (r2 = 0.96) (r2 = 0.99) ) ) -1

-1 3 1

2

0.5 N-Kjeldahl (g kg N-Kjeldahl

N-Kjeldahl (g kg N-Kjeldahl 1

0 0 00.511.5 01234 N-analyzer (g kg-1) N-analyzer (g kg-1)

c) L+M d) All systems 4 4 NKJ = 0.99(±0.03)NAN - 0.01(±0.04) NKJ = 0.96(±0.05)NAN - 0.003(±0.05) (r2 = 0.99) (r2 = 0.99) ) )

-1 3

-1 3

2 2

N-Kjeldahl (g kg N-Kjeldahl 1

N-Kjeldahl (g kg N-Kjeldahl 1

0 0 01234 01234 N-analyzer (g kg-1) N-analyzer (g kg-1)

Figure 2 - Relationship between total N concentrations obtained by the Kjeldahl semimicro method and the dry combustion method (analyzer) in (a) bare soil (n = 30), (b) P+M system (n = 60), (c) L+M system (n = 60) and (d) all systems (n = 150). For

P+M and L+M, results of both N levels are included. “NKJ” means N-Kjeldahl, “NAN” means N-analyzer. The values in parenthesis following the ± signal refer to the confidence interval (P < 0.05) of the angular or linear coefficient of the regression. combustion could be related to a non quantitatively means that the interpretations of the effect of soil release of some refractory N, or of N in N-O struc- management on the N concentrations and stocks, as - tures of NO3 (McGeehan & Naylor, 1988; Bremner, well as the conclusions drawn from these interpre- 1965). tations, are comparable irrespective if analysis were Similar to C, the N concentrations in the performed through dry combustion or Kjeldahl two top layers as assessed by dry combustion were method. significantly higher for the P+M and L+M systems C:N ratios as compared to bare soil, and for fertilized soils than One of the basic information in soil organic non fertilized treatments (Table 5). About the same matter characterization studies is the C:N ratio. There statistical trend was observed for results of the 2 was a poor correlation (R = 0.42) between the C:N Kjeldahl method, except for the top layer of P+M and ratios calculated from results of C and N obtained the second layer of L+M, where no differences were through dry combustion and those calculated from observed between fertilized and non-fertilized treat- results of Walkley-Black and Kjeldahl methods (Fig- ments (Table 5). In a general view, the statistical ure 3a). The same trend was observed when the trends observed for the results of N concentration Walkley-Black-C concentrations were corrected by are relatively similar, either for dry combustion or the respective correction equation (C = 1.05CW.Black Kjeldahl (Table 5). Similar statistical trends of the two + 0.47) (Figure 3b) or when the Walkley-Black-C methods were also observed for the results of total concentration was multiplied by the correction fac- N stocks in the 0-27.5 cm layer (Table 6). This tor of 1.12, as commonly performed for soils analy-

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Table 5 - Total nitrogen concentration in bare soil, pigeon pea plus maize (P+M) and lablab plus maize (L+M) samples as measured by dry combustion and Kjeldahl. NNo N With Soil layer BMare soil PM+ LM+ PM+ L+ cm ------g kg-1a ------Dry combustion 0B- 2.5 1b.20 2b.61 A 2a.63 A 3a.04 3.14 2.5 - 7.5 0.84 B 1.28 ABb 1.36 Ab 1.54 a 1.56 a 7A.5 - 17.5 0a.73 0a.89 A 0a.91 A 0a.98 0.94 17.5 - 27.5 0.68 A 0.81 Aa 0.76 Aa 0.81 a 0.84 a 2A7.5 - 37.5 0a.65 0a.76 A 0a.69 A 0a.74 0.69 37.5 - 47.5 0.61 B 0.72 Aa 0.65 ABa 0.68 a 0.68 a 4B7.5 - 57.5 0a.61 A 0a.67 A 0a.59 B 0a.69 0.64 57.5 - 67.5 0.54 A 0.65 Aa 0.59 Aa 0.67 a 0.62 a 6A7.5 - 87.5 0a.52 0a.59 A 0a.49 A 0a.54 0.54 87.5 - 107.5 0.50 A 0.50 Aa 0.44 Aa 0.50 a 0.48 a Kjeldahl 0 - 2.5 1.15 B 2.50 Aa 2.67 Ab 2.74 a 3.08 a 2B.5 - 7.5 0b.78 1a.24 A 1a.33 A 1a.52 1.45 7.5 - 17.5 0.65 B 0.88 Aa 0.92 Aa 0.96 a 0.92 a 1A7.5 - 27.5 0a.61 0a.70 A 0a.71 A 0b.77 0.78 27.5 - 37.5 0.60 A 0.70 Aa 0.70 Aa 0.71 a 0.72 a 3A7.5 - 47.5 0a.58 0a.66 A 0a.64 A 0a.65 0.66 47.5 - 57.5 0.56 A 0.61 Aa 0.59 Aa 0.62 a 0.62 a 5B7.5 - 67.5 0a.50 0a.58 A 0a.56 AB 0a.58 0.57 67.5 - 87.5 0.48 A 0.53 Aa 0.49 Aa 0.54 a 0.50 a 8A7.5 - 107.5 0a.44 0a.46 A 0a.43 A 0a.43 0.48 aValues in rows followed by the same capital letter do not differ (Tukey test, P = 0.05). This is a comparison between cropping systems. Values in rows followed by the same small letter, in the same cropping system, do not differ (Tukey test, P = 0.05). This is a comparison between N levels.

Table 6 - Nitrogen stocks in the 0-27.5 cm layer of bare soil, values, which possibly amplifies the distortions be- pigeon pea plus maize (P+M) and lablab plus maize tween the two methods. (L+M) systems as measured by dry combustion There was a tendency for the C:N ratio cal- and Kjeldahl methods. culated from non-corrected Walkley-Black-C and NNo N With Kjeldahl-N to underestimate the ratio calculated from Method BMare soil PM+ LM+ PM+ L+ dry combustion, above the value of 9.5 (Figure 3a). On the other hand, the C:N ratio calculated from the ------t N ha-1 a ------corrected Walkley-Black-C (C = 1.05CW.Black + 0.47) Dry combustion 3.3 B 4.8 Aa 4.9 Aa 5.3 a 5.4 a and Kjeldahl-N overestimated the C:N ratio from dry KBjeldahl 3a.0 4a.5 A 4a.8 A 5a.1 5.2 combustion, for values below 14.0, which are the com- aValues in rows followed by the same capital letter, in the same mon C:N ratios for soils. These results are indications N level, do not differ (Tukey test, P = 0.05). This is a comparison that the absolute C:N ratio values should be interpreted between cropping systems. Values in rows followed by the same with caution, since under- or overestimations can oc- small letter, in the same cropping system, do not differ (Tukey cur in relation to the real value according to the em- test, P = 0.05). This is a comparison between N levels. ployed methodological procedure. But in spite of this, sis in Rio Grande do Sul (Tedesco et al., 1995) (Fig- there is a clear tendency for samples with high C:N ure 3c). This poor correlation for the C:N ratio as ratios analyzed according to wet methods, also show compared to the high correlations for C and N may high C:N ratios analyzed according to dry combustion, be a result from the division operation to obtain C:N and vice versa.

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a) Non-corrected Walkley-Black-C b) Corrected Walkley-Black-C and Kjeldahl-N and Kjeldahl-N 15 15 C:Nwet = 0.70(±0.11)C:NAN + 2.86(±1.75) 2 14 (r = 0.42) ) 14 -1 13 13 12 12 11 11 10 10 9 9

8 C-Walkley-Black (g kg 8 C:Nwet = 0.75(±0.16)C:NAN + 3.51(±2.43) 2

C:N-Walk le y-Black : Kje ldahl (r = 0.42) 7 7 7 9 11 13 15 7 9 11 13 15 C:N-analyzer C:N-analyzer

c) Corrected Walkley-Black-C (f=1.12) and Kjeldahl-N (Tedesco et al., 1995) 15 14

13 12 11 10 9 8 C:Nwet = 0.79(±0.12)C:NAN + 3.18(±1.94) 2

C:N-Walk le y-Black : Kje ldahl (r = 0.42) 7 79111315 C:N-analyzer

Figure 3 - Relationships between the C:N ratio values obtained by the Walkley-Black and Kjeldahl methods and the C:N ratio values obtained by dry combustion, considering (a) the non-corrected Walkley-Black C and the Kjeldahl N, (b) the corrected

Walkley-Black C (C = 1.053CW.Black + 0,470) and the Kjeldahl N, and (c) the corrected Walkley-Black C using the factor 1.12

suggested by Tedesco et al. (1995) for soils of Rio Grande do Sul and the Kjeldahl N (n = 150). “C:Nwet” means C:N ratio

calculated from Walkley-Black and Kjeldahl, “C:NAN” means C:N-analyzer. The values in parenthesis following the ± signal refer to the confidence interval (P < 0.05) of the angular or linear coefficient of the regression.

CONCLUSIONS that both methodological approaches provide compa- rable conclusions in soil organic matter studies in dif- The C concentration determined by Walkley- ferent management systems. Black and the N concentration determined by The C:N ratio results obtained after Walkley- Kjeldahl methods obtained for this kaolinitic Paleudult Black and Kjeldahl methods have a poor correlation to profile highly correlate to the respective results of results obtained after dry combustion technique. Pos- C and N concentrations determined through the sibly the deviations between results of Walkley-Black automated dry combustion method (Elementar C and dry combustion C and the deviations between Vario EL), and, the correction equation (C = results of Kjeldahl N and dry combustion N were mag-

1.05CW.Black + 0.47) is suggested to be applied on re- nified after the calculation of C:N ratios. sults of C obtained by Walkley-Black. The results of N obtained by Kjeldahl methods did not need correc- ACKNOWLEDGEMENTS tion. The methods for C and N determination did To J. A. Zanatta, for her collaboration on sta- not affect the interpretation of the effects of manage- tistical analysis and to CNPq for providing scholarships ment systems on soil C and N stocks, indicating to J. Mielniczuk, C. Bayer and D.P. Dick. CAPES and

Sci. Agric. (Piracicaba, Braz.), v.64, n.5, p.532-540, September/October 2007 540 Dieckow et al.

DAAD are acknowledged for supporting the Probral GEE, G.W.; BAUDER, J.W. Particle-size analysis. In: KLUTE, A. project, in the framework of which this study was car- (Ed.). Methods of soil analysis. Part 1. Physical and mineralogical methods. Madison: SSSA, 1986. p.383-411. ried out. To FAPERGS, for financial support in the INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE - framework of the project PRONEX. IPCC. Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories: The workbook. Geneva: IPCC, 1996. REFERENCES McGEEHAN, S.L.; NAYLOR, D.V. Automated instrumental analysis of carbon and nitrogen in plant and soil samples. BAYER, C. Dynamics of soil organic matter in management Communications in Soil Science and Plant Analysis, v.19, systems. Porto Alegre: UFRGS, 1996. 241p. (Thesis Ph.D.). p.493-505, 1988. BLAKE, G.R.; HARTAGE, K.H. Bulk density. In: KLUTE, A. (Ed.). MIKHAILOVA, E.A.; NOBLE, R.R.P.; POST, C.J. Comparison of Methods of soil analysis. Part 1. Physical and mineralogical soil organic carbon recovery by Walkley-Black and dry methods. Madison: SSSA, 1986. p.363-375. combustion methods in the Russian Chernozem. BRASIL. Ministério da Agricultura. Levantamento de Communications in Soil Science and Plant Analysis, v.34, reconhecimento dos solos do Rio Grande do Sul. Recife: p.1853-1860, 2003. Convênio MA/DPP – SA/DRNR, 1973. 431p. NELSON, P.W.; SOMMERS, C. E. Total C, organic C and organic BREMNER, J.M. Organic forms of nitrogen. In: BLACK, C.A. matter. In: PAGE, A.L. (Ed.). Methods of soil analysis. Part (Ed.). Methods of soils analysis. Part 2. Chemical and 2. Chemical methods. Madison: SSSA, 1996. p.539-579. microbiological properties. Madison: SSSA, 1965. p.1238-1255. PÉREZ, D.V.; ALCANTARA, S.; ARRUDA, R.J.; MENEGHELLI, BREMNER, J.M.; SHAW, K. Denitrification in soil. I. Methods of N.A. Comparing two methods for soil carbon and nitrogen investigation. Journal of Agricultural Science, v.51, p.22- determination using selected Brazilian soils. Communications 39, 1958. in Soil Science and Plant Analysis, v.32, p.295-309, 2001. BRYE, K.R.; SLATON, N.A. Carbon and nitrogen storage in a STEWART, B.A.; PORTER, L.K.; CLARK, F.E. The reliability of Typic Albaqualf as affected by assessment method. a micro-dumas procedure for determining total nitrogen in soil. Communications in Soil Science and Plant Analysis, v.34, Soil Science Society of America Proceedings, v.27, p.377- p.1637-1655, 2003. 380, 1963. DIAZ-ZORITA, M. Soil organic carbon recovery by the Walkley- TEDESCO, M.J.; GIANELLO, C.; BISSANI, C.A.; BOHNEN, H.; Black method in a Typical Hapludoll. Communications in VOLKWEISS, S.J. Análise de solo, plantas e outros Soil Science and Plant Analysis, v.30, p.739-745, 1999. materiais. 2.ed. Porto Alegre: UFRGS, 1995. 174p. DIEKOW, J.; MIELNICZUK, J.; KNICKER, H.; BAYER, C.; DICK, D.P.; KÖGEL-KNABNER, I. Soil C and N stocks as affected by cropping systems and nitrogen fertilization in a southern Brazil Acrisol managed under no-tillage for 17 years. Soil and Tillage Received November 30, 2005 Research, v.81, p.87-95, 2005. Accepted June 22, 2007

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