Effect of retention of run-off water and grazing on soil and on vegetation of a temperate humid grassland

Agricultural Water Management, 23 (1993) 233-246 © 1993 Elsevier Science Publishers B.V. All rights reserved. 0378-3774/93/$06.00

233

Effect of retention of run-off water and grazing on soil and on vegetation of a temperate humid grassland M. Alconada 1, O.E. Ansin 2, R.S. Lavado, V.A. Deregibus, G. Rubio and F.H. Guti6rrez Boem Facultad de Agronomla, Universidad de Buenos Aires, Buenos Aires, Argentina (Accepted 29 December 1992)

ABSTRACT A 4-year field trial was carded out on a Typic Natraqualf to modify the surface run-off, to change the soil water regime and improve forage productivity. Water was retained by earth banks which were built along contour lines. The area was grazed by cattle at a density of six animal units per hectare during five or six occupation periods per year. To study the effect of cattle trampling, 1 ha within the water retention area was excluded from grazing. It was found that surface accumulation of water led to higher soil water contents and prevented salt ascension by capillarity from the water table (Electrical Conductivity of At horizon, 1.4 dS. m - ~against 3.4 dS-m-~ in the control area). Soil salinization in the control area was associated with soil water evaporative losses and the water table depth, when it was less than 1.5 m deep. Soil alkalinity (pH and SAR) showed variations closely related to salinity. The already impaired soil physical properties were not significantly affected by livestock trampling in the water retention area. A dramatic change in plant community composition was observed. Most halophitic species disappeared and the area was covered by hydrophilous grasses. This determined a 4-fold increase in higher quality forage. Run-offwater retention proved to be a promising way to change temporarily the status of the soil and to cause a large change in grassland characteristics and productivity.

INTRODUCTION

Control of run-off water on salt-affected soils is a technique designed to grow crops or increase their yields in regions with severe seasonal or year long drought. Moisture conditions are improved and soil salinity and alkalinity reduced. The technique of the management of surface water involves "water harvesting" (Kamra et at., 1986) or the control of run-off water along the slopes causing water storage in soil profiles together with salt leaching (SanCorrespondence to: R.S. Lavado, Facultad de Agronomia, Universidad de Buenos Aires, Avenida San Martin 4453, 1417 Buenos Aires, Argentina. Present address: ~Departamento de Suelos, Ministerio de Asuntos Agrarios, 1900 La Plata, Argentina. 2Facultad de Agronomia, Universidad Nacional de La Plata, 1900 La Plata, Argentina.

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M. ALCONADA ET AL.

doval et al., 1961; Soiseth et al., 1974; Nulsen, 1986). Ponding of run-off water has been utilized in the reclamation and natural revegetation of eroded saline soils with exposed B horizon (Ringrose-Voase et al., 1989 ). The Flooding Pampa occupies the eastern portion of the Argentinean Pampas. Characterized by its fiat topography, low altitude and temperate humid climate, the area experiences water deficits during mid-summer and excesses in late winter. The latter causes waterlogging and even floods on low grounds (Lavado and Taboada, 1988). Around 60% of the region's soils, predominately Natraquolls, Natraqualfs and other halomorphic soils, are affected by different degrees of alkalinity and salinity (I.N.T.A., 1990). The top soil salinity is usually low, but is subjected to periodic salt increases (Lavado and Taboada, 1988 ). All these characteristics have determined the livestock industry to be the main economic activity of the region. Cow-calf operations utilize native grasslands as their main forage source (Leon et al., 1984). Grass productivity is high in spring and summer and low in autumn and winter (Sala et al., 1981 ). During summer, the drought period often coincides with the occurrence of saline stress caused by increases in the soil surface salt content (Lavado and Taboada, 1988). This happens during periods of high atmospheric water demand, as water moves upwards carrying salts that are then deposited on bare soil surface as evaporation takes place (Lavado and Taboada, 1988). Cattle grazing is an important anthropic disturbance on grassland and soils of the region. Cattle tread and trample in the field irrespective of the soil water content. In wet conditions the hoof stress sensibility is high (Mullins and Fraser, 1980; Scholefield and Hall, 1986 ). The most common hoof disturbance is soil compaction, but when soil is trampled wet it undergoes consolidation and poaching. In the Flooding Pampa soil compaction in the soil surface and poaching were observed (Taboada and Lavado, 1988 ). Waterlogging is a physical disturbance that modifies the structure and floristic composition of plant communities, as flood tolerance varies widely among species (White, 1979; Kozlowski, 1984). Important increases in forage availability have been obtained by water harvesting through floristic changes and higher productivity (Houston, 1960). Variations in vegetation cover, floristic richness, and aerial biomass availability have also been reported (Kincaid and Williams, 1966; Miller et al., 1969; and Bayley et al., 1985). On a nonhalomorphic community of the Flooding Pampa native grasslands, Chaneton et al. (1988) observed a substitution of exotic species of a low foraging value by other native grasses with a higher value after large natural floods. In a controlled experiment with the same plant community, a 2-fotd increase in total cover and aerial biomass was caused by flooding (Insausti, personal communication). In the present research, the combined effect of run-off water control, with the subsequent water accumulation on soil surface, and direct cattle grazing

EFFECT OF RETENTION OF RUN-OFF WATERAND GRAZING ON SOIL AND VEGETATION

235

in a conspicuous halomorphic soil with a typical plant community of the Flooding Pampa, was studied. The main hypothesis of this work was that, by means of increasing water availability through the modification of the soil salt regime and the eventual transformation of the halomorphic plant community into an hydromorphic one, a substantial increase on forage production should be provided. MATERIALS AND METHODS

Site and experimental treatments The study was carded out in the northeast of the Flooding Pampa (57 ° 30' W, 35 ° 30' S), Argentina. Treatments were located inside a 30 ha basin with a general slope around 0.3%, with predominance of a Typic Natraqualf. The soil's main characteristics and chemical properties are shown in Table 1. The annual rainfall since 1986 to 1990 amounted to 874, 958, 1418,867 and 1061 mm, respectively. Plant communities that grow on these soils were characterized by Le6n et al. ( 1979 ). The dominant grass species (Distichlis spicata, D. scoparia, Sporobolus pyramidatus, Paspalum vaginatum ) provide very little and low quality summer forage. Other frequent species are Hordeum stenostachys, an annual cool season grass, and Chaetotropis elongata. The experimental area was divided into a "water retention area" and a "control area". Surrounded by two earth banks, run-off water was retained inside an area of 4.5 ha. The banks were built along contour lines with a difference of elevation between each other of 0.30 m, and its height averaged 0.50 m. They were built in August 1986, using farm and road machinery. Both areas were grazed periodically and simultaneously by cattle according to forTABLE1 Soil characteristics Horizon

Depth (em)

Structure

pH a

EC b (dS.m-l)

Clay (%)c

CaCO3 (%)d

Of e (%)

Pf ppm

Aj BI B21 B22 B31 B32

00-08 08-15 15-44 44-54 54-54 65-83

Massive Subangular Primatic Prismatic Blocky Massive

7.7 9.8 9.5 9.5 9.5 9.0

1.52 1.2 1.05 2.03 0.75 0.85

27.7 41.7 58.3 59.2 33.8 -

0 0 0 0 2.6 7.3

0.98 0.62 0.44 -

5.9 3.8 -

apH in paste (Rhoades, 1982). bElectrical conductivity in saturation extracts (Rhoades, 1982). cParticle size analysis by pipette method (Gee and Bauder, 1986 ). aLime content by the Allison method (Nelson, 1982 ). eOrganic carbon by the Walkley & Black method (Nelson and Sommers, 1982 ). fAvailable phosphorus by the Bray & Kurtz method (Olsen and Sommers, 1982 ).

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age availability. Five or six occupation periods occurred during the year at a stock density of six animal units (AU) per hectare. The average stocking rate over the entire ranch was 2 A U - h a - a-yr- 1. To study the effect of cattle trampling on the waterlogged soil, 1 ha was excluded from grazing by large herbivores within the "water retention area". Thus, three different treatments were defined through which the "grazed control" was compared with "water retention grazed" and "water retention ungrazed" treatments.

Evaluation of soil properties Sampling of the surface horizon (A1) was performed from spring 1986 to summer 1990, totalling 13 seasonal samplings. On some occasions, subsurface horizons were also sampled. Samples consisted of six replicates randomly taken on each treatment up to a distance of 50 m from the earth bank that divided the water retention area from the control. All soil samples were used to obtain the gravimetric water content by oven drying (Gardner, 1986 ), pH in paste, the electrical conductivity on saturation extracts (EC), the Sodium Adsorption Ratio (SAR) from the concentrations of Na, Ca and Mg in the saturation extracts, measured by atomic absorption spectrophotometry (Rhoades, 1982 ). In one summer and one winter, four samples were specially taken. Two replications of each were used for determining the hydraulic conductivity in saturated soil by the constant head method, and structural stability by the water-alcohol-benzene treatment (Burke et al., 1986), and six replications were used for determining the bulk density by the clod method (Blake and Hartge, 1986 ). To study in detail the effect of cattle trampling in wet conditions, the soil bulk density was measured at the same time on the water retention grazed and ungrazed treatments. In the latter the determinations were performed on the soil surface between tussocks. Within the grazed treatment the bulk density was measured at the bottom of the hoofsteps and in the soil which was removed by them and flowed around the tread. In addition, the penetration resistance, an indicator of soil strength, was measured with the Proctor penetrometer (Davidson, 1965 ). Thirty replicates were performed on each occasion. In order to eliminate the influence of water content, the data were adjusted to an average soil water content (Christensen et al., 1989). At the end of the study (summer 1990) infiltration rate was measured three times in each treatment with a cylinder infiltrometer (Bouwer, 1986). On every sampling date, the water table depth and the water salinity were evaluated in two observation wells installed in the basin.

Evaluation of vegetation characteristics During three summers ( 1987, 1989 and 1990) the total and specific cover of the vegetation were visually estimated using the Braun Blanquet method

EFFECT OF RETENTION OF RUN-OFF WATER AND GRAZING ON SOIL AND VEGETATION

2 37

(Braun Blanquet, 1950) whose cover scale was modified by Ares and Leon (1972), within two randomly located permanent stations of 25 m 2 per treatment. This method gave an estimate of the cover and floristic richness in both water retention and control areas (Goryainova and Rodman, 1984). The specific diversity (H) was estimated through the Shanon-Weaver index (Margalef, 1977). In the summer of 1990, the above-ground biomass availability was estimated by measuring the dry matter in graminoids, non-graminoids and litter in rectangular sample areas of 0.5 m 2 with 6 replications per treatment. The plant material was cut at soil surface, and the litter was manually picked up. Plant compartments were then separated in the laboratory, washed, oven dried at 60-70°C and weighted. The soil and plant data from each sampling date were statistically analyzed by an Analysis of Variance. When significant differences were found (p< 0.05 ), values were compared by using the Tukey test (Steel and Torrie, 1985). RESULTS AND DISCUSSION

Soil-water regime Soon after the earth banks were built, run-off water area became ponded. The extension of the waterlogged area created upstream varied throughout the study period according to rainfall events and it covered 15% of the total retention area on average. When intensive rainfall events occurred, the earth banks treaded by the cattle suffered some damage. The intakes and outlets of water were not quantified. Outside the earth banks, in the control treatment, rainwater followed its previous run-off pattern towards a nearby stream. The water infiltration varied between 0.05 and 0.2 cm.h-~ and showed no differences between treatments. It is evident that, with such a low infiltration rates, even water from less intense rainfalls would run-off down along the slopes. Since the earth bank building, the mean water content in the A~ horizon was higher where run-off water was retained (Fig. 1 ). Statistically significant differences among treatments were found during springs and in one summer. During those seasons the soil water content in the water retention grazed treatment was 148-275% of the controls. In the B horizon, the water content showed no significant differences between treatments on any of the sampling times. As expected by the infiltration rate, the hydraulic conductivity of surface and subsurface horizons was in all cases very low: between 0.015 and 0.199 cm-h-~ in the AI horizon (Table 2, between 0.00 and 0.25 cm-h -~ in BI horizon, and between 0.08 and 0.25 cm* h - 1 in the B2 horizons. These coincidental and extremely low values in conjunction with entramped air, as found in other soil of the region (Lavado

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Fig. 1. W a t e r c o n t e n t o f A~ h o r i z o n . S i g n i f i c a n t d i f f e r e n c e : * p < 0 . 0 5 , ** p < 0 . 0 1 .

TABLE 2 Soil: p h y s i c a l p r o p e r t i e s Date W i n t e r 1988

S u m m e r 1989

1.352 a 1.435 a 1.513 a

1.506 a 1.489 a 1.441 ~

0.065 a 0.199 a 0.064 a

0.035" 0.016 a 0.015 a

1.47 a 3.22 b 2.17 a

4.12 a 0.94 a 5.50 a

Bulk density

(g.cm -3 ) Control Water retention grazed Water retention ungrazed

Hydraulic conductivity

( c m - h - ~) Control Water retention grazed Water retention ungrazed

Structural stability index Control Water retention graze Water retention ungrazed S u p e r i o r letters: s i g n i f i c a n c e p < 0.05.

EFFECT OF RETENTION OF RUN-OFF WATER AND GRAZING ON SOIL AND VEGETATION

239

and Taboada, 1988 ) would explain why the B horizons were not water saturated in spite of the free water accumulation on the soil surface.

Soil halomorphism The salt dynamics in the top soil horizon differed between treatments. The average EC of the AI horizon showed little variation in the water retention grazed treatment while sharp summer peaks occurred in the control (Fig. 2 ). The EC of the control was between 30 and 140% higher than in the water retention grazed treatment. As it was found in other soils of the area during summer (Lavado and Taboada, 1988), capillary water ascending from the bottom of the profile and from the water table carry soluble salts towards the surface horizon. The phreatic water EC varied between 0.81 and 1.86 d S . m during the period of this experiment. When water is retained, the salt rise is prevented (Fig. 2). Although the soil water content differed greatly amongst treatments in spring 1989 and summer 1990, similar EC values were observed. In the control, the expected salinity peaks did not occur during that summer, although the soil water content was low, coinciding with the seasonal high evaporation rate. This behavior may be attributed to the deepening of the water table level during 1989. Being around 1 m deep during the first 2 years of the experiment, the water table dropped to 2.5 m deep at the beginning of 1989 (Fig. 3 ). Its rise from autumn 1989 was slow and in autumn 1990 the water table was 1.42 m deep. This could have prevented water (and salts) ascension to the top horizon, as happened in the three previous summers. From this result it is interesting to point out that salt peaks are not always related with water stress,

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Fig. 2. Electrical conductivity of A~ horizon. Significant difference: *p < 0.05, ** p < 0.01.

240

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but may also be related to the water table depth. Thus, water stress is more c o m m o n than saline stress. Both alkalinity parameters (SAR and p H ) showed a close relationship with salt pulses. In the A horizon, SAR showed a pattern similar to the EC, being almost constant in the soil of the water retention grazed treatment (22.2 to 28.1 ), but it varied from 22.1 to 56.6 in the control soil. Only summer peaks ( 1988 = 56.6 and 1989 = 44.9 ) were significantly higher (p < 0.01 ). Concerning the pH, a significant difference amongst treatments (p < 0.01 ) was found only in summer 1988, showing the higher value in the control soil. pH values fluctuated from 7.26 to 8.23 in the water retention grazed treatment soil and from 7.49 to 9.33 on the control soil. The high values of both parameters are related to the present salts type provided by the groundwater rich in HCO3-, CO~- and sodium as their main ions (average groundwater content are HCO3-, 14.79 m E q . l - '; CO~-, 0.84 mEq.1- '; Na, 16.26 mEq.l -~ ). The soil EC of the water retention ungrazed treatment showed identical values than those observed in the water retention grazed treatment. Both water retention treatments showed differences with the control (data not shown ). In the case of the B horizon, significant differences between treatments were found in the EC, SAR and pH values in summer 1988. The soil of the control area showed the higher values [control EC: 2.46 vs. 1.22 d S - m - ~of the water retention grazed treatment soil ( p < 0 . 0 5 ) ; control SAR: 83.69 vs. 37.9 ( p < 0 . 0 5 ) ; and pH: 9.99 vs. 8.78 ( p < 0 . 0 5 ) ] . No differences among treatments were found on the remaining dates.

Soil physical properties The water-alcohol-benzene instability index, which proved to be an efficient methods to evaluate the structural stability in the alkaline soils of the

EFFECT OF RETENTION OF RUN-OFF WATER AND GRAZING ON SOIL AND VEGETATION

241

area (Alconada and Lavado, in prep. ), showed that these soils have low structural stability. The values obtained and seasonal variation coincided with those found for the same soil in a nearby site (Alconada and Lavado, in prep.). Significant differences (p < 0.01 ) among treatments were observed in winter 1988 (Table 2). The soil of the control and that from the water retention ungrazed treatment showed to be more stable than the soil under the water retention grazed treatment. In the summer of 1989, no significant differences were observed due to the great variation in the results (Table 2 ). At least on one occasion, grazing affected the soil structural stability in the water retention area. The hydraulic conductivity was low on all the observation dates and there were no significant differences among the treatments. This showed that trampling in waterlogged conditions did not further reduce this soil property (Table 2). The bulk density was high and showed no significant differences amongst sampling dates and treatments (Table 2). Clods taken from the bottom of the hoofstep, from the soil mass displaced by it and from the ungrazed treatment showed no difference in its high bulk density (average 1.69, 1.67 and 1.64 g-cm -3, respectively). These results suggest that changing the water regime under grazing or ungrazing conditions causes no changes in the current high bulk density of the top horizon. Similarly, there was no significant differences in soil strength on any of the sampling dates. The annual average and standard deviation was: 5.13 Mpa, a = 2.68 for the control area; 3.75, a = 2.56 for the water retention grazed treatment, and 3.45 Mpa, a = 1.08 for the water retention ungrazed treatment. Physical conditions agreed with those expected for such an alkaline soil. The A~ horizon showed high levels of exchangeable sodium, massive structure, no visible macropores, fine texture (clay+ silt = 89%) and low organic carbon content, near 1%, and low structural stability and hydraulic conductivity and high bulk density.

Grassland characteristics Plant cover was significantly modified by the water retention treatment. While, in the control area, plant cover maintained a low value similar to the initial one (45.5%), it was substantially increased as a consequence of the water retention treatments (Fig. 4a). During the summer of 1990, 62% plant cover ( p < 0 . 0 5 ) was observed in the water retention grazed treatment and 71% ( p < 0 . 0 1 ) in the water retention ungrazed treatment (Fig. 4a). These values were lower than those found by Insausti (personal communication) when another non-halomorphic plant community of the Flooding Pampa was flooded. Important floristic changes and dramatic variations in specific cover occurred as a consequence of waterlogging. The graminoids cover increased 53%

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1990

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years

@ NON GRAZED WRT

WRT: WATER RETENTION TREATMENT

Fig. 4(a) Total plant cover. Different letters: significancep<0.01. (b) Species number. Different letters: significancep < 0.01. (c) Species diversity. Different letters: significancep < 0.01. with respect t o initial cover, changing significantly the specific dominance (Table 3 ). Distichlis spp. maintained its initial cover (10.5%) under control conditions (grazed), was significantly reduced to 5% ( p < 0.05 ) in the water retention ungrazed treatment, and disappeared ( p < 0 . 0 1 ) in the adjacent water retention grazed treatment (Table 3 ). T w o warm season hydrophilous grasses, Leersia hexandra and Echinochloa helodes, significantly increased

EFFECT OF RETENTION OF RUN-OFF WATERAND GRAZING ON SOIL AND VEGETATION

243

TABLE 3 Total cover (%) by different plant species and compartments

1987 Initial data 1989 Control Water retention grazed Water retention ungrazed 1990 Control Water retention grazed Water retention ungrazed

Grasses

Other spp.

Distichlis spp.

Graminoid

Litter

35.5

10.5

10.5

1.5

33.5

13.5

13.0

1.0

45.0

7.5

5.0

20.0**

50.5*

11.5

5.0

15.5**

31.5

10.0

13.0

1.0

5.0*

54.5*

7.0

0.0"*

49.5**

0.5

52.0**

4.0*

5.0"

47.0**

15.0"*

Treatment differences are: *significant (p < 0.05); **highly significant (p < 0.01 ).

their initial joint cover from 1.5% to almost 50% in both water retention treatment (p < 0.01 ) (Table 3). Chaneton et al. ( 1988 ) found similar floristic changes and specific cover variations on a nearby lowland non-halomorphic community after a long lasting natural flooding. The number of species was reduced to one-third by the water retention treatments, reaching significantly low values (p < 0.01 ) during the summer of 1990 (Fig. 4b ). Only two species were found in all censuses made in the water retention treatments; they were E. helodes and Marsiela concinna. In agreement with the changes observed in cover and number of species, the species diversity (H) showed a significant reduction (p<0.05) in the water retention grazed treatment, that became highly significant (p< 0.01 ) in the water retention ungrazed treatment (Fig. 4c). According to Lewis et al. ( 1988 ) the H index is good to evaluate the impact of an exogenous factor. In this experiment, H clearly indicates the flood effect over the grassland structure, expressing the hydrophilous species domination. Watedogging dramatically increased forage availability (Fig. 5). In the water retention treatments the total above-ground biomass was estimated to be 3837 and 4487 kg-DM-ha -~ during the summer of 1990, for grazed and ungrazed conditions, respectively. This implies an 190 to 240% increase with respect to 1323 k g - D M - h a - 1available in the control. Almost all available dry matter in water retention treatments was provided by graminoid species and in the ungrazed treatment a significant litter accumulation occurred (Fig. 5 ).

244

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Fig. 5. Availabledry matter. Differentletters: significancep < 0.01. The significant differences (p< 0.01 ) encountered among treatments in dry matter availability is enhanced when its forage value is considered. Hydrophilous grasses such as E. helodes and L. hexandra are highly palatable when maintained short and tender. Distichlis spp., the main forage contribution in the control treatment, is almost unpalatable. Its contribution in the water retention treatments decreased in the ungrazed and disappeared when grazed. Cattle trampling may have caused this result, as submerged Distichlis spp. plants died from anoxia. Similar results were obtained many years ago by Davis and Martin (1949) who reported that plants could endure prolonged floods if their tops emerged over the water level. CONCLUSIONS

According to the results obtained, the water retention of run-off, even in humid areas like the Flooding Pampa, is a technique which should deserve attention in order to increase forage production on salt-affected soils. This occurred through an increase in the soil water content and the avoidance of salinity peaks in the top horizon. These changes coincided with important changes in vegetation composition, which were expressed in a significant increase of soil cover, simplification of plant structure, dominance of few hydrophilous grasses and a drastic reduction of halophilous species.

EFFECT OF RETENTION OF R U N O F F WATER AND GRAZING ON SOIL AND VEGETATION

245

Practical aspects such as ways of construction and maintenance of earth banks, amounts of water to be retained, species seeding and grazing strategies, need to be tested in order to develop an optimum technique.

REFERENCES Ares, J. and Leon, R.J.C., 1972. An ecological assessment of the influence of grazing on plant community structure. J. Ecol., 60: 333-342. Bayley, S.E., Zoltek, J., Hermann, A.J., Dolan, T.J. and Tortora, L., 1985. Experimental manipulation of nutrients and water in a fresh water marsh: Effects on biomass, descomposition and nutrient accumulation. Limnol. Oceanogr., 30:500-512. Blake, G.R. and Hartge, K.H., 1986. Bulk density. In: A. Klute (Editors), Methods of Soil Analysis, Part 1, Physical and Mineralogical Methods, 2nd edn. Madison, Wisconsin, USA, pp. 363-375. Bouwer, H., 1986. Intake rate: cylinder inffltrometer. In: A. Klute (Editor), Methods of Soil Analysis, Part 1, Physical and Mineralogical Methods, 2nd edn. Madison, Wisconsin, USA, pp, 825-844. Braun Blanquet, J., 1950. Fitosociologia vegetal. Acme (Editor), Bs.As., 820 pp. Burke, W., Gabriels, D. and Bouma, J., 1986. Soil structure assessment. Balkema (Editor), Rotterdam, The Netherlands, 92 pp. Chaneton, E.J., Facelli, J.M. and Leon, R.J.C., 1988. Floristic changes induced by flooding on grazed and ungrazed lowland grasslands in Argentina. J. Range Manage., 41: 495-499. Christensen, N.B., Sisson, J.B. and Barnes, P.L., 1989. A method for analyzing penetration resistance data. Soil Till. Res., 13: 83-91. Davidson, D.T., 1965. Penetrometer measurements. In: C.A. Black, D.D. Evans, J.L. White, L.E. Ensminger and F.E. Clark (Editors), Methods of Soil Analysis, Part l, Physical and Mineralogical Properties, Including Statistics of Measurement and Sampling. Madison, Wiscosin, USA, pp. 472-484. Davis, A.G. and Martin, B.F., 1949. Observations on the effect of artificial flooding on certain herbage plants. J. Br. Grassl. Soc., 4: 64-65. Gardner, W.H., 1986. Water content. In: A. Klute (Editor), Methods of Soil Analysis, Part l, Physical and Mineralogical Methods, 2nd edn. Madison, Wisconsin, USA, pp. 493-544. Gee, G.W. and Bauder, J.W., 1986. Particle-size analysis. In: A. Klute (Editor), Methods of Soil Analysis, Part 1, Physical and Mineralogical Methods, 2nd edn. Madison, Wisconsin, USA, pp. 383-411. Goryainova, I.N. and Rodman, L.S., 1984. Some aspects of elaboration of the floristic classification of meadows (as exemplified by the meadows of the Volga-Kkhtuba floodplain-URSS). Izv Timiryazev s-khakad, 6: 35-46. Houston, W.R., 1960. Effects of water spreading on range vegetation in eastern Montana. J. Range Manage., 13: 289-293. I.N.T.A. (Instituto Nacional de Tecnologia Agropecuaria), 1990. Atlas de Suelos de la Repfiblica Argentina I, Bs.As., pp. 83-202. Kamra, S.K., Dhruvanarayana, V.V. and Rao, K.V.G.K., 1986. Water harvesting for reclaiming alkali soils, Agric. Water Manage., I h 127-135. Kincaid, D.R. and Williams, G., 1966. Rainfall effects on soil surface characteristics following range improvement treatments. J. Range Manage., 19:346-351. Kozlowski, T.T., 1984. Extent, causes and impacts of flooding. In: T.T. Kozlowski (Editor), Flooding and Plant Growth. Academic Press, New York, pp. 1-7.

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