Raúl S. Lavado

Impact of grazing on soil nutrients in a Pampean grassland.

Cattle exclusion induced dramatic changes in the plant community and modifications in nutrient cycling in grazed native grasslands of the Flooding Pampa (Argentina). The study was carried out to analyze the effect of grazing on the status and spatial variability of soil organic matter, nitrogen and phosphorus. Sampling was performed in the late summer and early spring. Geostatistical methods were used to study the spatial dependence of these soil properties. Organic carbon (OC) and total nitrogen (TN) showed spatial structure only in the ungrazed area with a similar range of dependence (39 m and 36 m respectively). The occurrence of litter in this area lead to a large and spatially homogeneous C input to the soil, which would be the key factor of the spatial structure of organic carbon and total nitrogen. Mineral nitrogen content 1(NO3(-1)-N + (NH4+)-N] was higher in the ungrazed area on both sampling dates. The mineral N content showed a large short-range variability (nugget variation) independent of grazing history. A significant decrease in the extractable P (Bray &Kurtz #1) in the grazed area was found. The extractable P exhibited spatial structure only in the ungrazed area. However, its spatial pattern was different from those of organic carbon and total nitrogen: the range of dependence was higher (57 m) and the spatial structure exhibited a great irregularity. The differences between C, N, and P variability were possibly related to their dynamics in the soil. No evidence of effects of animal excrete on nutrient content or spatial variability was found.

Adaptations and biomass production of two grasses in response to waterlogging and soil nutrient enrichment

We analysed the response of two grass species, Danthonia montevidensis and Paspalum dilatatum to waterlogging, soil-nutrient enrichment and the combination of both factors. Waterlogging did not affect total biomass of D. montevidensis, but it slightly promoted growth of P. dilatatum. Most analysed variables showed no significant interaction between fertilization and waterlogging. Therefore, waterlogging does not produce a detrimental effect either in the growth of these species or in their response capacity to stimulating growth factors, such as fertilization.

Mechanisms for the increase in phosphorus uptake of waterlogged plants: soil phosphorus availability, root morphology and uptake kinetics

Abstract Waterlogging frequently reduces plant biomass allocation to roots. This response may result in a variety of alterations in mineral nutrition, which range from a proportional lowering of whole-plant nutrient concentration as a result of unchanged uptake per unit of root biomass, to a maintenance of nutrient concentration by means of an increase in uptake per unit of root biomass. The first objective of this paper was to test these two alternative hypothetical responses. In a pot experiment, we evaluated how plant P concentration of Paspalum dilatatum, (a waterlogging-tolerant grass from the Flooding Pampa, Argentina) was affected by waterlogging and P supply and how this related to changes in root-shoot ratio. Under both soil P levels waterlogging reduced root-shoot ratios, but did not reduce P concentration. Thus, uptake of P per unit of root biomass increased under waterlogging. Our second objective was to test three non-exclusive hypotheses about potential mechanisms for this increase in P uptake. We hypothesized that the greater P uptake per unit of root biomass was a consequence of: (1) an increase in soil P availability induced by waterlogging; (2) a change in root morphology, and/or (3) an increase in the intrinsic uptake capacity of each unit of root biomass. To test these hypotheses we evaluated (1) changes in P availability induced by waterlogging; (2) specific root length of waterlogged and control plants, and (3) P uptake kinetics in excised roots from waterlogged and control plants. The results supported the three hypotheses. Soil P avail-ability was higher during waterlogging periods, roots of waterlogged plants showed a morphology more favorable to nutrient uptake (finer roots) and these roots showed a higher physiological capacity to absorb P. The results suggest that both soil and plant mechanisms contributed to compensate, in terms of P nutrition, for the reduction in allocation to root growth. The rapid transformation of the P uptake system is likely an advantage for plants inhabiting frequently flooded environments with low P fertility, like the Flooding Pampa. This advantage would be one of the reasons for the increased relative abundance of P. dilatatum in the community after waterlogging periods.

The K/Na and Ca/Na ratios and rapeseed yield, under soil salinity or sodicity

Rapeseed (Brassica napus) is a crop relatively tolerant to salt and sodium. Our objective was to study the interactions between Na, K and Ca and their relationship with its yield under the isolated effects of soil salinity or sodicity.Two experiments were carried out using pots filled with the Ah horizon of a Typic Natraquoll. There were three salinity levels (2.3 dS m-1; 6.0 dS m-1 and 10.0 dS m-1) and three sodicity levels, expressed as sodium adsorption ratios (SAR: 12; 27 and 44). The soil was kept near field capacity.As soil salinity increased, the K/Na and Ca/Na ratios in the tissues decreased markedly but yields and aerial biomass production were not affected. As soil SAR value increased, the K/Na and Ca/Na ratios in plants and K-Na and Ca-Na selectivities decreased. Plants could not maintain their Ca concentration in soil with a high SAR. The grain yield and biomass production diminished significantly in the highest SAR treatment. Our results are consistent with those showing detrimental osmotic effects of salts in Brassica napus. Conversely, under sodicity, the K/Na and Ca/Na ratios in plant tissues decreased considerably, in accordance with grain and biomass production. These results show that the effects of sodicity are different from those of salinity.

Sunflower nitrogen requirement and 15N fertilizer recovery in Western Pampas, Argentina

In order to avoid nitrogen overfertilization, fertilizer rates must be adjusted to meet crop requirements. Two field experiments with sunflower (Helianthus annuus L.) were performed in the western part of the Pampas, Argentina, to: (i) assess nitrogen fertilization effects on seed yield, grain oil content, and plant lodging, (ii) determine N requirement per unit of yield, crop recovery of fertilizer N, and whether these two parameters were affected by N and other nutrient additions. Nitrogen fertilization increased the seed yield only by 17% at one site. Crop nitrogen requirement per unit yield (b-value) increased from 37 to 42 kg Mg − 1 due to nitrogen fertilization only at the site where there was not a yield response. Therefore, if a yield response is expected, it is not necessary to use different b-values for non fertilized or fertilized crop. Reduction of seed oil content due to N addition was relatively small (2–5%), and was overcompensated by the seed yield increase at the responsive site. Recovery of fertilizer 15 N was of 51%. This efficiency of absorption should be considered for making fertilizer recommendations. Application of further nutrients including P and K had no influence on seed yield.

Heavy metals in soils of Argentina: Comparison between urban and agricultural soils

Abstract Trace metals, including heavy metals, can be harmful to the biota and human beings. This leads to study the accumulation of those elements in soils. In the Pampean region (Argentina) this knowledge is scarce. Our objectives were to (i) determine the trace metal concentration in soils of Buenos Aires City and agricultural areas, (ii) start to establish the soil trace metals baseline concentration, and (iii) find relationships between soil properties and those elements Topsoil samples were taken in Buenos Aires City and on farms along an arc 50 to 250 km away from the metropolis. All studied soils were Mollisols. Soil samples were analyzed for their cadmium (Cd), copper (Cu), zinc (Zn), chromium (Cr), cobalt (Co), lead (Pb), and nickel (Ni) contents by acid extraction. Soil properties were determined using standard methods. The soils of Buenos Aires City show the highest average concentrations of Cd, Cu, Pb, and Zn. The further the sampling sites were from Buenos Aires, the lower the metal concentr...

Note on the effects of winter and spring waterlogging on growth, chemical composition and yield of rapeseed

Abstract In regions where climatic conditions are adequate for rapeseed production soils may suffer waterlogging of varying duration. A pot trial was conducted to determine the effects of waterlogging on the growth, nutrient absorption and yield of rapeseed. As the effect of anoxia is known to depend on temperature the study was carried with winter or spring floods of 3, 7 or 14 days duration compared with a control without flooding. Seed yield was affected by 3 or more days of waterlogging. Winter waterlogging decreased the number of seed per plant, due to fewer branches, siliques and seeds per silique. Spring waterlogging, by contrast, reduced individual seed weight and seed oil content. The uptake of N, P, K and Ca decreased significantly with flooding but that of Na increased with spring waterlogging. Yield decline was greater with winter than with spring flooding. Temperature during the flooded period was not the only factor determining the effects of waterlogging: the stage of development, when waterlogging occurred, is also an important factor.

Grazing effects of the bulk density in a Natraquoll of the flooding pampa of Argentina.

The lnfiuence of grazing by cattle on soll bulk density was studied in a typic Natraquoll of the Ploodlng Pampa of Argentina for a period of 33 months, by comparing a grazed sltuatlon to an enclosure deferred from grazing for 7 years. Pioods took piace ln this period as usual. Bulk den&y (BD retention varled from 1.00 to 1.11 Mg m’ d at -33.3 kPa of water ln the ungraaed soil and ln the grued soil from 1.04 to 1.16 Mg mm3. Environmental factors were the primary agent controlling BD; only ln some periods were there sign&ant differences between treatments. Slight increases in BD occurred under graalng after the recession of the flood water, and sign&ant decreases occurred in the ungrazed soil during the large and sudden falls ln water content. In this case the effect of trampling, therefore, would con&t malnly of impeding the decrease in BD. No compaction was observed ln periods when no flood occurred or while soli remahred submerged ln water. The results indicated that the variations of buik density caused by cattle trampilng were superimposed on those produced by floods and showed an interaction between the effects of land-use and the particular environmental condltlons of the region. The Flooding Pampa is a subhumid low, very flat plain. Its 90.000 km* are mostly covered by halo-hydromorphic soils, the most conspicuous being Natraquolls (INTA 1977). The region is affected by recurrent floods caused by an excess of water: percolation, surface movement, and evaporation are less than rainfall. Soils remain saturated and ponded most years from winter to late spring, and exceptionally in autumn; they are dry in some summers. Floods and droughts sometimes follow each other. Because of several constraints, agriculture is limited and the dominant land-use in the region is the production of beef cattle on natural grasslands. Cattle remain over the field all year, including those periods when soil is very wet. Trampling in these conditions is usually considered a severe damage factor in surface structure of grassland soils (Davies 1985, Scholefield and Hall 1986). Trampling by cattle is reported by the literature as causing increases in bulk density by compaction in the soil surface (Heady 1975, Lull 1959, Van Haveren 1983, Warren et al. 1986, Willat and Pullar 1983). Nevertheless some results are in disagreement with this opinion. Laycock and Conrad (1967) found that some increases in bulk density may be ascribed to the lower water content that grazed soils usually have (Gifford and Hawkins 1978); Van Haveren (1983) showed that not ail soils may be compacted by trampling because of the high sand content of some soils; and Lagocki (1978) even found that bulk density decreased when trampling was performed on a soil with high water table. Another subject under controversy is the time required for the recovery of soil structure, as measured by the decrease of bulk density after compaction. For example, Braunack and Waiter (1985) found long periods for recovery, while Warren et al. (1986) showed that shorter terms were required. The factors causing these differences are obviously related to different soil properties, environmental conditions, and grazing intensities. They make it difficult to transfer results from one site to Authors arc wit! the Dcpartamento de Ecolo sidad de Buenos Ames, Avenida San Martin 44 f ia, Facukad de A

Late season nitrogen fertilization of soybeans: effects on leaf senescence, yield and environment

ronomia, UniverManuscript accepted 24 May 1988. 3,1417 Buenos Awes, Argentina. another. In the case of the Flooding Pampa, despite the economic importance of grazing, local literature is very scarce. Rusch and Leon (1983), for instance, attributed changes in the floristic composition of a natural grassland to soil compaction by trampling. Bulk density is a commonly used parameter of soil porosity, giving an effective indication of the compaction-regeneration processes (Bullock et al. 1985). The objective of this study was to evaluate the effects of cattle grazing upon soil bulk density in the Flooding Pampa region of Argentina. The implications for stock management are also considered. Materials and Methods Study Area Research was carried out in an area located in the middle of the Flooding Pampa near the town of Casalins (Buenos Aires Province), in which floods occur most years. In the period under study (from October 1983 to July 1986) floods submerged the soils with a few centimeters of water in winter-spring 1984 and 1985 and in autumn 1986. There were no floods in 1983. Vegetation of the area is a natural grazed grassland community characterized by Rptochaetium montevidensis, Briza subaristata, Eclipta bellidioides, and Menthapulegium (Perelman et al. 1982). It covers more than 7% of soil surface. Grasses are sparsely distributed over the soil and are stratified in the first 10 cm of plant canopy (Sala et al. 1986). The range is mainly devoted to the production of beef cattle. Dominant soil is a typic Natraquoll, General Guido series, moderately saline phases. The soil is characterized by a tough clayey and natric B2t horizon. Conversely, the 0.12 m thick Al horizon is loamy (clay %: 23.60), slightly acid (pH 6.20) with high organic carbon percentage (3.20). This horizon has a high swell-shrink capacity as its clay fraction has 30 to 50% of smectitic materials, causing bulk density to fluctuate with changes in soil water content. Taboada et al. (1988) related soil bulk density to moisture content by the function: BD = 1.20 + 0.0008 GW 0.00016 GW* r = -0.95 (1) where BD is soil bulk density and GW is gravimetric soil water percentage. Other morphological, mineralogical, and chemical properties of this soil have been published elsewhere (Lavado and Taboada 1985, Taboada and Lavado 1986, Taboada et al. 1987). Treatments The bulk density and water content of soil were measured in a grazed location and at a location excluded from grazing for 7 years prior to initiation of the study. The locations were not replicated . The grazed field was a natural grassland which had never been ploughed, but had been grazed year-round for more than a century. During the study period the mean stocking rate was 1.06 cattle ha-’ year-‘. This rate is representative of most ranges of the region. The ungrazed enclosure was a 4-ha field fenced and surrounded by the cattle range; grazing had been excluded from it since 1976. Grazing exclusion resulted in the replacement of a large number of small tussocks by a few large ones. Total biomass, mainly standing dead, increased steadily; litter accumulated. Another major effect was the disappearance of some native planophile species and most of the exotics from the enclosure (Sala et al. JOURNAL OF RANGE MANAGEMENT 41(6), November 1988 1986). Nevertheless the soil only showed significant reductions in salt content, but not in organic matter, total nitrogen, and available phosphorus contents (Lavado and Taboada 1985, 1987). Sampling and Analysis Sampling was carried out in the Al horizon. Five undisturbed soil cores (9 cm in diameter and 10 cm in depth) were taken at random from each treatment on 22 dates during the period under study. Sampling was performed carefully because of the extreme soil water content found along the time in each core. Bulk density was determined by the core method (Blake 1965) and gravimetric soil water content by the oven-dry method. Volumetric soil water content (VW) was calculated from them. The depth affected by animal hooves was evaluated by additional bulk density samples taken at the O-4,4-8, and 8-12 cm depths of the Al horizon using 6 cm diameter and 4 cm depth cores. Five samples were taken at random from each treatment in November 1984 and June 1985. In order to separate the effect of compaction by animal hooves from that caused by decreases in soil water content (Laycock and Conrad 1967), measured values of bulk density were adjusted to a fixed soil water content (Perrier et al. 1959). In this case bulk density was standardized at -33.3 kilo Pascal (kPa) of water retention (29.96% in this horizon), by means of the slopes of equation (1). This procedure was judged to be reliable because of the very high and statistically significant (a10.001) correlation coefficient found for equation (1). Soil surface strength was measured in September, November, and December 1984, using a Proctor penetrometer (Davidson 1965). In each month, 20 measurements were performed following a zig zag path over uncovered soil surfaces in each treatment. Results were statistically appraised by analysis of variances (ANOVA). When significant differences in BD were found between dates, the Tukey test was used to separate the means. Results and Discussion Volumetric water content and bulk density at -33.3 kPa in the Al horizon are shown in Figure 1. Flood periods are also included. Mean VW ranged from 17.06 to 40.79% in the ungrazed soil, and from 19.09 to 42.44% in the grazed soil. Only in the first summer of the studied period (November 1983 to January 1984) was soil dry. The rest of the time it remained with moderate to very high water contents. According to Scholefield and Hall (1986), under these conditions soil surface damage by cattle trampling is likely to occur. Mean BD ranged from 1.00 to 1.11 Mg me3 in the soil of the enclosure where cattle were removed from 1976, and from 1.04 to 1.16 Mg me3 in the soil under grazing. These values were somewhat low, but consistent (De Kimpe et al. 1982) with the high organic carbon content of the Al horizon. Different behavior was observed in BD with time in both treatments. In the ungrazed soil there were lower and significant values on 3 dates (November 1983, January and December 1985). They were reached after the soil underwent great loss of water as its VW showed a sudden fall from a previous very wet condition (it was saturated with water in 1983 and flooded both in 1984 and 1985). The sudden decreases in soil water content seemed to be associated with the parallel decreases in BD. This relationship cannot be explained

Effects of Sewage-Sludge Application on Soils and Sunflower Yield: Quality and Toxic Element Accumulation

Nitrogen demand from soybean seeds during seed filling is very high and has been proposed as the cause of nitrogen remobilization and leaf senescence. Previous research has not shown consistent effects of late season fertilization on seed yield, while its effects on leaf senescence have not been evaluated. Two field experiments were performed to determine the effects of a late season N fertilization on leaf senescence and fall, seed yield and its components, and residual soil nitrate, and to evaluate the potential risk of groundwater contamination. Two rates of nitrogen (50 and 100 kg N ha−1) were applied at the R3 and R5 development stages. Nitrogen fertilization, either at R3 or R5, increased soil nitrate availability during the seed-filling period. Seed yield, seed number and protein content were not affected by N fertilization. The addition of 100 kg N ha−1 produced a small delay of 1–2 days in the leaf fall, and slightly increased seed size (3.6%). Our results suggest that increasing soil N availability during the seed-filling period is not an effective way to delay leaf senescence or to increase seed growth and yield of soybean. Nitrogen fertilization increased the level of residual nitrate in the top soil at one site (the one with lowest seed yield), increasing the risk of nitrate leaching during subsequent fallow.