Humic substances in a catena of estuarine soils: distribution of organic nitrogen and carbon

The Science of the Total Environment, 81/82 (1989) 363-372 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

363

HUMIC SUBSTANCES IN A CATENA OF ESTUARINE SOILS: DISTRIBUTION OF ORGANIC NITROGEN AND CARBON S.J. GONZALEZ PRIETOl , M.A. LISTAl , M. CARBALLAS2 and T. CARBALLASl I I n s t i t u t o de Investigaciones Agrobiol6gicas de Galicia (C.S.I.C.), Apartado 122, 15080 Santiago de Compostela (Spain) 2Departamento de Edafolog~a y Quimica Agricola, Universidad de Santiago, Santiago de Compostela (Spain) SUMMARY This paper describes the organic fractions of a catena comprising two eutric f l u v i s o l s , one (F~) in the intertidal zone of an estuary and the other (F~) on i t s bank, and a d~stric cambisol (C) that is not affected by seawater. The humus of the fluvisols has features inherited from the t e r r e s t r i a l humus together with others acquired during i t s evolution in the marine environment. All the samples had l i t t l e unhumified organic matter, a f a i r amount of humin and a major fraction that is soluble in alkalis. Exposure to seawater changes the kind of organo-metallic complexes present and alters the relative s o l u b i l i t y of humic and f u l v i c acids in alkaline reagents, thereby modifying the degree of polymerization of the humus. The distribution of organic nitrogen also changes with exposure to seawater: ammonium content gradually rises, amide content remains pract i c a l l y constant and aminoacids f a l l gradually, while hexosamines, hydrolysable unknown nitrogen and hydrolysable organic nitrogen all decrease sharply. INTRODUCTION Rias, river valleys that have been p a r t i a l l y or t o t a l l y invaded by the sea, are customarily considered as estuaries, though geomorphologically they are different (ref. l ) . Permanently or almost permanently waterlogged by a mixture of salt and fresh water, the beds of rias are transition zones between the marine and t e r r e s t r i a l environments, and are zones of high primary productivity (ref. 2). Their organic matter is partly produced in situ or of a marine origin~ and partly derived from the basin draining into the estuary (ref. 2). This paper describes the organic fractions of a catena comprising two eutric fluvisols, (ref. 3), one (Fl) in the intertidal zone of a r i a and the other (F2) on i t s bank, and a d i s t r i c cambisol (C) that is not affected by seawater. The degree of marine influence increases in the order C< F2< FI. MATERIAL The three profiles studied are located in N.W. Spain in the Ria de Noya, the r i a originated by the River Tambre. Their characteristics are listed in Table 1. Fl,am eutric fluvisol located in the tidal zone, was described by Cabaneiro (ref. 4) as a sandy clay sediment of saprope] type (ref, 5) that is periodically flooded by tides and permanently waterlogged; i t has no vegetation. F2, another 0048-9697/89/$03.50

©

1989 Elsevier Science Publishers B.V.

364

TABLE 1 Main characteristics of the soils studied (a) Sample

pH

O.M. (%)

C AuI Bw F~AuI "Au~ FiIi~!

4.70 9.71 4.40 2.00 6 . 1 0 lO.17 6 . 6 0 8.88 6 . 5 0 8.78 6.90 8.69 7 . 0 0 lO.21

Ill

N (%) 0.29 0.09 0.50 0.36 0.36 0.32 0.33

NH~-N Ca Mg K Na CEC Fe203 AI203 Clay (%) tr. tr. O.OOl 0.002 0.002 0.004 0.007

(meq. I00 g-l) 2 l 7 7 lO II 9

0 0 lO 15 16 15 15

0 0 3 2 3 2 2

0 0 31 16 21 18 17

32 15 79 65 75 70 59

(%)

(%)

(%)

1.58 1.42 2.34 0.97 0.66 0.56 0.56

1.36 0.98 1.23 0.99 0.90 0.80 0.58

20 21 33 22 20 19 19

EC*

O.l O.l 14.2 9.6 ll.O 12.3 9.1

(a) C, Distric cambisol; F2 and Fl , Eutric f l u v i s o l s . *Electric conductivity expressed in mmhos cm-l eutric f l u v i s o l , is a sandy clay marsh soil

(refs. 5-6), located close to Fl;

i t has developed from a former sapropel that s i l t i n g of one of the r i a ' s channels has l e f t above the tidemark except at spring tides; with high humidity and waterlogged deep layers throughout the year, i t supports rushes and other halophytes. C, a d i s t r i c cambisol located not far from F2, is an autochthonous sandy clay soil developed from mica schists; seawater only reaches i t in the form of spray;

formerly under oakwood, i t

now supports pinewood with heather under-

growth. METHODS The distribution of carbon in various fractions was determined by the fractionation method of Bruckert and Metche (ref. 7), which is based on successive extraction with soda plus sodium tetraborate at pH 9.7, sodium pyrophosphate at pH I0.2, and soda at pH 12.4, after which soluble humin was separated from insoluble humin by treating the residue f i r s t with sodium dithionate and then with hydroflouric acid, followed in both cases by extraction with soda at pH 12.4. Prior to organic matter extraction, the soils were treated with a mixture of ethanol and bromoform of density 1.8 to separate humified organic matter from unhumified compounds (ref.

8). The main humus s t a b i l i z i n g agents, Ca, Fe and

Al, were determined in each extract. The d i s t r i b u t i o n of the various forms of organic nitrogen was determined by a stepwise acid hydrolysis procedure that included a modification introduced by GonzAlez Prieto and Carballas (ref. 9) and consisting in brief hot acid hydrolysis of neutralized hydrolysates in a reaction bomb in the presence of propionic acid. Other soil characteristics were determined by the methods described by Guitian Ojea and Carballas (ref. lO).

365

RESULTS AND DISCUSSION Distribution of organic carbon The content of organic matter (Table l) was similar in the three soils but it

distribution along the profile was very different, with Fl being an "inver-

ted" profile with respect to t e r r e s t r i a l soils. The organic fraction (Fig. l) had quite small quantities of unhumified organic matter (though increasingly more as marine influence increased), abundant humin (especially in the unsubmerged horizons) and an alkali-soluble major fraction composed of humic and f u l v i c acids (with the former predominant except in the deep horizons of Fl). In FI, unlike t e r r e s t r i a l soils, the quantity of unhumified organic matter and humin increased with depth while percentage extraction and percentage humification decreased (Table 2), which together with the depthwise trend of the FA/HA ratio reflects the sedimentary origin of this profile. The humin and alkali-soluble compounds of different soils differed as regards their s o l u b i l i t i e s in the various reagents (Table 2). In C, soluble humin and humic compounds soluble in soda of pH 9.7 predominated, whereas in Fl the major fraction consists of pyrophosphate-soluble compounds followed closely by the pH 12.4 soda fraction, which predominates in F2. These differences are due to d i f ferent mineral environments, which gives rise to organo-metallic complexes of

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Fig. I. Distribution of carbon in the humus fractions: unhumified compounds (NH) f u l v i c acids (FA), humic acids (HA) and humin (H), of a cambisol (C), and two f l u v i s o l s (Fl and F2).

366

TABLE 2 Characteristics of the organic matter of the soils studied (a) Sample

Soluble C (% Ct) NaOH+Na2B407 Na4P207 (pH 9.7)

C AuI F Au 2Au~ Fl I L "II Ill

Humin NaOH

Sol.

(pH I0.2) (pH 12.4) % C

24 15 6 lO 8 lO

15 14 19 28 27 17

13 19 22 23 23 25

Insol.

Insol. residue FA/HA %C

30 16 19 5 12 13

%C

5 20 19 16 I0 13

13 16 15 18 20 22

%

%

extr. humif. 0.61 0.46 0.38 0.65 1.09 0.97

52 48 47 61 58 52

87 84 85 82 80 78

(a) C, d i s t r i c cambisol; Fl and F2, eutric f l u v i s o l s . d i f f e r i n g composition (Table 3) and hence of d i f f e r i n g s o l u b i l i t y , mobility and s t a b i l i t y . The highest levels of organo-calcium complexes were found in Fl followed by F2, with much lower levels in C. Conversely, the t e r r e s t r i a l soil C had the highest levels of organo-aluminium complexes, followed by F2 and FI. The greatest organo-ferric content appeared in the surface horizon of F2 and in C, followed by the deep F2 horizon and Fl, in which i t decreases with increasing depth. The metal/carbon ratio is smaller (except for Ca) for the fraction extracted at pH 9.7 than for those extracted at pH I0.2 or pH 12.4; hence mobile complexes with small metal/carbon ratios predominate in the t e r r e s t r i a l

soil,

whereas immobile, condensed complexes with high metal/carbon ratios predominate in the profiles exposed to seawater. The high proportion of soluble humin in the t e r r e s t r i a l soil may also reflect the abundance of compounds that have been insolubilized by Al in this s o i l . Judging by the FA/HA ratios and the s o l u b i l i t i e s of the humic compounds in the different solvents (Table 2), the most highly polymerized humus is that of F2 and the least highly polymerized that of the terrestrial

s o i l , with the various horizons of Fl having different intermediate

TABLE 3 C, Fe, Al and Ca (a) solubilized with the various extractants (b) Sample

C Au~ F Au" 2Au~ FIIILI III

NaOH+Na2B407 C Fe Al Ca 1.38 0.86 0.31 0.50 0.40 0.56

40 21 13 14 19 9

213 41 19 44 44 29

40 57 27 69 87 92

C

Na4P207 Fe Al

0.83 320 0 . 8 1 388 0.96 387 1 . 4 2 348 1 . 3 4 331 1 . 0 2 325

450 205 196 195 174 ll5

Ca l 94 lOl 135 138 Ill

C

NaOH Fe Al Ca

0.73 40 1 . 1 5 146 l.ll 65 1.16 66 l . l O 58 1.51 60

353 384 310 285 236 174

42 50 59 61 55 57

C

Total Fe Al

2.94 472 2 . 8 2 555 1 . 8 8 457 3 . 0 8 428 2 . 9 2 399 3 . 0 9 395

974 630 525 524 455 318

Ca 83 201 187 265 270 260

(a) C expressed in g.lO0 g ~ s o i l ; Fe, Al and Ca expressed in mg.lO0 g-l s o i l . (b) C, d i s t r i c cambisol; F2 and Fl, eutric f l u v i s o l s .

367

degrees of polymerization. Distribution of organic nitrogen The proportion of organic N recovered as NH; (Fig. 2) increased with exposure i

to seawater (though to a lesser extend than did the amount of inorganic N: Table l ) , with 4.0-5.0% in C and F2, and 5.6-6.3% in Fl . These values are within the range obtained for a wide variety of soils by the analytical method used here (ref. l l ) ,

those of C and F2 being below the mean for all soils (5.7%) and those

of Fl s l i g h t l y above. The extraction sequence of Fl exhibited average soil beha+ viour, while F2 and C had smaller proportions of NH4 nitrogen (though both C and the surface F2 horizon had above-average proportions of easy hydrolysable NH;-N. The proportion of organic N recovered as amides (Fig. 2) hardly varied down the catena, and was generally close to the mean value for other soils (4.9%), the only exceptions being the surface F2 horizon with 6.1% and, to a lesser extent, F i I I I with just a l i t t l e more than 4.5%. The three Fl horizons differed markedly from F2 and C as regards hexosamine -content (Fig. 3). Except for F i l l , which with 2.2% was poor in aminosugars, the former had levels close to the mean for other soils (2.9%), which is considera-

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Fig. 2. Organic N of a cambisol (C) and two fluvisols (Fl and F~). I) amide N; IIa) ammonium in the pooled hydrolysates (zH) and in each individually (Hi); lib) percentage of ammonium recovered in each hydrolysate.

368

bly exceeded by the 3.9-4.6% of F2 and C. Puzzled by finding hexosamine levels generally no more than I-2% of the total N content in young marine sediments (in spite of the fact that the marine environment produces more chitin -in the form of animal exoskeletons- than cellullose), Jocteur Monrozier (ref. 12) put forward three hypotheses: l) that the hexosamines' biodegradation is extremely rapid, 2) that they are rapidly incorporated in polycondensed humic compounds, and 3) that they are decomposed into sugars and ammonium in the course of the hydrol y t i c analysis of the soil. Since destruction of hexosamines can be assumed neg l i g i b l e in the present study, being minimal with the stepwise hydrolytic method employed (ref. 9), the hexosamine solubilization curves for Fl (Fig.3) suggest that rapid biodegradation is the predominant process near the surface (since the extremely high percentage extraction by the f i r s t hydrolysis implies great lability),

while rapid incorporation in polycondensed humic compounds appears to

predominate in the lower horizons (since the aminosugars' resistance to hydrolysis increases with depth). The largest hydrolysable organic N fraction was composed of ~-aminoacids, whose level f e l l with increasing marine influence:35-37% in F2AuI and C, 30% in F2Au2 and 24-27% in Fl

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Fig. i3n Organic N of a cambisol (C) and two fluvisols (F1 and F~). a) Hexosami-

ne-N

the pooled hydrolysates (~-H) and in each individu~tlly (H~'); b) percent_a

ge of hexosamine-N recovered in each hydrolysate.

369

other soils, while the values for C and F2AuI are among the highest recorded. The proportions of ~-aminoacids in Fl and F2 are nevertheless both in the lowto-medium segment of the range observed by Jocteur Monrozier (ref. 12) in young marine sediments (5-80% of the total nitrogen content); the cited author a t t r i buted this enormous range to the great diversity of the sources of organic depos i t s , and considered that the extreme v a r i a b i l i t y in the abundance of aminoacids and hydrolysable unknown nitrogen differentiated young marine sediments from terrestrial

soils. The aminoacids of the horizons least influenced by seawater,

C and F2Aul , were not only greater in quantity but also more readily hydrolysed (Fig. 4) than those of Fl (with the relative exception of F i l l ) , though throughout the catena aminoacids, like hexosamines and hydrolysable unknown N, exhibited below-average s u s c e p t i b i l i t y to the f i r s t stage of the hydrolysis process. The proportion of

hydrolysable unknown nitrogen (HUN) varied i r r e g u l a r l y

among the profiles and horizons considered, as was to be expected of so complex and heterogeneous a fraction (Fig. 5). The only clear distinction can be drawn between, on the one hand, the 28% and 31.2% of C and F2AuI respectively, which are above the average for other soils (27.5%), and on the other the below-average 18.8-24.5% of the other horizons, with the surface horizon of Fl having the lowest figure. The Fl and F2 values l i e in the middle, most normal segment of

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Fig. 4. Organic N of a cambisol (C) and two f l u v i s o l s (F 1 and F~). a) -aminoacid-N in the pooled hydrolysates (zH) and in each i n d i v i d u a l l y '(Hi) ; b) percentage of a-aminoacid-N recovered in each hydrolysate.

370

the range observed by Jocteur Monrozier (ref. 12) in young marine sediments, 0-50%. Again, the results of the f i r s t stage of the hydrolysis procedure, and to a lesser extent the second, largely determine the overall differences among the six horizons studied. The HUN solubilization curves of Fig. 5 show belowaverage susceptibility to hydrolysis throughout the catena, with resistance to hydrolysis increasing with depth and with increasing exposure to seawater. Finally, i t may be pointed out that the total hydrolysable organic N figures (and also those of each hydrolysate), like the two major fractions (a-aminoacid N and HUN), differentiate between, on the one hand, the 78.7 and 79.7% of the more t e r r e s t r i f i e d horizons C and F2Aul ,

both of which figures are above the

average for all soils (67.3%), and on the other the below-average 58.7-65.5% of the other horizons. The greater resistance to hydrolysis of Fl and F2Au2, the horizons that are most exposed to seawater, might be due to the organic N of sediments being stabilized as the result of selection or concentration of nitrogen-bearing molecules during sedimentation, and/or to the incorporation of the nitrogen in stable organic compounds during rapid diagenesis (ref. 12). SYNTHESIS The humus of the t e r r e s t r i a l soil exhibits indirect humification (ref. 13)

1-C Aul

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z

401 30

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.Fig. 5. Organic N of a cambisol (C) and two fluvisols (F~ and F~). a) Hydrolysable unknown N in the pooled hydrolysates (zH) and in eacB indiv'fdually .(Hi); b) percentage of hydrolysable unknown N recovered in each hydrolysate.

371

in which i t s precursors are transformed into humin via intermediate compounds, f u l v i c and humic acids. The humus of the f l u v i s o l s has features inherited from the t e r r e s t r i a l humus together with others acquired during i t s evolution in the marine environment.

Inherited characteristics include the type of humification

and the d i s t r i b u t i o n of the humus fractions, which d i f f e r only s l i g h t l y in quantity

from one soil to another. Acquired characteristics include a rise in pH,

s a l i n i t y and exchange capacity, high levels of exchangeable Ca, Mg and Na, and a fall

in the C/N ratio (Table l ) . The marine environment changes the kind of

organo-metallic complexes present and alters the relative s o l u b i l i t y of humic and f u l v i c acids in alkaline reagents, thereby modifying the degree of polymerization of the marine humus with respect to the t e r r e s t r i a l humus. The t e r r e s t r i a l soil is also differentiated from the two marine soils by the d i s t r i b u t i o n of organic N. The changes in the levels of NH~ anda-aminoacids are gradual, but the changes in hydrolysable unknown N (HUN), hexosamine and hydrolysable organic N levels are sudden. The amount of organic N recovered as NH4 increases with exposure to seawater, but to a lesser extend

than the amount of

inorganic N. Am~de content remains practically constant, while hexosamines are leastin F2. The level ofa-aminoacids, which make up the largest organic N fraction, f a l l s gradually as exposure to seawater increases, while their resistance to hydrolysis increases. HUN, though varying i r r e g u l a r l y among the various profiles

and horizons, is greatest in the t e r r e s t r i a l

soil,

in which i t is also

least resistant to hydrolysis. Total hydrolysable organic N reflects the beha-

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Fig. 6. Organic N of a cambisol (C) and two f l u v i s o l s (F and Fg). Percentage of solubilized organic N in the pooled hydrolysates (zH) an~ in each i n d i v i d u a l l y (Hi ).

372

viour of the two major fractions (aminoacids and HUN), as do the separate hydrolysates. REFERENCES l

J. GraBa and F. Macias, Equilibrios de oxidaci6n-reducci6n en sistemas de estuario. Cuadernos da Area de Ciencias MariSas, Seminario de Estudos Galegos, l (1984) 46-65. 2 L.M. Mayer, Geochemistry of humic substances in estuarine environments, in G. R. Aiken, D.M. McKnight, R.L. Wershaw and P. MacCarthy (Eds.), Humic substan ces in s o i l , sediment and water, J. Wiley, New York, 1985. 3 F.A.O.-U.N.E.S.C.O., Soil Map of the World, Vol. I, Legend, Roma-Paris, 1974. 4. A. Cabaneiro, Estudio de los materiales limnico-saprop61icos de los estuarios gallegos, Doctoral Thesis, University of Santiago de Compostela, Spain, 1978. 5 W.L. Kubiena, Claves sistemAticas de suelos, C.S.I.C., Madrid, 1953. 6 M.C. Leir6s and F. Guitian Ojea, Suelos de la zona hOmeda espafiola. XI. ContribuciSn al estudio de los suelos hidromorfos de Galicia. I. Suelos costeros. An. Edaf. Agrobiol., 40 (1982) 1707-1734. 7 S. Bruckert and M. Metche, Dynamique du fer et de l'aluminium en milieu podzolique: caract~risation des complexes organo-m~talliques des horizons spodiques. Bull. ENSAIA Nancy, 14 (1972) 263-275. 8 G. Monnier, L. Turc, C. Jeanson-Luusinang, Une m6thode de fractionnement den sim6trique par centrifugation des mati~res organiques du sol, Ann. Agron., 13 (1962) 55-63. 9 S.J. GonzAlez Prieto and T. Carballas, Modified method for the fractionation of soil organic nitrogen by successive hydrolyses, Soil Biol. Biochem., 20 (1) (1988) I-6. IO F. Guitian Ojea and T° Carballas, T~cnicas de anAlisis de suelos, 2nd edn., Ed. Pico Sacro, Santiago de Compostela, 1976.' II S.J.GonzAlez Prieto, Influencia de la composici6n de la fracci6n orgAnica ni trogenada sobre la cin~tica de mineralizaci6n de los suelos gallegos. Doctoral thesis, University of Santiago de Compostela, Spain. In preparation, 1988. 12 L. Jocteur Monrozier, Nature et ~volution de l'azote organique dans les sols et les s~diments marins r~cents, Doctoral thesis, University of Nancy I, France, 1984. 13 Ph. Duchaufour, Edafologia. I. Edafog~nesis y clasificaciSn. Ed. Masson, Bar celona, 1984.