437

Patterns of Organic Matter Accumulation in Soils of the Semiarid Argentinian Pampas

Daniel E. Buschiazzo*), Alberto R. Quiroga') and Karl Stahr")

*) Laboratorio de Suelos, Facultad de Agronomia, Universidad Nacional de La Pampa, 6300 Santa Rosa, Argentina **) Institut fur Bodenkunde und Standortslehre, Univemitat Hohenheim, 7000 Stuttgart 70, FRG

Angenommen: 19. Juli 1991

Summary - Zusammenfassung A-horizons of 48 Haplic Phaeozems and Kastanozems in plain sites of

the Semiarid Argentinian Pampas under three contrasting management systems (virgin bushland. permanent pasture and continuous agriculture) were studied. Morphological characteristics, organic carbon and total ni- trogen levels and WE6 ratios were determined on the assumption that both quantity and quality of organic matter should be related to soil textwe of parent materials in this region since other soil forming facton an uniform.

The more intensive land use produced an averaged decrease of 7 cm in A-horizon thickness and degradation of soil structure, but little changes in color and propenies of boundaries to AC horizons. In virgin soils organic carbon and nitrogen contents correlate with silt + clay fractions (t = 0.92 and 0.99. respectively), while WMratios are related to clay contents (r = -0.69') confirming the strong influence of soil texture. Agricultural- management and pasture use of soils lead to dispersion of these correla- tions probably because of differential influence of varying intensities of land use within each management system. Soils under continuous agricul- ture show lower organic carbon contents than virgin soils only at clay + silt > 40 8. Eragrosris curvulu-pastures seems to be more effective in restoring organic matter and nitrogen levels in sandy but not in loamy soils with respect to virgin soils. This may be due to a better efficiency of E . cuvu&-mots in coarse than in fine textured soils to produce humific- able residues than the autochthonous grass species.

Introduction

The soils of the semiarid Argentinian Pampas, mainly weakly developed Haplic Phaeozems and Kastanozems (INTA et al., 1980). are considered to be homogeneous since the soil forming factors are assumed to be uniform throughout the region. However, preliminary studies showed the existence of spatial variations of organic matter contents, although the parameters which determined these variations could not be established.

Assuming that climate (Fig. 1). time of formation (the soils developed on holocene materials) and biotic material (originally Prosopis caldenia bushland) are uniform, and taking into consideration identical topographic positions, the

Humusverteilung in Boden der semiariden argentinischen Pampas

Die A-Horizonte von 48 Haplic Phaeozems und Kastanozems in ebener Lage der semiariden argentinischen Pampas wurden untersucht. Sie ver- teilten sich unter die drei unterschiedlichen Landnutzungssysteme natUrli- ches Buschland, Dauenveide und permanenter Ackerbau. Die morphob gischen Eigenschaften. die organischen Kohlenstoff- und Gesamtstick- stoffgehalte sowie die E4:E6-VerhlUtnisse wurden bestimmt unter der An- nahme. daB sowohl Quantitili wie auch Qualitiit der organischen Substanz zur Bodenan des Ausgangssubseats und Bodennutzung in diesem Gebiet in Beziehung stehen. alle anderen bodenbildenden Faktoren konnen als einheitlich betrachtet werden.

Die intensivere Landnutzung eneugte im Durchschnitt eine Abnahme der Miichtigkeit der A-Horizonte von 7 cm und eine Degradierung der Bodenstruktur. aber nur geringe Ve-derungen der Bodenfarbe und der Ausbildung der Grenze zu den AC-Horizonten. Die org. Kohlenstoff- und Stickstoffgehalte korrelieren mit den Schluff + Tongehalten (r = 0.92 bzw. 0.99). w w n d die E4:E6-Quotienten nur mit der Tonfmktion in den jungMulichen W n in Beziehung stehen (r = -0.69). Dies besutigt die Eingangshypothese. In ackerbaulich und weidewinschaftlich genutzten BWen nimmt die Straffheit dieser Korrelationen a b wahrscheinlich, weil der EinfluB verschiedener Intensitliten der Landnutzung innerhalb eines Landnutzungssystems differenzierend wirkt. Die Erugrosiis curvulu- Wei- den waren effektiv im Erhalten der organischen Substanz und des Stick- stoffniveaus in sandigen aber nicht in lehmigen B(Men, wiederum bezo- gen auf die jungftlblichen BtMen. Wahrscheinlich sind Eragrosris runw- la-Wurzeln in grtiberen eher als in feineren B&n in der Lage. mehr humifizierbare R U c k s W zu bilden als die autochthonen G&r.

variations could be ascribed to either 1) variability in the texture of the parent material, or 2) different magnitude of erosion processes caused by distinct soil management prac- tices and/or cultivation history. The first hypothesis is sup- ported by the fact that the parent material varies in texture even within short distances, because of different sedimenta- tion patterns of the loess, caused by different wind veloc- ities (Buschiuzzo, 1985).

The influence of soil texture on the organic matter levels of soils of arid and semiarid regions has been extensively studied (Haas et al., 1957; Burke et al., 1989). Some factors which determine this influence are the greater water-holding capacity of finer textured soils, favouring organic matter accumulation ( M u m et al., 1978) or the association between

2. Pflanzenerndhr. Bodenk.. 154.437-441 (1991) OVCH Verlagsgesellschaft mbH. W-6940 Weinheim, 1991 0044-326319 110612-0437 S 3.50 + .25/0

43 8 Buschiazzo. Quiroga. and Stahr

humic substances and clay particles which can protect the organic matter against decomposition (Dulal and Muyer, 1986).

Another interrogation is how soil texture affects the soil organic matter quality. Zhung et al. (1988) found that the E4:E6 ratios of humic acids extracted from different soil fractions tend to decrease as the particle size of the soil fractions decreases indicating a larger size of the humic acids in the finer soil fractions.

If a direct influence of texture on organic matter contents and -quality could be confirmed, the possible effect of tex- ture on the degradation rate of soils under similar manage- ment systems should be taken into account.

The objective of the present study is to evaluate the influ- ence of soil texture and land use on the quality and quantity of soil organic matter in the semiarid Argentinian Pampas.

Materials and Methods The semiarid Argentinian Pampa Region has a climate with a mean

annual temperature of 16°C. annual rainfall average of 600 +/- 50 mm with maximum intensities in spring and autumn (Fig. 1).

In an area of approximately 20.000 square km, 48 A-horizon samples of plain soils (mostly Haplic Phaeozems and Kastanozems) were collected

60Omm 70Omm I . \

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36'

37'

50 km

- mean annual temperature ra in fa l l - -

Figure 1: Location map of sampled soils in the semiarid Argentinian Pam- pas (virgin, pasture, agriculture) together with main soil types (Hh = Haplic Phaeozems, Kh = Haplic Kastanozems, Zo = Orthic Solonchaks). mean annual temperature and rainfall isolines. Abblldung 1: hersichtskarte der beprobten BBden in der semiariden ar- gentinischen Pampa (jungfrtiuliche Naturstandorte, Weiden. Acker). Zu&- lich sind die wichtigsten Bodentypen sowie lsolinien der mittleren Jahres- temperatur und der Niederschlagsmengen eingeuagen.

(Fig. 1). These soils present an A-AC-C (Ck) horizon sequence. Tosca (calcrete) appears in plains commonly at a depth of about 1 m, shallower at hills (at surface) and deeper at basins (more than 2 meters). The dominant hue of the A-horizons is IOYR and their moist values and chromas vary between 6/5 and 2/2. Boundaries between horizons are commonly smooth and clear to gradual.

On each sample the following determinations were canied out: bulk density using core samples; organic carbon (Walkley and Black); total nitrogen (semimicro Kjeldahl method); texture by the combined method of the pipette and wet sieving, separating the Q-, 2-50- and 50-2000 pm-frac- tions (Schlichring and Blwne, 1966); quality of soil organic matter by the determination of the WE6 ratios of the whole c 2 mm soil samples using 1 m NaHCO3 as extractant (Kononova. 1966).

Results and Discussion

The main morphological characteristics of the studied A- horizons (Table 1) show that the thickness of A-horizons varies between 13 and 36 cm. Average A-horizon thickness of virgin soils is similar to that of pasture soils (24.6 and 24 cm) but they are larger than that of agricultural soils (17.2 cm). The more intensive utilization of soils leads to a de- crease in A-horizon thickness (about 7 cm) caused by ero- sion. Similar trends were found by Aguilur and Heil(l988) and Gregorich and Anderson (1985).

The boundaries between A and AC horizons are similar in all soil management systems (clear and smooth). Dry and moist colors are darker in A-horizons of virgin

and pasture soils than in agricultural ones probably because of larger amounts of organic matter in the first ones.

The structure is more developed in A-horizons of virgin and pasture soils (medium and strong to moderate subangu- lar blocky) than in agricultural soils (fine-weak subangular blocky) according to the larger rate of degradation of the latter.

No significant morphological differences were found be- tween different textured soils under similar management systems.

Table 1: Main morphological characteristics of A-horizons of Haplic Kas- tanozems of the Semiarid Argentinian Pampas under three contrasting man- agement systems Tabelle 1: Wichtige morphologische Eigenschaften von A-Horizonten von Haplic Kastanozemen der semiariden argentinischen Pampas unter 3 ver- schiedenen Nutzungssystemen

VIRGIN PASTURE AGRICULTURE (n = 14) (n = 20) (n = 14)

Thickness 13-36 (3 24.6) 18-35 (8 24) 15-19 (0 17.2) (cm)

Boundaries clear-diffuse clear clear smooth smooth smooth

Colour dry 10YR5/3-4/1 4/2-3/2 5/3-34 moist 1OYR3/3-34 3/2-2/2 3/3-24

Structure subangular blocky subangular blocky subangular medium, strong to medium, moderate blocky, fine, moderate to weak weak

Organic matter in soils 439

The organic carbon (0C)- and total nitrogen (N)-contents of the studied A-horizons increase with increasing propor- tions of the < 50 pm mineral fraction (Fig. 2a and b). This tendency, which is observed in all three types of land use, confirms the assumption that soil parent material, through texture, is the most important factor controlling OC-content of soils in this zone, if relief is considered to be constant. That agrees with the results of many authors (Allison, 1973; Brady, 1984; Foth, 1984). Finer textures of the parent ma- terial produced a higher degree of humification, probably because of a better water balance of these soils, favoured biological activity and thus accumulation of larger amounts of humified organic residues and organo-mineral com- pounds (Nichols, 1984; Mc Daniel and Munn, 1985).

Virgin soils have mostly higher OC- and N-contents than either pasture or agricultural soils. This might be explained by higher mineralization rates and more intensive erosion processes caused by the frequency of tillage in the latter. This fact has been corroborated by numerous authors (Weaver and Forcella, 1979; Sims and Nielsen, 1986; Lyres and Tatarko, 1986).

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b 0

0 10 20 30 40 50

C L A Y + S I L T ( % I

F l p r e f: Relationship between a) organic carbon and b) total nitrogen and the clay + silt fraction (< 50 pn) of plain soils under three contrasting management systems in Ihe semiarid Argentinian Pampas. Abblldung 2: Beziehung zwischen a) organischem Kohlenstoff und b) Ge- samtstichtoff und der Ton- + Schlull Fraktion von B W n der Ebenen unter drei verschiedenen Landnutzungssystemen in den semiariden argen- tinischen Pampas.

Considering the regression equations, cultivated soils al- ways show lower OC-contents than virgin ones and the slope of the regression equation is steeper in the latter. Cal- culation of curved regression lines instead of linear ones did not improve the regression coefficient. Nevertheless abso- lute OC-losses are significantly different (P c 0.05) only in fine textured soils (clay + silt > 40 9%). Greater OM-losses, after cultivation in fine than in coarse textured soils don't agree with results of other authors (Dalal and Muyer, 1987; Aguilar and Heil, 1988; Blank and Fosberg, 1989). Never- theless relative OC-losses tend to be larger in coarse than in fine textured soils if the regression equations are taken as a basis for this calculation. The lack of significant differences between OC-contents of cultivated and virgin soils at coarse textures is related to larger dispersion of the points in the first regression, as it can be deducted from its lower regres- sion coefficient value. Larger dispersion can be a product of a differential influence of management, even within culti- vated soils, or of variable textural changes of the originally finer, cultivated A-horizons, after residual accumulation of sand by wind erosion. This last process may lead to an underestimation of OC-losses of cultivated soils. It could be avoided if texture of parent materials are taken as a basis for this comparison.

OC-contents of pasture soils and N-contents of both culti- vated and pasture soils are not significantly different from that of virgin soils. Nevertheless relative differences in OC- and N-contents at coarse textures tend to be greater in pas- ture than in virgin soils. This could be due to a better effi- ciency of Eragrostis curvula to produce larger root residues in relation to the spontaneous grass species on sandy virgin soils (Elyonurus muticus, Hyalis argentea, Aristida mendo- c im) than on sandy-loam textured virgin soils (Stipa te- nuissimu, Stipa brachychaetu, Stipa tenuis).

Lower regression coefficients of the relations between the mineral fraction c 50 pm and N-contents than between < 50 pm and OC-contents in all three management systems, indi- cate a larger dispersion in the first. A possible explanation would be that management practices and their effect on OM decomposition on the studied soils have been different. Even within each soil management system, this may affect with varying intensities the labile organic N-contents in agreement with results of other authors (Blank and Fosberg, 1989). On the other hand it is known that a relationship between N-mineralization and the C/N ratio exists (Schefer and Schachtschubel, 1989, p. 264). That explains the disper- sion of points in Fig. 2b because a great variation of the C/N ratio exists when contrasted against the < 50 pm fraction (Fig. 3).

The slope of the regression equations of the relation be- tween the c 50 pm fraction and N-contents in the cultivated soils is less pronounced than that of the relation between the < 50 pm-fraction and OC-contents. This indicates a greater influence of more intense land use on the organic nitrogen fractions of finer soils. Chichester (1969) found that the N-mineralization rate of finer fractions was higher than that of coarse ones in an incubation study.

Buschiazzo. Quiroga. and Siahr

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6

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0,-V I R; i b! 0 ?ASTURE

AGiRLCULTU?E

0 10 20 30 40 50 60

C L A Y + S I L T ( % I

Figure 3: Relationship between C/N-ratio and clay + silt content (c 50 pm) of soils under three contrasting management systems of the semiarid Argen- tinian Pampas. Abblldung 3: Beziehung zwischen C/N-Vemtllmis und Ton- + Schluff- Fraktionen in BWen unter drei verschiedenen hdnutzungssystemen der argentinischen Pampas.

It also demonstrates that the type of management system has no significant influence on C/N ratios, as stated pre- viously by Dulul and Muyer (1987) and Srrebel et al. (1988).

I y.-3.012.3.5?9 x ?:ll,, r.-O.L9*

0 10 20 30 40

C L A Y ( % I

Figure 4: Relationship between FAE6-ratios and the clay fraction of soils under three contrasting management systems in the semiarid Argentinian Pampas. 34 yrs = undisturbed soil under Eragrostis curvuln since 34 years. Abblldung 4: Beziehung zwischen WE6-VerNUtnis und der Tonfraktion von BMen unter drei verschiedenen Landnutzungssystemen in den semi- ariden argentinischen Pampas. 34 yrs = Boden der seit 34 Jahren ungestort unter Eragroslis curvula liegt.

In virgin soils the E4:E6 ratios decrease with increasing clay contents (Fig. 4). This probably means that higher con- densed organic compounds accumulate in finer textured vir- gin soils, probably because of higher complexation with clays (Chen et al., 1977). These tendencies agree with the results of Zhung et al. (1988) who found higher amounts of

condensed humic acids in finer fractions. On the other hand the E4:E6 ratio shows no clear tendencies in pasture- and agricultural soils, probably because of variable influences of soil management on the size of those organic compounds. Nevertheless the E4:E6 ratios of these soils are mainly lower than those of virgin soils, which indicates a residual accumulation of higher condensed organic matter, probahly because of a loss of the most labile aliphatic compounds. That also indicates that soils once degraded and currently under pasture, have still not improved their OM-quality.

An exception is a pasture soil that remained undisturbed for the last 34 years in the INTA-Anguil Experimental Sta- tion. This soil shows the highest E4:E6 values, probably because of an accumulation of young and small aliphatic organic substances produced by the decomposition of the E. curvulu roots during the 34-year period.

Conclusions

From these results it can be concluded that soil evolution in the semiarid Argentinian Pampas depends greatly on the texture of the parent material: finer textured soils have higher contents of organic matter because of more intense humification rates under better soil water regimes and/or formation of organo-mineral complexes.

Intensive land utilization (continuous agriculture) reduces the thickness and aggregation of A-horizons. Organic car- bon-contents of cultivated soils are significantly different from that of virgin ones only at fine textures because of increasing dispersion of the values in cultivated soils prob- ably due to varying influence of different management sys- tems or to textural changes (residual sand accumulation) after wind erosion of variable intensities.

Nitrogen contents of cultivated and pasture soils are statis- tically not different from those of virgin soils because of variable influence of management, even within each man- agement system, on the N-contents of the organic and mine- ral fraction.

Eragrostis curvulu pastures seems to increase OM- and N-levels in coarse but not in fine textured soils in relation to agricultural soils, probably because of more intense produc- tion of humificable rests by E. curvulu roots in relation to autochthonous grass species in coarse soils.

Agriculture and pasture soils show great variations in OMquality because of variable land use within each man- agement system. Nevertheless these soils seem to show higher contents of larger and more stable aromatic com- pounds than virgin ones, indicating their residual accumula- tion after the mineralization of the less stable aliphatic com- pounds. Undisturbed pasture soils may have larger accumu- lation of young and poorly polymerized organic compounds with respect to virgin soils.

Acknowledgments This work was supported by the Stiftung Volkswagenwerk. We thank

Ing. E l k Noellemeyer for her translation into the English language.

Organic matter in soils 441

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