- Email: [email protected]
N biochemical diversity as a factor of soil diversity
Pergamon
003&0717(94)00159-6
Soil Biol. Biochem. Vol. 27, No. 2, pp. 2OS-210, 1995 Copyright 0 1995Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-0717/9S $9.SO+O.O0
N BIOCHEMICAL DIVERSITY AS A FACTOR OF SOIL DIVERSITY S. J. GONZALEZ-PRIETO
and T. CARBALLAS*
Instituto de Investigaciones Agrobiologicas de Galicia (CSIC), Apartado 122, 15700 Santiago de Compostela, Spain (Accepted
10 July 1994)
Summary-As lithospherebiosphere interphases, soils have some characteristics (mainly biochemical characteristics) highly influenced by living organisms and therefore these characteristics vary according to soil biological activity and diversity. The variability of these characteristics could be used for the differentiation of soil types. This potential utility was successfully analysed for N biochemical fractions in temperate-humid region soils. The fractions of medium-to-low hydrolytic resistance and among them hydrolysable organic N, a-aminoacid N, hydrolysable unidentified N and, to a lesser extent, hexosamine N, were the main organic N forms to supplement the physical and chemical characteristics in soil classification systems.
Soil classifications are usually based on soil properties or occasionally on characteristics related to genesis and evolution of the profile. In all cases the main variables considered--soil moisture and temperature regimes, colour, texture, structure, depth, pH, organic matter, exchangeable cations, weatherable minerals, CaC03, Fe and Al oxides, etc.-have a physical or chemical character (FAO/UNESCO, 1974; Soil Survey Staff, 1975). Nevertheless, as lithospherebiosphere interphases, soils have some characteristics highly influenced by living organisms. These characteristics, therefore, usually have a biochemical character and vary according to biological activity and diversity. The variability of these characteristics could be useful for the differentiation of the types of soil but they have been scarcely used for this aim. Variables related to carbonaceous fractions (type of humus, fulvic-to-humic acid:; ratio) have been considered (Kubiena, 1952; Duchaufour, 1984) as well as the usefulness of aminoacid N to establish paleosol horizons (Goh, 197211. The high interdependence between living organisms and the N cycle (Sprent, 1987), together with the documented relationships between N cycle microorganisms, or organic N distribution, and soil physical and chemical charac:teristics (Acea and Carballas, 1989; Gonzalez-Prietlo and Carballas, 199 1) suggest that biochemical N fractions could be used to differentiate soil types. Like the case of soils, biochemical N fractions are the result of the biosphere-lithosphere interphase. We have examined the potential use of N fractions to differentiate soils
*Author for correspondmcnce.
from a temperate-humid region and determined the main N fractions responsible for soil diversity.
MATERIALS AND MJSTHODS The soils used were those described by GonzalezPrieto and Carballas (1991). They comprised 23 soil horizons coming from a temperate-humid region (Galicia, NW Spain). Their main physical and chemical characteristics as well as their organic N forms were described in the same paper. The method used for the hydrolytic fractionation of organic N (Gonzalez-Prieto and Carballas, 1988) consists basically of four successive hydrolyses of increasing hydrolytic strength with 1~ HCI for 3 h (hydrolysate 1 or Hl), 3N HCl for 3 h (H2), 6~ HCI for 4 h (H3) and 6~ HCI for 20 h (H4), being CH=Hl+HZ+H3+HC The 23 samples considered (16 surface horizons and 7 subsurface horizons) were grouped according to the similarity of: (A) the 15 main physical and chemical characteristics; (B) the organic N fractions; and (C) the physical and chemical characteristics and the organic N fractions together. The hierarchical clustering of the samples was performed using the program “BMD.PZM Cluster Analysis of Cases” [Biomedical Computer Programs, University of California, Berkeley, CA (Engelman, 1990)]. Similarity between samples was established according to their Euclidean distance (i.e. the squared root of the sum of the squared differences of the values for each variable), most alike samples having the shortest Euclidean distances between them. The organic N compounds responsible for most of the soil organic N variability were found by a factor analysis using the program “BMD.P4M Factor
205
S. J.
206
Gonzhlez-Prieto and T. Carballas
Analysis of Cases” [Biomedical Computer Programs, University of California, Berkeley, CA (Frane et al., 199011.This program, which is analogous to a multiple regression model with several dependent variables, was applied to the data matrix formed by the 23 soil samples and the 25 variables related to organic N distribution. By a varimax rotation, the factor analysis finds the factors (the axes on the plots), which are a few weighted averages of the original variables, that summarize, as much as possible, the information that the variables contain. The distribution of the variables is plotted in the plane defined by each pair of factors according to the corresponding regression coefficients (factor loadings) of the variables on each factor. In Figs 4 and 5, the circles contain the more important variables for the definition of each axis, i.e. the variables whose regression coefficients on the axis were >oso.
RJEWLTS AND DISCUSSION
The cluster obtained according to the similarity of the main physical and chemical characteristics (Fig. 1) showed a soil grouping close to the FAO/UNESCO (1974) classification. Nevertheless, they were not identical because the cluster program considers that all the variables have the same category, whereas the FAO/UNESCO classification uses certain characteristics to define diagnostic horizons, or diagnostic properties, which have a higher systematic value than the other characteristics. It must be pointed out that in the cluster analysis some profiles were not well-differentiated. This is the case of the Ferralic Cambisol A-23 that appears between the Rendzina
6
5
4
9
e
and the Rankers, although the organic matter of the three profiles is clearly different: Fe, Ca and Al, respectively, being the cations that influence their organic matter evolution (Duchaufour, 1984). Moreover, the degree of evolution of the Ferralic Cambisol is much greater than that of the others, this soil being the only one that has an altered horizon (Cambic B horizon). It is also noteworthy the poor discrimination between the horizons from Podzol, Cambisols and Atlantic Ranker (sensu Carballas, 1982) three types of soil that have different genesis and profile differentiation. In the Podzol the dominant process is emigration of Fe and Al complexed by organic compounds (Spodic horizon), whereas the Cambisol is formed by in situ weathering processes (Cambic horizon). The Atlantic Ranker, with an Umbric horizon, is a polycyclic soil formed by deposition of very stable organic material coming from other places in the slope (FAO/UNESCO, 1974; Carballas, 1982). Grouping the samples according to the similarity of the biochemical N fractions (Fig. 2) showed high coherence with the FAOjUNESCO (1974) classification. This is in keeping with the relationships between organic N fractions and different physical and chemical characteristics found by Gonzalez-Prieto and Carballas (1991). Moreover, with regard to Fig. 1, Fig. 2 shows that the Ferralic Cambisol A-23 and the Ranker GM4 were correctly separated from the Rendzina and grouped with the other Cambisols. Figure 2 also shows that the nature of the distribution of organic N differentiated the Humic Cambisols SE-8 and SP-13 from the other Cambisols. In Fig. 2 the discrimination among the horizons from Podzol, Atlantic Ranker and Humic Cambisols was also
!
0
Key
Type of soil
CA-24 A-23 GM-4 PZ-19 Ml-22 Sl
Rendzina Ferralic Cambisol Ranker Ranker Eutric Fluvisol Eutric Fluvisol Eutric Fluvisol Eutric Fluvisol Eutric Fluvisol Ferralic Cambisol Atlantic Ranker Humic Cambisol Humic Cambisol Humic Camblsol Humlc Cambisol Humic Cambisol Atlantic Ranker Humic Cambisol Atlantic Ranker Humic Cambisol Orthic Podzol Atlantic Ranker Orthlc Podzol
:‘3
K-1 R-17 5
GC-32 SE-9 SP-13 G P-5 GR-19 R3 GE-20 R2 ET-21 PBh R4 PAo-2
Fig. 1. Cluster of soil samples grouped according to the similarity of their main physical and chemical characteristics. The scale shows the amalgamation distances between samples and/or clusters. The key is the same used by GonzBlez-Prieto and Carballas (1991) to identify the samples. Types of soil according to FAO/UNESCO (1974); Atlantic Ranker, sensu Carballas (1982).
N biochemical diversity as a factor of soil diversity 6
7
5
2
3
4
207
t1
-l
Key
Type of soil
CA-24 PZ-1 a SE-0 SP-13 R-l 7 R3 R4 R2 M2
Rendzina Ranker Humic Cambisol Humic Camblsol Atlantic Ranker Atlantic Ranker Atlantic Ranker Atlantic Ranker Eutrlc Fluvisol Eutric Fluvisol Eutric Fluvisol Eutric Fluvisol Humic Cambisol Humic Cambisol Humic Cambisol Eutric Fluvisol Ferralic Cambisol Ranker Ferralic Camblsol Humlc Cambisol Humic Cambisol Orthic Podzol Orthic Podzol
:il Sl GC-32 GP-5 ET-21 Ml-22 A-23 GM-4 SG-15 GE-20 GR-19 PBh PAo-2
Fig. 2. Cluster of soil samples grouped according to the similarity of their organic N fractions. The scale shows the amalgamation distances between samples and/or clusters. The key is the same used by Gonzalez-Pneto and Carballas (1991) to identify the samples. Types of soil according to FAOjUNESCO (1974); Atlantic Ranker, sense Carballas (1982).
improved compared to Fig. 1. On the other hand, the horizons of the Atlantic Ranker were grouped because the distribution of N forms is similar in all of them and very different from that in the other soils. Finally, the singularity of MI-212 horizon from the Eutric Fluvisol [marsh according to the classification of Kubiena (1952)] was recognized. Thus, this horizon appears associated to the reference terrestrial profile, which is
10 :
8
7
6
5
in keeping with the terrestrification of its organic matter shown by Gonzalez-Prieto et al. (1989); nevertheless, its separation from the other Fluvisols could be excessive according to the other edaphic characteristics. Figure 3 shows the grouping samples cluster due to the similarity of the main physical and chemical characteristics and the organic N fractions together.
4 I
3
2 Key
Type of soil
CA-24 PZ-10 Ml-22
Rendzina Ranker Eutric Fluvisol Eutric Fluvisol Eutric Fluvisol Eutric Fluvisol Eutric Fluvisol Ferralic Cambisol ;tu&Cambisol
:: s3 !!23 SP-13
Humic Cambisol Humic Cambisol g:-_532 Humic Cambisol ET-21 GE-20 ::I:; R-17 R4 R3 ._ ;:h PAo-2
Humic Cambisol Humic Cambisol Humlc Cambisol Ferrallc Cambisol Atlantic Ranker Atlantic Ranker Atlantic Ranker Atlantic Ranker Orthic Podzol Orthlc Podzol
Fig. 3. Cluster of soil samples grouped according to the similarity of their main physical and chemical characteristics and organic N fractions. The scale shows the amalgamation distances between samples and/or clusters. The key is the same used by Gonzalez-Prieto and Carballas (1991) to identify the samples. Types of soil according to FAOjUNESCO (1974); Atlantic Ranker, sensu Carballas (1982).
208
S. J. Gonzalez-Prieto and T. Carballas
0.8 -
0.6
-
0,
lion-48
.+“A
oHA.ON
Fig. 4. Factor analysis of the organic N fractions. Location of the variables in the plane defined by factors I and II. Scales show the factor loadings (regression coefficients) of the variables on each axis. HA, hydrolysable NH:-N; AM, amide N; HEX, hexosamine N; AA, cc-aminoacid N; HUN, hydrolysable unidentified N; HON, hydrolysable organic N; ON total organic N. Numbers l-4 indicate the hydrolysate number from which the fractions originate and ZH indicates the cumulated amount of the corresponding N form from the four hydrolysates (see Materials and Methods). Amide N was completely solubilized in Hl.
This cluster revealed the supplementarity of both types of variables to differentiate types of soil in a logical and efficient way, even without the establishment of diagnostic characteristics. Like the preceding analyses, the first type of soil separated was the Rendzina. This was followed, although at greater distance than in the other analyses, by the Ranker PZ- 18 and the Fluvisol group, all of them characterized by a low degree of differentiation of the profile. Then the Ferralic Cambisol A-23 was separated from the Humic Cambisols group between which it appears the Ranker GM-4, which is an intergrade soil between Rankers and Cambisols, that cannot be classified as Humic Cambisol only because it does not satisfy some FAO/UNESCO (1974) requirements for Cambic horizons. The soil SG-15 appears less differentiated from Humic Cambisols, possibly due to its less marked ferralic characteristics (FAO/UNESCO, 1974). Finally, the horizons from the Atlantic Ranker and the Podzol are well-separated from each other and from those of the Humic Cambisols. Considering the utility of the variability of the biochemical organic N fractions to differentiate with more accuracy some groups of soils, the most important N-containing compounds from this point of view were determined by factor analysis according to the variance explained. The first three factors accounted for 80% of the total variance, factors I, II
and III amounting to 43,23 and 14%, respectively, of the variance. Figure 4 shows the distribution of the variables in the plane defined by axes I and II, which together explained 2/3 of the total variance. The positive extreme of the first factor (axis I) was defined by the N fractions solubilized during the hydrolysis with 3~ HCl for 3 h (H2) as well as a-aminoacid N, hydrolysable unidentified N and the total organic N hydrolysable with 6~ HCl for 4 h (H3), all of them with high factor loadings (i.e. with regression coefficients > 0.50 on factor I). Axis II was defined at its positive extreme by the variables from Hl . Moreover, a group of variables with high factor loadings on the positive extreme of both axes I and II, occupies intermediate positions between both axes. This group includes the cumulated percentages from all the hydrolysates (ZH) of each organic N form, excepting NH:-N; hexosamine tentatively coefficients considered solubilized included in each of the hydrolysable N always remarkable
N was included in this group only because of its lower (cO.50) regression on both axes. Amide N, which can be a variable of Hl and CH because it is only in the first hydrolysate, was also this group. It must be pointed out that for three groups considered, a-aminoacid N, unidentified N and hydrolysable organic have higher factor loadings. It is also that the variables related to hexosamine N
209
N biochemical diversity as a factor of soil diversity contents and to the different N-containing compounds of H4 showed a poor resolution on the plane defined by the first two factors. Only NH:-N from H4 and total organic N have negative, although low, factor loadings on both axes. It was therefore shown that the important direction of variation determined by axis I differentiates samples according to organic N content of medium resistance to hydrolysis, whereas the direction defined by axis II differentiates samples according to the most labile N content. Specifically, the N-containing fractions most strongly associated with both factors were aaminoacid N, hydrolysable unidentified N and hydrolysable organic N, the cumulated values of each organic N fraction !for all the hydrolysates occupying intermediate positions between axes I and II with important factor loadings on both axes. Figure 5 shows tlhe distribution of the variables in the plane defined by axes I and III. Both axes accounted for 57% of the total variance. Axis III defines a direction of variation based mainly on hexosamine N content. Nevertheless, three variables of a-aminoacid N, which are located in an intermediate position in the first quadrant of the plane defined by axes I and II, also have high factor loadings. The only variables with negative although low factor loadings on both axes were total organic N and hydrolysable unidentified N from HI. Factors I and III are therefore determined by the variables of H2 and aminosugars, respectively, although the factor loadings of aaminoacid N and II3 variables are not negligible.
In summary, factor analysis attributed the most importance to fractions of low and medium resistance to acid hydrolysis and among them to hydrolysable organic N, a-aminoacid N, hydrolysable unidentified N and to a lesser extent to hexosamine N. These forms were, for the soils studied, the main variables to supplement the physical and chemical characteristics in soil classification systems. Confirmation of these results in others types of soil is difficult because of the different hydrolytic methods successive stepwise used by various authors: hydrolyses in the study by Gohlez-Prieto and Carballas (1991) and continuous hydrolysis in most of the papers published on this matter. Nevertheless, although it was impossible to compare the resistance to hydrolysis of the different N fractions, because there have been no other studies on this topic, it was possible to compare the importance of the different N-containing compounds from the point of view of soil differentiation. With this aim, two factor analyses were carried out, one on the cumulated values of each N form from all the hydrolysates of the 23 soils from Gonzhlez-Prieto and Carballas (199 1) and the other on values of each N form from the unique hydrolysate of 100 soils from many parts of the world performed by Keeney and Bremner (1964), Kadirgamathaiyah and MacKenzie (1970), Sowden et al. (1976), Cheng et al. (1977), Dalal(l978), Rose11 et al. (1978), Singh et al. (1978) and Goh and Edmeades (1979). As Table 1 shows, for both analyses the variables with higher factor loadings were
0.6 -
0.6 -
0.2
*AA-l
I
c
I H&-4
.HUN-2
a
"tN-4 aAM
HEX-2
?? HON-1 ,HA-4
0
?? YiHA ?? SHUN
I
t -’ 0.2
OHA-
I
I
- 0.2
I
I 0
1
1 42
1
1 0.4
’
1 0.6
1
’ 0.6
’
1 1.0
Fig. 5. Factor analysis of the organic N fractions. Location of the variables in the plane defined by factors I and III. Scaks show the factor loadings (regression coefficients) of the variables on each axis. Abbreviations as given in Fig. 4.
S. J. GonzPlez-Prieto and T. Carballas
210
Table 1. Loadings of the variables on factors I and II in the factor analyses of the organic N fractions from: (A) the cumulated values of the four hydrolysates from the soil samples of Gontilez-Prieto and Carballas (1991); (B) the values of the unique hydrolysate obtained by Keeney and Bremner (1964), Kadirgamathaiyah and MacKenzie (1970). Sowden ef al. (1976). Cheng er al. (1977), Dalal(1978). Rose11ef al. (1978). Singh et al. (1978) and Goh and Edmeades (1979) Fraction
Factor loadings (A) Factor II Factor I
Factor loadings (B) Factor I Factor II
Amide N NH:-N Hexosamine N cr-Aminoacid N Hydrolysable unidentified N Hydrolysable organic N
0.749 - 0.040 0.725 0.937 0.770 0.986
ND 0.716 0.124 0.687 - 0.027 0.896
-0.163 - 0.976 0.404 - 0.077 0.239 -0.053
hydrolysable organic N and u-aminoacid N. Moreover, for both analyses hydrolysable unidentified N was more important than hexosamine N, although both variables appeared associated to the first factor for the samples from Gonzklez-Prieto and Carballas (1991) and to the second factor for the other soils. The main discrepancy between both groups was for NH:-N, which is easily explained by the differences between the hydrolytic methods applied (GonzilezPrieto and Carballas, 1988) and because in the data of GonzBlez-Prieto and Carballas (1991) amide N was solubilized separately, whereas in the data of the other authors, where continuous hydrolysis was used, amide N is included with NH:-N. Therefore, it could be concluded that hydrolysable organic N, a-aminoacid N, unidentified hydrolysable N and hexosamine N are, in this order, the organic N forms that explain most of the diversity within a wide variety of soils from many parts of the world. Among these organic N forms, the more labile fractions (those extracted by Hl and H2) explain the highest percentage of the variance, at least for soils of the temperate-humid zone. Moreover, the biochemical fractions mentioned were shown to be supplementary of physical and chemical characteristics used in classification systems for soils from temperate-humid regions.
ND -0.186 0.715 0.336 -0.784 0.233
volcanic and non volcanic tropical soils. Soil Science 125, 178-180. Duchaufour Ph. (1984) Edafologia. 1 Edafoginesis y Clasificacibn. Masson, Barcelona. Engelman L. (1990) Cluster analysis of cases. In BMDP Statistical Software Manual, Vol. 2 (W. J. Dixon et al., Eds), pp. 817-827. University of California Press, Berkeley, CA. FAO/UNESCO (1974) Soil Map of the World, Vol. I. FAO, Rome. Frane J., Jennrich R. and Sampson P. (1990) Factor analysis. In BMDP Statistical Software Manual, Vol. 1 (W. J. Dixon et al., Eds), pp. 312-329. University of California Press,
Berkeley, CA. Goh K. M. (1972) Amino acid levels as indicators of paleosols in New Zealand soil profiles. Geoderma 7, 3347. Goh K. M. and Edmeades D. C. (1979) Distribution and partial characterization of soil hydrolyzable organic nitrogen in six New Zealand soils. Soil Biology & Biochemistry 11, 127-132. GonzLlez-Prieto S. J. and Carballas T. (1988) A modified method for the fractionation of soil organic nitrogen by successive hydrolyses. Soil Biology & Biochemistry 20, l-6. GonzBlez-Prieto S. J. and Carballas T. (1991) Composition of organic N in temperate humid region soils (NW Spain). Soil Biology & Biochemistry 23, 887-895.
Gontilez-Prieto S. J., Lista M. A., Carballas M. and Carballas T. (1989) Humic substances in a catena of estuarine soils: distribution of carbon and nitrogen. The Science of the Total Environment 81182, 363-372.
Kadirgamathaiyah S. and MacKenzie A. F. (1970) A study of soil nitrogen organic fractions and correlation with yield response of Sudan-sorghum hybrid grass in Quebec soils. Plant and Soil 33, 12&128.
Acknowledgements-We
thank Mr G. Hermida for technical assistance and Mr J. Caballo for drawing the figures. This research was supported by Conselleria de Educaci6n of the Xunta de Galicia (Spain) and by the Ministerio de Educaci6n y Ciencia of Spain through a fellowship to S. J. GonzBlez-Prieto.
REFERENCES Acea M. J. and Carballas T. (1989) Relationships among microbial groups in various humid zone soils and the factors controlling their distribution. Agrochimica 34, 1-14. Carballas M. (1982) Estudio de la GPnesis de1 Ranker Arlintico. Universidad de Santiago, Servicio de Publica-
ciones, Santiago de Compostela. Cheng Y., Sowden F. J. and Schnitzer M. (1977) Nitrogen in Mediterranean soils. Agrochimica 21, 7-14. Dalal R. C. (1978) Distribution of organic nitrogen in organic
Keeney D. R. and Bremner J. M. (1964) Effect of cultivation on the nitrogen distribution in soils. Soil Science Society of America Proceedings 30, 714719.
Kubiena W. L. (1952) Claves Sistemciticas de Suelos. CSIC, Madrid. Rose11 R. A., Salfeld J. C. and Sochtig H. (1978) Organic compounds in Argentine soils: 1. Nitrogen distribution in soils and their humic acids. Agrochimica 22, 98-105. Singh B. R., Uriyo A. P. and Lontu B. J. (1978) Distribution and stability of organic forms of nitrogen in forest soil profiles in Tanzania. Soil Biology & Biochemistry 10, 10>108. Soil Survey Staff (1975) Soil Taxonomy: a Basic System of Soil ClassiJication for Making and Interpreting Soil Surveys.
U.S. Department of Agriculture, Soil Conservation Service, Washington, DC. Sowden F. J., Griffith S. M. and Schnitzer M. (1976) The distribution of nitrogen in some organic tropical volcanic soils. Soil Biology & Biochemistry 8, 5540. Sprent J. I. (1987) The Ecology of the Nitrogen Cycle. Cambridge University Press.
003&0717(94)00159-6
Soil Biol. Biochem. Vol. 27, No. 2, pp. 2OS-210, 1995 Copyright 0 1995Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-0717/9S $9.SO+O.O0
N BIOCHEMICAL DIVERSITY AS A FACTOR OF SOIL DIVERSITY S. J. GONZALEZ-PRIETO
and T. CARBALLAS*
Instituto de Investigaciones Agrobiologicas de Galicia (CSIC), Apartado 122, 15700 Santiago de Compostela, Spain (Accepted
10 July 1994)
Summary-As lithospherebiosphere interphases, soils have some characteristics (mainly biochemical characteristics) highly influenced by living organisms and therefore these characteristics vary according to soil biological activity and diversity. The variability of these characteristics could be used for the differentiation of soil types. This potential utility was successfully analysed for N biochemical fractions in temperate-humid region soils. The fractions of medium-to-low hydrolytic resistance and among them hydrolysable organic N, a-aminoacid N, hydrolysable unidentified N and, to a lesser extent, hexosamine N, were the main organic N forms to supplement the physical and chemical characteristics in soil classification systems.
Soil classifications are usually based on soil properties or occasionally on characteristics related to genesis and evolution of the profile. In all cases the main variables considered--soil moisture and temperature regimes, colour, texture, structure, depth, pH, organic matter, exchangeable cations, weatherable minerals, CaC03, Fe and Al oxides, etc.-have a physical or chemical character (FAO/UNESCO, 1974; Soil Survey Staff, 1975). Nevertheless, as lithospherebiosphere interphases, soils have some characteristics highly influenced by living organisms. These characteristics, therefore, usually have a biochemical character and vary according to biological activity and diversity. The variability of these characteristics could be useful for the differentiation of the types of soil but they have been scarcely used for this aim. Variables related to carbonaceous fractions (type of humus, fulvic-to-humic acid:; ratio) have been considered (Kubiena, 1952; Duchaufour, 1984) as well as the usefulness of aminoacid N to establish paleosol horizons (Goh, 197211. The high interdependence between living organisms and the N cycle (Sprent, 1987), together with the documented relationships between N cycle microorganisms, or organic N distribution, and soil physical and chemical charac:teristics (Acea and Carballas, 1989; Gonzalez-Prietlo and Carballas, 199 1) suggest that biochemical N fractions could be used to differentiate soil types. Like the case of soils, biochemical N fractions are the result of the biosphere-lithosphere interphase. We have examined the potential use of N fractions to differentiate soils
*Author for correspondmcnce.
from a temperate-humid region and determined the main N fractions responsible for soil diversity.
MATERIALS AND MJSTHODS The soils used were those described by GonzalezPrieto and Carballas (1991). They comprised 23 soil horizons coming from a temperate-humid region (Galicia, NW Spain). Their main physical and chemical characteristics as well as their organic N forms were described in the same paper. The method used for the hydrolytic fractionation of organic N (Gonzalez-Prieto and Carballas, 1988) consists basically of four successive hydrolyses of increasing hydrolytic strength with 1~ HCI for 3 h (hydrolysate 1 or Hl), 3N HCl for 3 h (H2), 6~ HCI for 4 h (H3) and 6~ HCI for 20 h (H4), being CH=Hl+HZ+H3+HC The 23 samples considered (16 surface horizons and 7 subsurface horizons) were grouped according to the similarity of: (A) the 15 main physical and chemical characteristics; (B) the organic N fractions; and (C) the physical and chemical characteristics and the organic N fractions together. The hierarchical clustering of the samples was performed using the program “BMD.PZM Cluster Analysis of Cases” [Biomedical Computer Programs, University of California, Berkeley, CA (Engelman, 1990)]. Similarity between samples was established according to their Euclidean distance (i.e. the squared root of the sum of the squared differences of the values for each variable), most alike samples having the shortest Euclidean distances between them. The organic N compounds responsible for most of the soil organic N variability were found by a factor analysis using the program “BMD.P4M Factor
205
S. J.
206
Gonzhlez-Prieto and T. Carballas
Analysis of Cases” [Biomedical Computer Programs, University of California, Berkeley, CA (Frane et al., 199011.This program, which is analogous to a multiple regression model with several dependent variables, was applied to the data matrix formed by the 23 soil samples and the 25 variables related to organic N distribution. By a varimax rotation, the factor analysis finds the factors (the axes on the plots), which are a few weighted averages of the original variables, that summarize, as much as possible, the information that the variables contain. The distribution of the variables is plotted in the plane defined by each pair of factors according to the corresponding regression coefficients (factor loadings) of the variables on each factor. In Figs 4 and 5, the circles contain the more important variables for the definition of each axis, i.e. the variables whose regression coefficients on the axis were >oso.
RJEWLTS AND DISCUSSION
The cluster obtained according to the similarity of the main physical and chemical characteristics (Fig. 1) showed a soil grouping close to the FAO/UNESCO (1974) classification. Nevertheless, they were not identical because the cluster program considers that all the variables have the same category, whereas the FAO/UNESCO classification uses certain characteristics to define diagnostic horizons, or diagnostic properties, which have a higher systematic value than the other characteristics. It must be pointed out that in the cluster analysis some profiles were not well-differentiated. This is the case of the Ferralic Cambisol A-23 that appears between the Rendzina
6
5
4
9
e
and the Rankers, although the organic matter of the three profiles is clearly different: Fe, Ca and Al, respectively, being the cations that influence their organic matter evolution (Duchaufour, 1984). Moreover, the degree of evolution of the Ferralic Cambisol is much greater than that of the others, this soil being the only one that has an altered horizon (Cambic B horizon). It is also noteworthy the poor discrimination between the horizons from Podzol, Cambisols and Atlantic Ranker (sensu Carballas, 1982) three types of soil that have different genesis and profile differentiation. In the Podzol the dominant process is emigration of Fe and Al complexed by organic compounds (Spodic horizon), whereas the Cambisol is formed by in situ weathering processes (Cambic horizon). The Atlantic Ranker, with an Umbric horizon, is a polycyclic soil formed by deposition of very stable organic material coming from other places in the slope (FAO/UNESCO, 1974; Carballas, 1982). Grouping the samples according to the similarity of the biochemical N fractions (Fig. 2) showed high coherence with the FAOjUNESCO (1974) classification. This is in keeping with the relationships between organic N fractions and different physical and chemical characteristics found by Gonzalez-Prieto and Carballas (1991). Moreover, with regard to Fig. 1, Fig. 2 shows that the Ferralic Cambisol A-23 and the Ranker GM4 were correctly separated from the Rendzina and grouped with the other Cambisols. Figure 2 also shows that the nature of the distribution of organic N differentiated the Humic Cambisols SE-8 and SP-13 from the other Cambisols. In Fig. 2 the discrimination among the horizons from Podzol, Atlantic Ranker and Humic Cambisols was also
!
0
Key
Type of soil
CA-24 A-23 GM-4 PZ-19 Ml-22 Sl
Rendzina Ferralic Cambisol Ranker Ranker Eutric Fluvisol Eutric Fluvisol Eutric Fluvisol Eutric Fluvisol Eutric Fluvisol Ferralic Cambisol Atlantic Ranker Humic Cambisol Humic Cambisol Humic Camblsol Humlc Cambisol Humic Cambisol Atlantic Ranker Humic Cambisol Atlantic Ranker Humic Cambisol Orthic Podzol Atlantic Ranker Orthlc Podzol
:‘3
K-1 R-17 5
GC-32 SE-9 SP-13 G P-5 GR-19 R3 GE-20 R2 ET-21 PBh R4 PAo-2
Fig. 1. Cluster of soil samples grouped according to the similarity of their main physical and chemical characteristics. The scale shows the amalgamation distances between samples and/or clusters. The key is the same used by GonzBlez-Prieto and Carballas (1991) to identify the samples. Types of soil according to FAO/UNESCO (1974); Atlantic Ranker, sensu Carballas (1982).
N biochemical diversity as a factor of soil diversity 6
7
5
2
3
4
207
t1
-l
Key
Type of soil
CA-24 PZ-1 a SE-0 SP-13 R-l 7 R3 R4 R2 M2
Rendzina Ranker Humic Cambisol Humic Camblsol Atlantic Ranker Atlantic Ranker Atlantic Ranker Atlantic Ranker Eutrlc Fluvisol Eutric Fluvisol Eutric Fluvisol Eutric Fluvisol Humic Cambisol Humic Cambisol Humic Cambisol Eutric Fluvisol Ferralic Cambisol Ranker Ferralic Camblsol Humlc Cambisol Humic Cambisol Orthic Podzol Orthic Podzol
:il Sl GC-32 GP-5 ET-21 Ml-22 A-23 GM-4 SG-15 GE-20 GR-19 PBh PAo-2
Fig. 2. Cluster of soil samples grouped according to the similarity of their organic N fractions. The scale shows the amalgamation distances between samples and/or clusters. The key is the same used by Gonzalez-Pneto and Carballas (1991) to identify the samples. Types of soil according to FAOjUNESCO (1974); Atlantic Ranker, sense Carballas (1982).
improved compared to Fig. 1. On the other hand, the horizons of the Atlantic Ranker were grouped because the distribution of N forms is similar in all of them and very different from that in the other soils. Finally, the singularity of MI-212 horizon from the Eutric Fluvisol [marsh according to the classification of Kubiena (1952)] was recognized. Thus, this horizon appears associated to the reference terrestrial profile, which is
10 :
8
7
6
5
in keeping with the terrestrification of its organic matter shown by Gonzalez-Prieto et al. (1989); nevertheless, its separation from the other Fluvisols could be excessive according to the other edaphic characteristics. Figure 3 shows the grouping samples cluster due to the similarity of the main physical and chemical characteristics and the organic N fractions together.
4 I
3
2 Key
Type of soil
CA-24 PZ-10 Ml-22
Rendzina Ranker Eutric Fluvisol Eutric Fluvisol Eutric Fluvisol Eutric Fluvisol Eutric Fluvisol Ferralic Cambisol ;tu&Cambisol
:: s3 !!23 SP-13
Humic Cambisol Humic Cambisol g:-_532 Humic Cambisol ET-21 GE-20 ::I:; R-17 R4 R3 ._ ;:h PAo-2
Humic Cambisol Humic Cambisol Humlc Cambisol Ferrallc Cambisol Atlantic Ranker Atlantic Ranker Atlantic Ranker Atlantic Ranker Orthic Podzol Orthlc Podzol
Fig. 3. Cluster of soil samples grouped according to the similarity of their main physical and chemical characteristics and organic N fractions. The scale shows the amalgamation distances between samples and/or clusters. The key is the same used by Gonzalez-Prieto and Carballas (1991) to identify the samples. Types of soil according to FAOjUNESCO (1974); Atlantic Ranker, sensu Carballas (1982).
208
S. J. Gonzalez-Prieto and T. Carballas
0.8 -
0.6
-
0,
lion-48
.+“A
oHA.ON
Fig. 4. Factor analysis of the organic N fractions. Location of the variables in the plane defined by factors I and II. Scales show the factor loadings (regression coefficients) of the variables on each axis. HA, hydrolysable NH:-N; AM, amide N; HEX, hexosamine N; AA, cc-aminoacid N; HUN, hydrolysable unidentified N; HON, hydrolysable organic N; ON total organic N. Numbers l-4 indicate the hydrolysate number from which the fractions originate and ZH indicates the cumulated amount of the corresponding N form from the four hydrolysates (see Materials and Methods). Amide N was completely solubilized in Hl.
This cluster revealed the supplementarity of both types of variables to differentiate types of soil in a logical and efficient way, even without the establishment of diagnostic characteristics. Like the preceding analyses, the first type of soil separated was the Rendzina. This was followed, although at greater distance than in the other analyses, by the Ranker PZ- 18 and the Fluvisol group, all of them characterized by a low degree of differentiation of the profile. Then the Ferralic Cambisol A-23 was separated from the Humic Cambisols group between which it appears the Ranker GM-4, which is an intergrade soil between Rankers and Cambisols, that cannot be classified as Humic Cambisol only because it does not satisfy some FAO/UNESCO (1974) requirements for Cambic horizons. The soil SG-15 appears less differentiated from Humic Cambisols, possibly due to its less marked ferralic characteristics (FAO/UNESCO, 1974). Finally, the horizons from the Atlantic Ranker and the Podzol are well-separated from each other and from those of the Humic Cambisols. Considering the utility of the variability of the biochemical organic N fractions to differentiate with more accuracy some groups of soils, the most important N-containing compounds from this point of view were determined by factor analysis according to the variance explained. The first three factors accounted for 80% of the total variance, factors I, II
and III amounting to 43,23 and 14%, respectively, of the variance. Figure 4 shows the distribution of the variables in the plane defined by axes I and II, which together explained 2/3 of the total variance. The positive extreme of the first factor (axis I) was defined by the N fractions solubilized during the hydrolysis with 3~ HCl for 3 h (H2) as well as a-aminoacid N, hydrolysable unidentified N and the total organic N hydrolysable with 6~ HCl for 4 h (H3), all of them with high factor loadings (i.e. with regression coefficients > 0.50 on factor I). Axis II was defined at its positive extreme by the variables from Hl . Moreover, a group of variables with high factor loadings on the positive extreme of both axes I and II, occupies intermediate positions between both axes. This group includes the cumulated percentages from all the hydrolysates (ZH) of each organic N form, excepting NH:-N; hexosamine tentatively coefficients considered solubilized included in each of the hydrolysable N always remarkable
N was included in this group only because of its lower (cO.50) regression on both axes. Amide N, which can be a variable of Hl and CH because it is only in the first hydrolysate, was also this group. It must be pointed out that for three groups considered, a-aminoacid N, unidentified N and hydrolysable organic have higher factor loadings. It is also that the variables related to hexosamine N
209
N biochemical diversity as a factor of soil diversity contents and to the different N-containing compounds of H4 showed a poor resolution on the plane defined by the first two factors. Only NH:-N from H4 and total organic N have negative, although low, factor loadings on both axes. It was therefore shown that the important direction of variation determined by axis I differentiates samples according to organic N content of medium resistance to hydrolysis, whereas the direction defined by axis II differentiates samples according to the most labile N content. Specifically, the N-containing fractions most strongly associated with both factors were aaminoacid N, hydrolysable unidentified N and hydrolysable organic N, the cumulated values of each organic N fraction !for all the hydrolysates occupying intermediate positions between axes I and II with important factor loadings on both axes. Figure 5 shows tlhe distribution of the variables in the plane defined by axes I and III. Both axes accounted for 57% of the total variance. Axis III defines a direction of variation based mainly on hexosamine N content. Nevertheless, three variables of a-aminoacid N, which are located in an intermediate position in the first quadrant of the plane defined by axes I and II, also have high factor loadings. The only variables with negative although low factor loadings on both axes were total organic N and hydrolysable unidentified N from HI. Factors I and III are therefore determined by the variables of H2 and aminosugars, respectively, although the factor loadings of aaminoacid N and II3 variables are not negligible.
In summary, factor analysis attributed the most importance to fractions of low and medium resistance to acid hydrolysis and among them to hydrolysable organic N, a-aminoacid N, hydrolysable unidentified N and to a lesser extent to hexosamine N. These forms were, for the soils studied, the main variables to supplement the physical and chemical characteristics in soil classification systems. Confirmation of these results in others types of soil is difficult because of the different hydrolytic methods successive stepwise used by various authors: hydrolyses in the study by Gohlez-Prieto and Carballas (1991) and continuous hydrolysis in most of the papers published on this matter. Nevertheless, although it was impossible to compare the resistance to hydrolysis of the different N fractions, because there have been no other studies on this topic, it was possible to compare the importance of the different N-containing compounds from the point of view of soil differentiation. With this aim, two factor analyses were carried out, one on the cumulated values of each N form from all the hydrolysates of the 23 soils from Gonzhlez-Prieto and Carballas (199 1) and the other on values of each N form from the unique hydrolysate of 100 soils from many parts of the world performed by Keeney and Bremner (1964), Kadirgamathaiyah and MacKenzie (1970), Sowden et al. (1976), Cheng et al. (1977), Dalal(l978), Rose11 et al. (1978), Singh et al. (1978) and Goh and Edmeades (1979). As Table 1 shows, for both analyses the variables with higher factor loadings were
0.6 -
0.6 -
0.2
*AA-l
I
c
I H&-4
.HUN-2
a
"tN-4 aAM
HEX-2
?? HON-1 ,HA-4
0
?? YiHA ?? SHUN
I
t -’ 0.2
OHA-
I
I
- 0.2
I
I 0
1
1 42
1
1 0.4
’
1 0.6
1
’ 0.6
’
1 1.0
Fig. 5. Factor analysis of the organic N fractions. Location of the variables in the plane defined by factors I and III. Scaks show the factor loadings (regression coefficients) of the variables on each axis. Abbreviations as given in Fig. 4.
S. J. GonzPlez-Prieto and T. Carballas
210
Table 1. Loadings of the variables on factors I and II in the factor analyses of the organic N fractions from: (A) the cumulated values of the four hydrolysates from the soil samples of Gontilez-Prieto and Carballas (1991); (B) the values of the unique hydrolysate obtained by Keeney and Bremner (1964), Kadirgamathaiyah and MacKenzie (1970). Sowden ef al. (1976). Cheng er al. (1977), Dalal(1978). Rose11ef al. (1978). Singh et al. (1978) and Goh and Edmeades (1979) Fraction
Factor loadings (A) Factor II Factor I
Factor loadings (B) Factor I Factor II
Amide N NH:-N Hexosamine N cr-Aminoacid N Hydrolysable unidentified N Hydrolysable organic N
0.749 - 0.040 0.725 0.937 0.770 0.986
ND 0.716 0.124 0.687 - 0.027 0.896
-0.163 - 0.976 0.404 - 0.077 0.239 -0.053
hydrolysable organic N and u-aminoacid N. Moreover, for both analyses hydrolysable unidentified N was more important than hexosamine N, although both variables appeared associated to the first factor for the samples from Gonzklez-Prieto and Carballas (1991) and to the second factor for the other soils. The main discrepancy between both groups was for NH:-N, which is easily explained by the differences between the hydrolytic methods applied (GonzilezPrieto and Carballas, 1988) and because in the data of GonzBlez-Prieto and Carballas (1991) amide N was solubilized separately, whereas in the data of the other authors, where continuous hydrolysis was used, amide N is included with NH:-N. Therefore, it could be concluded that hydrolysable organic N, a-aminoacid N, unidentified hydrolysable N and hexosamine N are, in this order, the organic N forms that explain most of the diversity within a wide variety of soils from many parts of the world. Among these organic N forms, the more labile fractions (those extracted by Hl and H2) explain the highest percentage of the variance, at least for soils of the temperate-humid zone. Moreover, the biochemical fractions mentioned were shown to be supplementary of physical and chemical characteristics used in classification systems for soils from temperate-humid regions.
ND -0.186 0.715 0.336 -0.784 0.233
volcanic and non volcanic tropical soils. Soil Science 125, 178-180. Duchaufour Ph. (1984) Edafologia. 1 Edafoginesis y Clasificacibn. Masson, Barcelona. Engelman L. (1990) Cluster analysis of cases. In BMDP Statistical Software Manual, Vol. 2 (W. J. Dixon et al., Eds), pp. 817-827. University of California Press, Berkeley, CA. FAO/UNESCO (1974) Soil Map of the World, Vol. I. FAO, Rome. Frane J., Jennrich R. and Sampson P. (1990) Factor analysis. In BMDP Statistical Software Manual, Vol. 1 (W. J. Dixon et al., Eds), pp. 312-329. University of California Press,
Berkeley, CA. Goh K. M. (1972) Amino acid levels as indicators of paleosols in New Zealand soil profiles. Geoderma 7, 3347. Goh K. M. and Edmeades D. C. (1979) Distribution and partial characterization of soil hydrolyzable organic nitrogen in six New Zealand soils. Soil Biology & Biochemistry 11, 127-132. GonzLlez-Prieto S. J. and Carballas T. (1988) A modified method for the fractionation of soil organic nitrogen by successive hydrolyses. Soil Biology & Biochemistry 20, l-6. GonzBlez-Prieto S. J. and Carballas T. (1991) Composition of organic N in temperate humid region soils (NW Spain). Soil Biology & Biochemistry 23, 887-895.
Gontilez-Prieto S. J., Lista M. A., Carballas M. and Carballas T. (1989) Humic substances in a catena of estuarine soils: distribution of carbon and nitrogen. The Science of the Total Environment 81182, 363-372.
Kadirgamathaiyah S. and MacKenzie A. F. (1970) A study of soil nitrogen organic fractions and correlation with yield response of Sudan-sorghum hybrid grass in Quebec soils. Plant and Soil 33, 12&128.
Acknowledgements-We
thank Mr G. Hermida for technical assistance and Mr J. Caballo for drawing the figures. This research was supported by Conselleria de Educaci6n of the Xunta de Galicia (Spain) and by the Ministerio de Educaci6n y Ciencia of Spain through a fellowship to S. J. GonzBlez-Prieto.
REFERENCES Acea M. J. and Carballas T. (1989) Relationships among microbial groups in various humid zone soils and the factors controlling their distribution. Agrochimica 34, 1-14. Carballas M. (1982) Estudio de la GPnesis de1 Ranker Arlintico. Universidad de Santiago, Servicio de Publica-
ciones, Santiago de Compostela. Cheng Y., Sowden F. J. and Schnitzer M. (1977) Nitrogen in Mediterranean soils. Agrochimica 21, 7-14. Dalal R. C. (1978) Distribution of organic nitrogen in organic
Keeney D. R. and Bremner J. M. (1964) Effect of cultivation on the nitrogen distribution in soils. Soil Science Society of America Proceedings 30, 714719.
Kubiena W. L. (1952) Claves Sistemciticas de Suelos. CSIC, Madrid. Rose11 R. A., Salfeld J. C. and Sochtig H. (1978) Organic compounds in Argentine soils: 1. Nitrogen distribution in soils and their humic acids. Agrochimica 22, 98-105. Singh B. R., Uriyo A. P. and Lontu B. J. (1978) Distribution and stability of organic forms of nitrogen in forest soil profiles in Tanzania. Soil Biology & Biochemistry 10, 10>108. Soil Survey Staff (1975) Soil Taxonomy: a Basic System of Soil ClassiJication for Making and Interpreting Soil Surveys.
U.S. Department of Agriculture, Soil Conservation Service, Washington, DC. Sowden F. J., Griffith S. M. and Schnitzer M. (1976) The distribution of nitrogen in some organic tropical volcanic soils. Soil Biology & Biochemistry 8, 5540. Sprent J. I. (1987) The Ecology of the Nitrogen Cycle. Cambridge University Press.