Surface activity of soil ecosystem engineers and soil structure in contrasted land use systems of Colombia

This study describes the impact of the conversion of native Colombian savannahs into crops and pastures on: (1) the quantity and diversity of the bio-structures produced by soil ecosystem engineers and (2) soil structure. Bio-structure diversity decreased in all agroecosystems (12 types in the savannah, four to six in pastures and three in crops). Bio-structures were mostly earthworm casts in native savannah and pastures, and ant mounds in crops. Compared with the savannah (750 cm³ m^–2), their volume increased in the old pasture (+48%) and decreased in recent pasture and crops (–65% to –97%). Soil structure was similar to savannah soil in the older pasture, but was sharply affected in annual crops. In contrast to crops, pastures appear to sustain soil structure and are also suitable for engineering activity.     

Seasonal differences in the soil water balance under perennial and annual pastures on an acid Sodosol in southeastern Australia

Replacement of native deep‐rooted grasses by shallow‐rooted ones has resulted in greater losses of water and nitrogen by drainage. To counter this effect we have tested the hypothesis that liming, and the conversion of annual grass pastures to perennial grass pastures, could improve the sustainability of grazing systems in the high rainfall zone (> 600 mm per annum) in southeastern Australia, through better use of water and nitrogen. A field experiment consisting of sixteen 0.135 ha (30 m × 45 m) grazed paddocks representing four pasture combinations (annual pasture (mainly Lolium rigidum) without lime (AP–); annual pasture with lime (AP+); perennial pasture (mainly Phalaris aquatica) without lime (PP–), and perennial pasture with lime (PP+)) was carried out from 1994 to 1997 on an acid Sodosol (Aquic Hapludalf) in southern New South Wales, Australia. Measurements were made of surface runoff, subsurface flow (on top of the B horizon) and soil water content. The results showed that perennial grass pastures, especially PP+, extracted approximately 40 mm more soil water each year than the annual grass pastures. As a result, surface runoff, subsurface flow and deep drainage were at least 40 mm less from the perennial pastures. These measurements were further supported by a simulation of soil water deficit and deep drainage for AP– and PP+ paddocks, using 10 years' past meteorological records. Overall, the results suggested that well‐grown, phalaris‐based pastures could reduce recharge to groundwater and make pastoral systems more sustainable in the high rainfall zone.     

Changes in Chemical Properties of Sandy Duplex Soils in 11 Paddocks over 21 Years in the Low Rainfall Cropping Zone of Southwestern Australia

Soil testing was conducted during 1985–2005 in 11 paddocks on sandy duplex soils on Newdegate Research Station, average annual rainfall of 377 mm, with about 70% falling in the May–October growing season, in the Mediterranean-type climate of southwestern Australia. The study was undertaken to determine lime and fertilizer requirements of eight crop species grown in rotation with one another (one crop each year in the typical May–October growing season, comprising wheat, Triticum aestivum L.; barley, Hordeum vulgare L.; oats, Avena sativa L.; lupin, Lupinus angustifolius L.; canola, Brassica napus L.; chickpea, Cicer arietinum L.; field pea, Pisum sativum L.; and subterranean clover-based pasture, Trifolium subterraneum L. All crops were sown using no-till. The study demonstrated that plant testing was required in conjunction with soil testing to confirm decisions based on soil testing and to assess management decisions for elements not covered by soil testing. Pasture dry-matter production seldom exceeds 2 t ha^?1 during the growing season in the region, but clover pasture is valued as a break crop for diseases and pests of grain crops and to facilitate control of herbicide-resistant weeds for cropping. Pastures had negligible impact on soil-test values. By contrast, grain crops typically produce more dry matter than pasture (4–8 t ha^?1) and consistently significantly resulted in soil pH, soil-test potassium (K), and organic carbon (C) of soil decreasing through time. Fertilizer phosphorus (P) was not applied to pasture but was applied while sowing most grain crops from 1985 to 1996, a common practice at the time, and soil-test P significantly increased through time in these years. Thereafter fertilizer P was only applied when soil-test P was less than the critical value for that soil and grain crop species resulting, in little P being applied in these years, and soil-test P significantly declined through time. Plant testing indicated P was adequate when soil testing indicated no fertilizer P was required. The soils only started to become K deficient in the mid-1990s because of the removal of indigenous soil K in grain, and fertilizer K was applied when soil-test K was less than the 50 mg kg^?1 critical value determined for wheat and canola. Plant testing indicated K was adequate when soil testing indicated no fertilizer K was required, and it indicated K was adequate after fertilizer K was applied, showing K levels applied were adequate for grain production. Plant testing indicated nitrogen (N), sulfur (S), calcium (Ca), magnesium (Mg), copper (Cu), zinc (Zn), manganese (Mn), iron (Fe), and boron (B) were adequate for grain production. Electrical conductivity (EC) of soil was very variable but EC values indicated soil salinity was unlikely to reduce grain yields of all the crop species grown. We conclude soil testing for pH is reliable for indicating paddocks requiring lime to ameliorate soil acidity and to monitor progress of liming. Soil testing proved reliable for determining when fertilizer P and K needed to be applied. Research has shown that for the low rainfall cropping areas of southwestern Australia laboratories need to measure and report soil pH, soil-test P, and soil-test K every 1–3 years and the P-buffering index (estimating P sorption of soil), organic C, and electrical conductivity every 3–5 years.     

Nitrate leaching loss under annual and perennial pastures with and without lime on a duplex (texture contrast) soil in humid southeastern Australia

Mineral N accumulates in autumn under pastures in southeastern Australia and is at risk of leaching as nitrate during winter. Nitrate leaching loss and soil mineral N concentrations were measured under pastures grazed by sheep on a duplex (texture contrast) soil in southern New South Wales from 1994 to 1996. Legume (Trifolium subterraneum)‐based pastures contained either annual grass (Lolium rigidum) or perennial grasses (Phalaris aquatica and Dactylis glomerata), and had a control (soil pH 4.1 in 0.01 m CaCl₂) or lime treatment (pH 5.5). One of the four replicates was monitored for surface runoff and subsurface flow (the top of the B horizon), and solution NO₃^– concentrations. The soil contained more mineral N in autumn (64–133 kg N ha^?1 to 120 cm) than in spring (51–96 kg N ha^?1), with NO₃^– comprising 70–77%. No NO₃^– leached in 1994 (475 mm rainfall). In 1995 (697 mm rainfall) and 1996 (666 mm rainfall), the solution at 20 cm depth and subsurface flow contained 20–50 mg N l^?1 as NO₃^– initially but < 1 mg N l^?1 by spring. Nitrate‐N concentrations at 120 cm ranged between 2 and 22 mg N l^?1 during winter. Losses of NO₃^– were small in surface runoff (0–2 kg N ha^?1 year^?1). In 1995, 9–19 kg N ha^?1 was lost in subsurface flow. Deep drainage losses were 3–12 kg N ha^?1 in 1995 and 4–10 kg N ha^?1 in 1996, with the most loss occurring under limed annual pasture. Averaged over 3 years, N losses were 9 and 15 kg N ha^?1 year^?1 under control and limed annual pastures, respectively, and 6 and 8 kg N ha^?1 year^?1 under control and limed perennial pastures. Nitrate losses in the wet year of 1995 were 22, 33, 13 and 19 kg N ha^?1 under the four respective pastures. The increased loss of N caused by liming was of a similar amount to the decreased N loss by maintaining perennial pasture as distinct from an annual pasture.     

Nitrogen dynamics in different types of pasture in the Austrian Alps

 Soil N dynamics were compared in Alpine pastures on two mountains. N-pool sizes and N fluxes were measured relative to N losses via leaching and denitrification in summer. On each mountain, four types of pasture were studied: (1) forest pastures, (2) recently developed pastures formed by forest clearance ("new pastures"), (3) older established pastures, and (4) pastures planted with clover. At both study sites (Scheuchegg and Teufelstein) we obtained similar results. Compared with forest pasture soils, open pasture soils were found to have greater microbial biomass and faster mineralisation potentials, but net field mineralisation rates were slower. In the forest pastures, highest N losses via denitrification were found. Higher potential leaching of NO₃ ^–, estimated by accumulation of NO₃ ^– on ion-exchange resins, in the forest pasture soils suggests lower N uptake by microbes and herbaceous plants compared with open pastures. N₂O-production rates of the forest pasture soils at the Scheuchegg site (11.54 μg N₂O-N m^–2 h^–1) were of similar magnitude to those reported for spruce forests without pastures, but at Teufelstein (53.75 μg N₂O-N m^–2 h^–1) they were higher. However, if forest pastures are not overgrazed, no elevated N loss through N₂O production and leaching of NO₃ ^– is expected. Denitrification rates in the open pastures (0.83–7.50 μg N₂O-N m^–2 h^–1) were low compared with reports on lowland pastures. In soils of the new pastures, rates of microbial N processes were similar to those in the established pastures, indicating a high capacity of soils to restore their internal N cycle after forest clearance. Received: 19 August 1999     

Soil organic matter,microbial properties,and aggregate stability under annual and perennial pastures

The use of annually sown pastures to provide winter forage is common in dairy farming in many regions of the world. Loss of organic matter and soil structural stability due to annual tillage under this management may be contributing to soil degradation. The comparative effects of annual ryegrass pastures (conventionally tilled and resown each year), permanent kikuyu pastures and undisturbed native vegetation on soil organic matter content, microbial size and activity, and aggregate stability were investigated on commercial dairy farms in the Tsitsikamma region of the Eastern Cape, South Africa. In comparison with soils under sparse, native grassy vegetation, those under both annual ryegrass and permanent kikuyu pasture had higher soil organic matter content on the very sandy soils of the eastern end of the region. By contrast, in the higher rainfall, western side, where the native vegetation was coastal forest, there was a loss of organic matter under both types of pasture. Nonetheless, soil organic C, K₂SO₄-extractable C, microbial biomass C, basal respiration, arginine ammonification and fluorescein diacetate hydrolysis rates and aggregate stability were less under annual than permanent pastures at all the sites. These results reflect the degrading effect of annual tillage on soil organic matter and the positive effect of grazed permanent pasture on soil microbial activity and aggregation. Soil organic C, microbial biomass C, K₂SO₄-extractable C, basal respiration and aggregate stability were significantly correlated with each other. The metabolic quotient and percentage of organic C present as microbial biomass C were generally poorly correlated with other measured properties but negatively correlated with one another. It was concluded that annual pasture involving conventional tillage results in a substantial loss of soil organic matter, soil microbial activity and soil physical condition under dairy pastures and that a system that avoids tillage needs to be developed.     

The chemical nature of organic phosphorus that accumulates in fertilized soils of a temperate pasture as determined by solution 31P NMR spectroscopy

Inefficiency of fertilizer phosphorus (P) use in grazing systems is often associated with the accumulation of inorganic and organic P in fertilized soil. However, the chemical nature of the accumulated organic P remains poorly understood. The aim of this study was to use solution ³¹P nuclear magnetic resonance (NMR) spectroscopy on sodium hydroxide–ethylenediaminetetraacetic acid (NaOH‐EDTA) extracts to identify the chemical nature of organic P in soils from a medium‐term (13 years) permanent pasture field experiment. This included an unfertilized pasture (P0), and treatments designed to maintain soil P fertility at near ‘optimum' (P1) and ‘supra‐optimum' (P2) levels for pasture growth; pastures at all levels of soil P fertility were continuously grazed with either a moderate or high stocking rate (SR09 and SR18). Approximately 20% of the fertilizer P added to pastures was recovered as organic P in NaOH‐EDTA extracts at the P1 level of soil P fertility in the 0–10 cm soil layer, and the majority (≈ 65%) of this was detected as the broad phosphomonoester signal. In addition, several specific forms of phosphomonoesters (myo‐ and scyllo‐inositol hexakisphosphate, α‐ and β‐glycerophosphate, and RNA mononucleotides) and phosphodiesters were detected across all soils but at low concentrations. This study shows that phosphate fertilization of pastures primarily results in the accumulation of complex forms of phosphomonoesters rather than that of specific forms of recognizable biomolecules (e.g ., myo‐inositol hexakisphosphate).     

Temporal variation of soil compaction and deterioration of soil quality in pasture areas of Colombian Amazonia

In the Amazon basin, tropical rainforest is being slashed and burned at accelerated rates for annual crops over a couple of years, followed by forage grasses. Because of poor management, the productivity of established pastures declines in a few years so that grazing plots are abandoned and new areas are deforested. Previous studies in the region report higher bulk density in soils under pasture than in similar soils under forest. The objective of this study was to detect changes in the physical quality of the topsoil of nutrient-poor Typic Paleudults in the colonisation area of Guaviare, Colombian Amazonia, and analyse the effect of soil deterioration on pasture performance. Temporal variation of soil compaction under pasture was analysed by comparing natural forest taken as control and pasture plots of Brachiaria decumbens (Stapf) grouped into three age ranges (<3, 3–9, >9 years). Evidence of soil compaction through cattle trampling, after clearing the primary forest, included the formation of an Ap horizon with platy structure and dominant greyish or olive colours, reflecting impaired surface drainage, the increase of bulk density and penetration resistance, and the decrease of porosity and infiltration rate. From primary forest to pastures older than 9 years, bulk density of the 5–10 cm layer increase was 42% in fine-textured soils and 30% in coarse-textured soils. Penetration resistance ranged from 0.45 MPa under forest to 4.25 MPa in old pastures, with maximum values occurring at 3–12 cm depth in pastures older than 9 years. Average total soil porosity was 58–62% under forest and 46–49% under pasture. Basic infiltration dropped from 15 cm h⁻¹ in the original forest conditions to less than 1 cm h⁻¹ in old pastures. Crude protein content and dry matter yield of the forage grass steadily decreased over time. No clear relationship between declining protein content as a function of pasture age and changes in chemical soil properties was found, but there was a high negative correlation (r=−0.81) between protein content and bulk density, reflecting the effect of soil compaction on pasture performance. After about 9–10 years of use, established grass did no longer compete successfully with invading weeds and grazing plots were abandoned. As land is not yet a scarcity in this colonisation area, degraded pastures are seldom rehabilitated.     

Potential loss of nutrients from different rearing strategies for fattening pigs on pasture

Nutrient load and distribution on pasture were investigated with fattening pigs that: (1) spent a proportion of or their entire life on pasture; (2) were fed either restrictively or ad libitum; and (3) were weaned at different times of the year. N and P retention in pigs decreased the longer they were kept on pasture. The contents of soil inorganic N and exchangeable K were significantly raised compared with the soil outside the enclosures but with no differences between treatments. Pig grazing did not affect extractable soil P. Regular moving of huts, feeding and water troughs was effective in ensuring that nutrients were more evenly distributed on the paddocks. Grass cover, as determined by spectral reflectance, was not related to the experimental treatments but only to the time of year. During spring and summer, grass was present in parts of the paddocks, whereas during autumn and winter, the pigs kept grass cover below 10%. Fattening pigs on pasture carry a high risk of nutrient loss and it is concluded that the most environmentally acceptable way of keeping them on pasture involves a combination of reduced dietary N intake, reduced stocking rate and seasonal rather than round the year production.     

Soil organic‐C accumulation and N availability under improved pastures established in Mediterranean oak woodlands

Long‐term effects of improved pasture establishment (with high proportion of legumes) on soil organic‐C status and N availability in Mediterranean cork oak (Quercus suber L.) woodlands were assessed. Soils were sampled beneath scattered crowns and in open areas, considering two systems: unmanaged and managed woodlands where improved pastures were installed 26 and 32 years ago. Total and labile C and N pools were measured and C and N mineralization were determined over 24 weeks laboratory incubation. Soils under improved pastures showed higher organic‐C, total‐N and net N mineralization than those under unmanaged pasture, mainly when established beneath trees. Potentially mineralizable C, C mineralization rate and microbial C were not statistically different between the unmanaged and improved pasture sites, but were higher closer to the tree than in the open area (1.8, 1.2 and 1.2 times, respectively). The qCO₂ was higher in improved pastures (1.7 times). Labile pool of C and N extracted with hot water increased under improved pasture (3.4 and 1.7 times, respectively). Results indicate that soil quality amelioration by improved pastures is stronger in the presence of oak trees. Management systems that favour oak tree maintenance and regeneration should be taken into account to reverse soil degradation.     

Assessing the organic phosphorus status of an Oxisol under tropical pastures following native savanna using 31P NMR spectroscopy

³¹P nuclear magnetic resonance (NMR) spectroscopy, P fractionation, and a P sorption experiment were used to follow the changes in P in the A horizons (0–10 cm) of acid savanna soils, Colombia, after little P fertilization and 15 years' continuous growth of a grass (Brachiaria decumbens) and a grass/legume (B. decumbens+Pueraria phaseoloides) pasture. Ready P supply as analyzed by Bray P was low under native savanna (1.3 mg kg^-1 soil) and responded moderately on pasture establishment. Concurrently, the affinity of the soil for inorganic P declined slightly after pasture establishment. ³¹P NMR spectroscopy revealed that P associated with humic acids was dominated by monoester P followed by diester P. Smaller proportions were observed for phosphonates, teichoic acid P, orthophosphate, and pyrophosphate. P associated with fulvic acids had lower proportions of diester P and higher contents of orthophosphate. Under native savanna the reserves of labile organic P species (phosphonates and diester P including teichoic acid P) associated with humic and fulvic acids were 12.4 and 1.1 kg ha^-1, respectively, and increased to 18.1 and 1.8 kg ha^-1 under grass pasture, and to 19.5 and 2.3 kg ha^-1 under grass/legume pasture. These data emphasize the importance of labile organic P species in the P supply for plants in improved tropical pastures, and further indicate that humic acid P in particular responds to land-use changes within a relatively short time-scale. Earthworm casts were highly abundant in the B. decumbens+P. phaseoloides plot and were enriched in labile organic P species. We conclude that earthworm activity improves the P supply in soil under tropical pastures by creating an easily available organic P pool.Dedicated to Professor J.C.G. Ottow on the occasion of his 60th birthday     

Soil carbon increases with long-term cattle stocking in northern temperate grasslands

Grassland management aimed at enhancing carbon (C) in soil is an important tool in mitigation of rising atmospheric CO₂, yet little is known of how grassland soil C changes with livestock stocking rate (SR). We relate soil organic and inorganic C mass (t ha⁻¹ to 60 cm depth) with cattle stocking over periods of 7–27 year for 32 paddocks distributed across nine community pastures in the mixed-grass prairie of Saskatchewan, Canada. Initial analysis comparing Akaike information criterion models showed that cattle SR explained a greater proportion of variance in soil C, particularly soil organic C, than rainfall. Soil organic C mass increased with cattle SR (R² = .293; p = .001), even when the latter was normalized to account for differences in vegetation composition and growing conditions among pastures. Normalized SR varied from 0.49 to 2.30 times recommended levels, over which SOC increased from 24.7 to 57.4 t ha⁻¹. Increases in soil organic C under greater stocking coincided with increased abundance of introduced vegetation, particularly the rhizomatous grass Poa pratensis. Inorganic soil C accounted for 34.6% of total soil C, being particularly large below 30 cm soil depth, but did not vary with stocking rate. These findings indicate that both organic and inorganic C are important pools of C in northern temperate grassland soils, with soil organic C positively associated with long-term cattle SR. Further studies are recommended to understand more fully the mechanisms regulating grazing impacts on soil C mass in northern temperate grasslands.     

Occurrence of barley yellow dwarf virus and cereal yellow dwarf virus in pasture grasses and spring cereals in Latvia

Abstract

The relative frequency of three viruses causing barley yellow dwarf disease (BYD) was assessed in spring cereals and pasture grasses at two regions in Latvia in 2000–2002. A total of 2589 leaf samples (367 from spring oats, 743 from spring barley, 1479 from predominant grass species) were collected from 44 fields of spring oats, 84 fields of spring barley, and 26 pastures. We found that isolates of barley yellow dwarf virus-PAV (BYDV-PAV), barley yellow dwarf virus-MAV (BYDV-MAV) and cereal yellow dwarf virus-RPV (CYDV-RPV) were present in these samples of spring cereals and pasture grasses. The most common isolates of barley yellow dwarf virus were BYDV-PAV and BYDV-MAV in both grasses and cereals, but there was a great difference between years and regions. The proportion of BYD symptomatic cereal samples that reacted positively in TAS-ELISA test was 9 to 15%. The overall BYDV/CYDV incidence in pasture grasses ranged from 2 to 19%. The incidence of BYDV/CYDV infection was higher in Festuca elatior than in other grass species. Isolates of CYDV-RPV were rather rare: only found in Lolium perenne and Dactylis glomerata among six grass species tested and more frequently in barley than oats. This paper reports the first quantitative survey of selected BYD-causing viruses in spring cereals and pasture grasses in Latvia and in the Baltic states. We conclude that three selected virus species are prevalent in spring cereals and pasture grasses in Latvia, although with great variation between years. Further studies are needed to obtain knowledge of the most critical factors that determine these fluctuations.     

Effects of Multiple Reference Plants,Season, and Irrigation on Biological Nitrogen Fixation by Pasture Legumes using the Isotope Dilution Method

Abstract

The popular and widely used ¹⁵nitrogen (N)–isotope dilution method for estimating biological N fixation (BNF) of pasture and tree legumes relies largely on the ability to overcome the principal source of error due to the problem of selecting appropriate reference plants. A field experiment was conducted to evaluate the suitability of 12 non‐N₂‐fixing plants (i.e., nonlegumes) as reference plants for estimating the BNF of three pasture legumes (white clover, Trifolium repens L.; lucerne, Medicago sativa; and red clover, Trifolium pratense L.) in standard ryegrass–white clover (RWC) and multispecies pastures (MSP) under dry‐land and irrigation systems, over four seasons in Canterbury, New Zealand. The ¹⁵N‐isotope dilution method involving field ¹⁵N‐microplots was used to estimate BNF. Non‐N₂‐fixing plants were used either singly or in combination as reference plants to estimate the BNF of the three legumes. Results obtained showed that, on the whole, ¹⁵N‐enrichment values of legumes and nonlegumes varied significantly according to plant species, season, and irrigation. Grasses and herb species showed higher ¹⁵N‐enrichment than those of legumes. Highest ¹⁵N‐enrichment values of all plants occurred during late summer under dry‐land and irrigation conditions. Based on single or combined non‐N₂‐fixing plants as reference plants, the proportion of N derived from the atmosphere (% Ndfa) values were high (50 to 90%) and differed between most reference plants in the MSP pastures, especially chicory (Cichorium intybus), probably because it is different in phenology, rooting depth, and N‐uptake patterns compared to those of legumes. The percent Ndfa values of all plants studied also varied according to plant species, season, and irrigation in the MSP pastures. Estimated daily amounts of BNF varied according to pasture type, time of plant harvest, and irrigation, similar to those shown by percent Ndfa results as expected. Irrigation increased daily BNF more than 10‐fold, probably due to increased dry‐matter yield of pasture under irrigation compared to dry‐land conditions. Seasonal and irrigation effects were more important in affecting estimates of legume BNF than those due to the appropriate matching of N₂‐fixing and non‐N₂‐fixing reference plants.     

Pasture type and fertilization effects on N2 fixation, N budgets and external energy inputs in western Canada

A grazing experiment was conducted in Brandon, Manitoba, Canada. The objectives were to examine the effects of including alfalfa and fertilizer management on N₂ fixation by alfalfa and plant N dynamics, and to compare N budgets in the four contrasting pasture systems and external energy inputs between fertilizer-N-based and legume-based pasture systems. Estimates of annual amounts of N₂ fixed, based on shoot herbage production in grazed mixed alfalfa/grass pastures, ranged from 40 to 118 kg N ha⁻¹ y⁻¹. The amounts would be in the range of 52-153 kg N ha⁻¹ y⁻¹, if the amounts of fixed N stored in the roots, were included. Compared to grass-only pastures, total amounts of N₂ fixed in the mixed pastures should be sufficient to improve total external N inputs, replace N fertilizer and sustain plant protein for grazing. The reliance of alfalfa (Medicago sativa L.) on N₂ fixation for growth was high (70-95%), and %N derived from the atmosphere by alfalfa (%Ndfa) was not affected by P fertilizer management. Thus, the amounts of N₂ fixed were predominantly regulated by alfalfa dry matter productivity. The data also indicated that alfalfa fixed 27 kg N t⁻¹ dry matter produced. In mixed alfalfa/grass pastures, high soil mineral N uptake by companion grasses, was essential to effectively utilize N that was fixed by alfalfa and returned to soils through the decomposition of alfalfa litter and roots. Compared to grass-only pastures with or without N fertilizer, alfalfa-based pastures could supply sufficient plant protein for grazing animals through N₂ fixation, and at same time, sustain animal productivity with only 28% of the external energy input of the grass-only pasture with N fertilizer.     

Earthworm communities in native savannas and man-made pastures of the Eastern Plains of Colombia

 The structure and seasonal changes of earthworm communities were evaluated in a natural savanna and in a improved grass-legume pasture in a Colombian oxisol over a period of 18 months. One plot of 90×90 m was isolated in each of the systems and each month five samples of 1 m²×0.5 m and ten of 20×20×20 cm were randomly selected from a stratified block design. Species richness was similar in the two evaluated plots (seven species), whereas diversity measured by the index, H (Shannon and Weaver 1949) was clearly different, i.e. H=2.89 in natural savanna and H=1.29 in pasture. This is explained by differences in earthworm community structure. The average annual density in the savanna was 49.8, ranging from 10.8 to 135.8 individuals (ind) m^–2, and biomass was 3.3 g m^–2 (hand-sorting method), ranging from 0.9 to 11.5 g m^–2. In the man-made pasture, density was 80.1 ind m^–2 on average, ranging from 24 to 215.8 ind m^–2 and biomass was more than tenfold higher, ranging from 29.2 to 110.4 g m^–2. This was especially due to the presence of a large glossoscolecid anecic species, Martiodrilus carimaguensis Jiménez and Moreno, which has been greatly favoured by conversion of savanna to pasture. Endogeic species were dominant in the natural savanna whereas the anecic species accounted for 88% of total earthworm biomass in the pasture. Total earthworm density and biomass were significantly different in the two systems studied (t-test). The results indicate a clearly positive response of earthworm communities to improved pastures, a type of land use that is being increasingly adopted in moist neotropical savannas. Received: 20 October 1997     

Distribution and Sorption of Phosphate in a Savanna Soil Under Improved Pastures in Northern Nigeria

Abstract

Land use patterns affect soil nutrient transformation and availability. The study determined the distribution of phosphorus (P) fractions and sorption in five pasture fields composed of Andropogon gayanus, Brachiaria decumbens, Chloris gayana, Digitaria smutsii, and Stylosanthes guianensis. The objectives were to characterize P fractions in improved pastures and to determine the effect of forage species on soil P lability. Total P (P_t) across the pastures was not significantly different. Organic P (P_o) accounted, on the average, for 64% of P_t. Resin‐P, considered the plant‐available P, ranged from 4 to 10 mg kg^?1, suggesting acute P deficiency in the pastures. The sum of P fractions extracted by 0.5 M NaHCO₃, 0.1 M NaOH, and 1.0 M HCl, together with the resin‐P, accounted for less than 35% of P_t. Factor analysis indicated that plant‐available P approximated by resin‐P was furnished by ?HCO₃‐P_o mineralization and HCl‐P. The highest concentrations of ?HCO₃‐P_o and ?OH‐P_o were maintained by Brachiaria decumbens. Grouping P_i and P_o fractions into labile and nonlabile fraction showed that Brachiaria decumbens maintained the greatest concentration of labile P as a proportion of its P_t. The pasture soils sorbed between 31 and 65% of added P from a standard concentration of 50 mmol kg^?1. Phosphorus sorbed by soils from the pasture fields was in the order: Digitaria smutsii=Stylosanthes guianensis>Brachiaria decumbens=Chloris gayana>Andropogon gayanus, whereas resin recovery of sorbed P was greater in Brachiaria decumbens than other pastures. Between 82 and 92% of sorbed P was bound irreversibly. It was concluded that the relatively high concentration of labile P maintained by soil under Brachiaria decumbens was probably related to its capacity to sequester more carbon than the other pastures.     

Inoculation responses of perennial forage legumes grown in fresh hill‐land ultisols as affected by soil acidity‐related factors

A growth chamber experiment was initiated with two field moist, marginal and acidic (pH 5.1–5.2) soils of the Lily series (Typic Hapludults) in order to determine the need for improved legume‐rhizobia symbioses for forage species of current, or potential, use in the renovation of Appalachian hill‐land pastures. One soil was from an abandoned pasture having broomsedge (Andropogon virginicus L.) as the predominant vegetation, whereas the other was from a minimally‐managed pasture dominated by orchardgrass (Dactylis glomerata L.). Treatments included inoculation (or no inoculation) and the addition of aluminum, nil, or lime to provide a range of soil acidities. Both soils contained effective populations of naturalized rhizobia for white clover (Trifolium repens L.) and red clover (Trifolium pratense L.), but low and/or ineffective naturalized populations of rhizobia for alfalfa (Medicago sativa L.), birdsfoot trefoil (Lotus corniculatus L.), bigflower vetch (Vicia grandiflora Scop.), and flatpea (Lathyrus sylvestris L.). Seed inoculation, by lime‐pelleting, was highly beneficial in establishing effective symbioses for all these latter species. The addition of low levels of aluminum or lime (1.5 and 2.0 cmol/kg soil, respectively) had little effect on any of the symbioses, with the exception of those for alfalfa. Thus, an improved legume rhizobia symbiosis would not seem to be a prerequisite for renovating pastures established on chemically similar ultisols with the forage legume species examined in this study, especially if the pasture has at least some history of management.     

Soil respiration rate and its sensitivity to temperature in pasture systems of dry-tropics

Abstract

Tree clearing is a topical issue the world over. In Queensland, the high rates of clearing in the past were mainly to increase pasture production. The present research evaluates the impact of clearing on some soil biological properties, i.e. total soil respiration, root respiration, microbial respiration, and microbial biomass (C and N), and the response of soil respiration to change in temperature.

In-field and laboratory (polyhouse) experiments were undertaken. For in-field studies, paired cleared and uncleared pasture plots were selected to represent three major tree communities of the region, i.e. Eucalyptus populnea, E. melanophloia, and Acacia harpophylla. The cleared sites were chosen to represent three different time-since-clearing durations (5, 11–13, and 33 years; n=18 for cleared and uncleared plots) to determine the temporal impact of clearing on soil biological properties. Experiments were conducted in the polyhouse to study in detail the response of soil respiration to changes in soil temperature and soil moisture, and to complement in-field studies for estimating root respiration.

The average rate of CO₂ emission was 964 g CO₂/m²/yr, with no significant difference (P<0.05) among cleared and uncleared sites. Microbial respiration and microbial biomass were greater at uncleared compared with those at cleared sites. The Q ₁₀-value of 1.42 (measured for different seasons in a year) for in-field measurements suggested a small response of soil respiration to soil temperature, possibly due to the limited availability of soil moisture and/or organic matter. However, results from the polyhouse experiment suggested greater sensitivity of root respiration to temperature change than for total soil respiration. Since root biomass (herbaceous roots) was greater at the cleared than at uncleared sites, and root respiration increased with an increase in temperature, we speculate that with rising ambient temperature and consequently soil temperature, total soil respiration in cleared pastures will increase at a faster rate than that in uncleared pastures.     

Partitioning and translocation of photosynthetically fixed 14C in grazed hill pastures

Information on carbon (C) flows and transformations in the rhizosphere is vital for understanding soil organic matter dynamics and modelling its turnover. We followed the translocation of photosynthetically fixed C in three hill pastures that varied in their phosphorus (P) fertility, using a ¹⁴C-CO₂ pulse-labelling chamber technique. Pasture shoot, root and soil samples were taken after 4h, 7 days and 35 days chase periods to examine the fluxes of ¹⁴C in the pasture plant-root-soil system. Shoot growth over 35 days amounted to 114, 179 and 182gm^–2 at the low (LF), medium (MF) and high (HF) fertility pasture sites, respectively. The standing root biomass extracted from the soil did not differ significantly between sampling periods at any one level of fertility, but was significantly different across the three levels of fertility (1367, 1763 and 2406gm^–2 at the LF, MF and HF pastures, respectively). The above- and below-ground partitioning of ¹⁴C was found to vary with the length of the chase period and fertility. Although most ¹⁴C (74%, 65% and 57% in the LF, MF and HF pastures, respectively) was in the shoot biomass after 4h, significant translocation to roots (23–39%) was also detected. By day 35, about 10% more ¹⁴C was partitioned below-ground in the LF pasture compared with the HF pasture. This is consistent with the hypothesis that, at limiting fertility, pasture plants allocate proportionally more resource below-ground for the acquisition of nutrients. In the LF site, with an annual assimilated C of 7064kgha^–1, 2600kg was respired, 1861kg remained above-ground in the shoot and 2451kg was translocated to roots. In the HF pasture, of the 17313kgha^–1 C assimilated, 7168kg was respired, 5298 remained in the shoot and 4432kg was translocated to the roots. This study provides, for the first time, data on the fluxes and quantities of C partitioned in a grazed pasture. Such data are critical for modelling C turnover and for constructing C budgets for grazed pasture ecosystems. Received: 31 July 1996