Fallow Sites Research Articles

Long-term effects of short-fallow on soil phosphorus pools in Costa Rica

Many Costa Rican farmers have shortened the fallow period associated with frijol tapado, a slash/mulch bean ( Phaseolus vulgaris L.) system, from 3 or more years to only 9 months. This study determined the long-term effects of continual short-fallow (9 months) frijol tapado on soil phosphorus (P) fractions and assessed the importance of these fractions for crop nutrition. It also examined the effect fallow length had on biomass production and mulch P, when fallow included established, dispersed trees. The study was conducted on three bean sites that had been cropped using short-fallow frijol tapado for more than four decades and on a virgin forest site. Prior to seeding in 1999, one bean site was fallowed for 9 months, another for 21, and the third for 33. There were established, dispersed trees within each. Samples of surface soil (four divisions to 30 cm), mulch cover, and living vegetation were collected before slashing, after seeding, and at harvest. Plant material and soil samples were analyzed for total P; soils were subjected to sequential P extractions. Results demonstrated that, compared to the virgin forest soil, soils which had been cropped with short-fallow frijol tapado for decades were depleted in top- and subsoil stable organic P (Po) and labile inorganic P. The bean soils had not accumulated P in other fractions, indicating that substantial fractional redistribution had not occurred. Of the soil P fractions, NaOH Po made the most important contribution to plant nutrition during the cropping season in the 1- and 2-year fallow bean sites. The 1- and 2-year fallows supplied similar quantities of mulch and mulch P. The dispersed trees probably played a role in this, and lower P concentrations in the older fallows’ mulches also helped equalize total mulch P values. Additionally, mulch variability, before and after slashing, in the 2- and 3-year fallow sites obscured what differences there may have been. The exceptionally steep slope of the 3-year fallow site (92%) overwhelmed other effects and prohibited meaningful comparisons with other sites.

Adaptative above-ground biomass, stand density and leaf water potential to droughts and clearing in Guiera senegalensis, a dominant shrub in Sahelian fallows (Niger)

Guiera senegalensis tolerates repeated cutting and pruning to which it is increasingly subjected at the onset of each cropping period, and one to three times during the annual cropping period (June–September). It responds by profusely branching at the shrub base. The topographic and geomorphic influences, and the effect of clearing on the relationships between shrub density, mean individual above-ground biomass and leaf water status were analysed for seven fallow sites. They were sampled 75 km east of Niamey in Niger. Surprisingly, shrub density and mean individual above-ground biomass were highest in the sites that had been recently in fallow and intensively cultivated prior to crop abandonment. Stand above-ground biomass was also positively related to mean predawn and daily minimum leaf water potentials. Consequently, in the studied G. senegalensis stands, above-ground biomass appeared to be controlled by water availability – rainfall, runoff, infiltration – rather than by cropping intensity. Anisohydric stomatal regulation, resulting in large safety margins from critical transpiration, was inferred from the diurnal amplitude of leaf water potential during the dry season. The plant physiological resistance to water stress combined with its population adjustment in density and growth contributes to the sustainable dominance of G. senegalensis.

Relationships between plant-parasitic nematode community, fallow duration and soil factors in the Sudano-Sahelian area of Senegal

Before studying the mechanisms by which changes in vegetation influence the nematode community following abandonment of a field, it is necessary to first assess the impact of environmental factors such as soil or climate on these organisms. The study was undertaken on sites of increasing fallow duration located in the same area in the Sudano-Sahelian zone of Senegal: a forest and 10 fallow sites ranging from 1 to 18 years old. Soil samples were collected over 3 years on 17 occasions, along 21 m fixed transects, located in representative vegetation zones. Plant-parasitic nematodes were extracted, identified and enumerated from soil samples. The statistical analysis showed that the sites could be split in three groups according to the plant-parasitic nematode communities. One group, corresponding to young fallows, was characterised by large populations of Scutellonema cavenessi and Tylenchorhynchus gladiolatus. A second group included most of the older fallows and was characterised by a more diversified nematode community dominated by Helicotylenchus dihystera. The forest hosted a particular community partly similar to both of the other groups. Soil physical and chemical analysis split the sites into two groups, the young fallow sites plus the forest, and the older sites. The study of the annual changes revealed contradictory tendencies, such as a very small increase in T. gladiolatus and in the soil clay content, which suggest that the sampling technique had probably slightly influenced the results. If this did occur it did not change the effect of fallow duration on the nematode community. The process of decline of certain nematode populations seemed to be extremely slow and not detectable over three successive years. Nematode species were apparently able to reproduce for a long time on poor host plants, even if one plant cohort disappeared abruptly, as was the case when the field was not replanted. This study demonstrated that the soil had a greater influence on the nematode community structure than the annual climatic variations. Consequently, the soil effect will have to be mathematically

Conservation tillage practices for winter wheat–fallow farming in the temperate continental climate of northwestern Iran

Soil water is the most limiting resource for crop growth in dryland areas. Conservation tillage has been proposed as a promising strategy to improve soil and water conservation in these areas. A 3-year field experiment to determine the feasibility of conservation tillage for wheat ( Triticum aestivum L.) production was initiated in 1999 on a clay loam (Vertic Calcixerepts) soil in the temperate continental climate of northwestern Iran, with an average annual precipitation of 375 mm. The tests were conducted using conventional (moldboard plow), reduced (chisel plow), minimum (sweep plow) and direct drilling (in standing stubble/total residue) tillage systems. In the first year of the experiment (1999–2000), treatments were applied as pre-seeding tillage on a fall chisel tilled fallowed site, whereas in the second and third years, the tillage treatments were applied during fallow with direct drilling of the following wheat. Wheat grain yields were significantly affected by tillage. The same yield trend was observed for the tillage treatments in all years. Yields under reduced tillage were consistently higher (35%) than yields from conventional tillage. Grain yields under direct drilling were similar to those obtained using the reduced-tillage system and superior to yields obtained by conventional tillage system. The minimum till system applied as pre-seeding or fallow tillage reduced crop yields compared to reduced tillage. The overall results showed that tillage intensity could be reduced to the level of reduced tillage or direct drilling with an increase in crop yields.

Inter-annual variation of species composition of fallow vegetation in semi-arid Niger

To monitor inter-annual changes of species composition, ten permanent plots were investigated on six fallow sites on different soil types in Niger, West Africa, from 1994 to 1998. Based on the behaviour of species occurrence, four groups of plant species were distinguished for each site: (i) a group of species that occurred in one of the years but was absent in others, (ii) a group of species that occurred in all years but changed enormously in abundance from year to year, (iii) a group of species of constant occurrence over the years and (iv) a group of species that could not be attributed to any of the other groups because they either occurred in too small numbers or showed a contradictory behaviour between the different sites. In total, 43 species, most of them restricted to one or two sites, showed inter-annual fluctuations in occurrence. This constitutes a considerable problem for vegetation studies in semi-arid environments, because for phyto-sociological approaches as well as for the analysis of indicator species, species that are abundant in some years but absent in others cannot be used as character species or differential species for the distinction of plant communities. For this purpose we identified five groups of species, that, in relation to soil type and grazing intensity, can be used as indicator, character or differential species groups for vegetation studies in the Sahel.

Contribution of dissolved organic nitrogen to N leaching from four German agricultural soils

AbstractDissolved organic nitrogen (DON) substantially contributes to N leaching from forest ecosystems. However, little is known about the role of DON for N leaching from agricultural soils. Therefore, the aim of our study was to quantify the contribution of DON to total N leaching from four agricultural soils. Concentrations and fluxes of DON and mineral N were monitored at two cropped sites (Plaggic Anthrosols) and two fallow plots (Plaggic Anthrosol and Gleyic Podzol) from November 1999 till May 2001 by means of glass suction plates. The experimental sites were located near the city of Münster, NW Germany. Median DON concentrations in 90 cm depth were 2.3 mg l—1 and 2.0 mg l—1 at the cropped sites and 1.6 mg l—1 and 1.3 mg l—1 at the fallow sites. There was only a slight (Anthrosols) or no (Gleyic Podzol) decrease in median DON concentrations with increasing depth. Total N seepage was between 19 kg N ha—1 yr—1 and 46 kg N ha—1 yr—1 at the fallow sites and 16—159 kg N ha—1 yr—1 at the cropped sites. For the fallow plots, DON seepage contributed 10—21 % to the total N flux (4—5 kg DON ha—1 yr—1), at the cropped sites DON seepage was 6—21 % of the total N flux (6—10 kg DON ha—1 yr—1). Thus, even in highly fertilized agricultural soils, DON is a considerable N carrier in seepage that should be considered in detailed soil N budgets.

A multi-scale approach of runoff generation in a Sahelian gully catchment: a case study in Niger

Runoff production conditions in a small gully catchment are studied at four different scales: the point scale (0.001 m 2), the local scale (1 m 2), the field scale (of the order of 100 m 2) and the catchment scale (0.2 km 2). At the point scale, infiltration measurements were conducted using a tension infiltrometer. At the local and the field scale, runoff plots were setup on typical soil surface conditions of the catchment (plateau bare soil, hillslope bare soil and fallow grassland). At the catchment scale, stream discharges were measured at two gauging stations. The overland flow yield is significantly nonuniform in space, due to the high spatial variability of infiltration capacities and the depressional storage of the soil surface. The runoff and the infiltration data collected confirmed the major role played by soil crusting on runoff generation in that part of Sahel. At the point scale, hydraulic conductivity measurements have shown that infiltration and runoff were driven by the hydraulic properties of the crust. At the field scale, microtopography and heterogeneity in the soil surface crusting decreased discharge volumes. The influence of vegetation growth on runoff yield was evident in the case of the fallow sites. Analysis of discharge data at the catchment scale highlights that infiltration through the bottom of the gully between two gauging stations leads to considerable runoff water transmission losses.

Potassium Studies on Some Sugarcane Growing Soils in Fiji

This study investigated the potassium (K) status of sugarcane growing soils in Fiji, using samples taken from fields under cultivation and also from sites that had not been cultivated for at least 30 years. Five sites were on highly weathered oxyhydroxide soils where the total K contents and K retention capacities were generally low. The two less weathered soils containing significant amounts of 2:1 expanding clay minerals had much higher total K and non-exchangeable K. Exchangeable K and soil solution K contents varied significantly, but were generally low for most soils. Exchangeable K contents were lower in the regularly cultivated fields at five sites out of the eight, were higher at 2 sites, while for one site there was no change. These variations may be due to different levels of K input, uptake by crop and losses due to leaching and erosion. The two less weathered soils (Sigatoka and Nawaicoba) had lower contents of both non-exchangeable and exchangeable K in the cultivated soil when compared with the fallow sites suggesting net loss of K due to cultivation. These soils, however, also had high surface negative charge and thus a better capacity to retain K. Since the K requirement of sugarcane is characteristically high and large amounts of K are removed in the harvested crop, K fertilizer practices need to be scrutinized carefully in Fiji.

Energy and Water Balance at Soil-Air Interface in a Sahelian Region

The aim of this work is an improvement of the parameterization of the soil moisture in the scheme of the Land Surface Process Model (LSPM) for applications over desert areas. In fact, in very dry conditions, the water vapour flux plays an important role in the evaporation processes and influences the underground profiles of humidity and temperature. The improved version of soil moisture parameterization in the LSPM scheme has been checked by using the data taken from the database of the field experiment HAPEX-Sahel (Hydrology-Atmosphere Pilot Experiment in the Sahel. 1990-1992). Model simulations refer to three different stations located in Niger (Fallow, Millet and Tiger sites) where input data for LSPM and observations were simultaneously available. The results of simulations, taking into account the water vapour flux in the soil model LSPM, seem to compare better with the observed behaviour of soil moisture and turbulent heat fluxes than those overlooking the water vapour flux, confirming the great importance of the water vapour in such dry conditions.

INFLUENCE OF DIFFERENT FALLOW MANAGEMENT SYSTEMS ON STABILITY OF SOIL AGGREGATES IN SOUTHERN NIGERIA1†

The effects of different fallow management systems on aggregate stability were studied on an Ultisol and an Alfisol in southern Nigeria. Aggregate stability was measured in natural regrowth, and planted fallows of Pueraria phaseoloides Benth. and Leucaena leucocephala Lam de Wit in a trial established in 1989 on an Alfisol at Ibadan (7°30′N, 3°54′E), southwestern Nigeria. Soil samples (0–30 cm depth at this instance) were wetted by immersion for 2 and 10 minutes before wet-sieving at 30 rpm for 5 to 35 minutes at 5 minute increments. Mean-weight diameter (MWD), geometric mean diameter (GMD) and proportions of water-stable aggregates (WSA) were calculated. GMD and WSA were not affected by the length of pre-wetting or by the length of the wet-sieving period. Thus, wet-sieving for more than 5 minutes at 30 rpm was not necessary for these sandy soils. The trends observed for soil aggregate stability differed between the Alfisol and Ultisol, and for the Alfisol, it differed between a degraded and a managed fallow site. Thus, soil aggregate stability was influenced by soil type and soil management. The GMDs for the natural regrowth at 0–10 cm soil depth (0–15 cm soil depth sampling at 5 cm increment in this case) were between 1.12 and 1.42 mm, 1.14 and 1.46 mm for the Pueraria system, and 1.12 and 1.33 mm for the Leucaena system. An adjacent forest soil (0–10 cm) had GMDs between 1.24 and 1.54 mm. On a continuously cropped Alfisol, aggregate stability was significantly higher in the Pueraria live mulch system than in Leucaena alley cropping and natural regrowth. Fallowing for 2 or 3 years after 1 year cropping was essential under any of the systems to keep aggregate stability within the range of the forest soil. The fallow management practices enhanced surface soil aggregate stability. †This is IITA Manuscript Number IITA/00/JA/35.

Variation in Post-dispersal Seed Predation in Mature Peruvian Lowland Tropical Forest and Fallow Agricultural Sites1

Variation in post-dispersal seed predation between mature forest and fallow swidden agriculture sites (young forest) was examined for 26 tree and liana species in Peruvian lowland tropical forest. Exclosures were also used to determine the relative importance of vertebrate and invertebrate seed predators. Although species differed significantly in levels of predation, predation of seeds exposed to vertebrates was high (>75%) for all species, while seed predation by invertebrates was lower and more variable. Seed and seedling predation by insects was significantly higher in mature forest, while seed predation by rodents was higher in fallow areas. Other sources of mortality (primarily desiccation) were higher for seeds in fallow areas. The percentage of seeds moved was significantly different among species but not between forests.

Alternative seeding dates (fall and April) affect Brassica napus canola yield and quality

Brassica napus L. canola production on the Canadian prairies often is limited by hot, dry growing conditions in early July and a short growing season. Brassica napus canola seeded in the fall just prior to freeze-up or in the early spring as soon as fields are passable may allow canola to avoid these adverse conditions. Our objective was to determine if late October (fall), or mid- to late April (April) seeding dates improve canola yield and quality relative to a mid-May (15 to 20 May) seeding date. Plant density and height, phenological development, seed yield, seed weight and seed oil content were assessed in plots sown to herbicide-tolerant B. napus canola at three seeding dates on five fallow sites and three stubble sites at Scott, SK, from 1994 to 1998. A thinner plant stand occurred for the fall compared with spring seeding dates; however, this difference rarely corresponded with less canola yield. Fifty percent flowering occurred 20 d earlier (June rather than July), reproductive growth (50% flowering to maturity) was 10 d longer, plants were 23 (fall) or 8 (April) cm shorter, and maturity occurred 13 d earlier when canola was seeded in the fall and April compared with mid-May seeding. Canola seed yield was 38% greater when seeded on the alternative dates rather than the more traditional mid-May seeding date. The yield advantage for alternative seeding dates was greater and more consistent on stubble than on fallow likely because of lack of soil crusting and temperature and wind protection from stubble. The response of seed weight to seeding date was similar to that for seed yield, indicating that a portion of the positive yield response to alternative seeding dates was associated with larger seed size. Oil content also was greater for the fall and April compared with mid-May seeding dates, but the improvement was smaller (6%) than that for seed yield. Fall- and April-seed-ed canola tolerated spring frosts and avoided adversely hot, dry weather during the flowering period, thus improving canola seed yield and quality. Alternative seeding dates provide canola producers in semi-arid regions with a sustainable option to diversify their cropping systems. Key words: Seeding date, dormant, stubble, fallow, herbicide tolerant, alternative cropping practice

Nitrous oxide emissions from frozen soils under agricultural, fallow and forest land

In a field study, N 2O emissions were measured in an agricultural, a fallow, and a forest system once a week from December 1995 to November 1996. Elevated N 2O emissions were detected during periods of both soil freezing and soil thawing. The dynamics of the N 2O winter emissions were influenced by the changes in soil temperatures. The highest emission rates were observed during soil thawing. The N 2O emissions during the entire winter period (December 1995 to March 1996) amounted to 2.8, 1.3, and 0.7 kg N 2O–N for the agricultural land, fallow and forest, respectively, and contributed to 58, 45 and 50% of the annual N 2O emissions from these systems. Differences in the winter emissions among the three sites could not be explained by means of nitrate concentration but rather by water-filled pore space (WFPS). Additionally, the upper organic layers of the forest and the grass vegetation of the fallow site delayed the time of soil freezing and reduced the depth of frost penetration. Both WFPS and vegetation control the N 2O emissions in winter.

Investigating soil and atmospheric plant water stress using physiological and micrometeorological data

The effect of drought on the parameters of commonly used models of plant stomatal conductance is investigated, based on a large number of data obtained at the leaf level by plant physiologists, and at the canopy level by micrometeorologists. Sixty-three case studies are analysed in order to understand the intra- and inter-specific variations of the conductance and photosynthesis parameters in both stressed and unstressed conditions. It appears that the intraspecific variability is as large as the interspecific variability, and that the stomatal sensitivity to air humidity may depend on soil water content. Soil resistance to rooting seems to be an important factor of the variability. Field studies are consistent with leaf-scale ones. A simple representation of these effects is implemented into an interactive vegetation model and then applied to simulate three annual vegetation cycles on a fallow site.

Influence of shrubs on soil characteristics and their function in Sahelian agro-ecosystems in semi-arid Niger

Soil beneath shrubs form ‘fertile islands’ in fallow sites and millet fields in semi-arid Niger. To gain more information about this phenomena different shrub species in fallow sites following a gradient from 350–650 mm precipitation were examined. For each shrub two different areas were distinguished: an area under the canopy of the shrubs and an area in the nearby open land. Soil samples were taken from a depth of 0–10 cm and analysed for Corg, Ntotal, PBray, pH(H2O), exchangeable cations, effective cation exchange capacity (ECEC) and soil texture. Significantly higher concentrations between 38–51% for C, N, P and 22% on ECEC for K+were found in the soil under the shrubs. The pH showed only slight but significant differences, whereas Al3+and H+rates on ECEC under the shrubs were increased by 44–55%. For Guiera senegalensis, the most common shrub of the studied area, enrichment ratios of most soil properties increased relatively more with increasing aridity. In general, enrichment ratios decreased with the age of the fallows, whereas concentrations showed no clear evolution. The chemical composition of the shrub litter seems to influence the degree of soil enrichment. The main step of fertile island formation takes place during the cultivation period by trapping wind-blown sediment. This work shows that shrubs are of vital importance for the accumulation of nutrients and maintenance of soil fertility within agro-ecosystems of Niger.

Soil seed bank and growth rates of an invasive species, Piper aduncum, in the lowlands of Papua New Guinea

Secondary fallow vegetation in parts of the Papua New Guinea lowlands is dominated by the shrub Piper aduncum L. that originates from South America. Here we report on its seed bank, growth rate and biomass accumulation. P. aduncum accounted for 69 % (408 m−2) of the seed bank in the forest and 53 % (1559 m−2) of the seed bank under fallow. About 90 % of the tree seed bank at the fallow site was dominated by P. aduncum whereas this was 78 % in the forest soil. Two-year-old P. aduncum had grown to 4.5 m height and had accumulated 48 Mg dry matter (DM) per ha of above ground biomass. The rate of biomass accumulation increased from 10 Mg DM ha−1 y−1 in the first year to 40 Mg DM ha−1 y−1 in the second year when 76 % of the biomass consisted of mainstems. The highest growth rate of 134 kg DM ha−1 d−1 occurred when P. aduncum was 17-mo-old. Aggressive invasion and monospecific stands of P. aduncum are explained by its dominance in the seed bank, fast growth, and high rates of biomass accumulation. P. aduncum is a major competitor to indigenous tree species and presents a threat to Papua New Guinea's rich biodiversity.

LABORATORY MEASUREMENTS AND MODELING N MINERALIZATION POTENTIAL IN VIRGINIA COASTAL PLAIN AGRICULTURAL, FALLOW, AND FOREST SOILS

A long-term aerobic incubation and leaching technique was used to measure N mineralization of surface andsubsurface soils (sandy loam) from agricultural, forest, and fallow sites in a Virginia Coastal Plain watershed. Nmineralization potential was measured to refine models used to describe this process in a watershed-scale nutrient exportassessment. Potentially mineralizable N (N 0 ) and reaction rate constants (k) were estimated using a first-order model anda nonlinear regression procedure. Large variations in cumulative N mineralized, N 0 , and k, were found for the surfacesoils from agricultural areas. Forest soils had much higher potentially mineralizable N than agricultural soils. Forsubsurface soils, the differences among land uses were less variable than those observed for the surface soils. The first-ordermodel (single-pool approach) was adequate for predicting N mineralization in surface soils from agricultural andfallow areas, but less suitable for forest surface soils. Consideration of a double exponential (two-pool approach) modeldid not improve the performance of N mineralization prediction for forested or agricultural soils. Large variationsoccurred in the field-predicted values of mineralized N due to temperature and moisture ranges commonly occurringthroughout the season. Variability in the N mineralization potential of soils in the watershed suggests that individual kand N 0 should be derived for soils with similar properties to obtain better predictions of N mineralization and thus Nmovement to groundwater.

Links between the detritivore and the herbivore system: effects of earthworms and Collembola on plant growth and aphid development.

Effects of Collembola (Heteromurus nitidus and Onychiurus scotarius) and earthworms (Aporrectodea caliginosa and Octolasion tyrtaeum) on the growth of two plant species from different functional groups (Poa annua and Trifolium repens), and on the development of aphids (Myzus persicae) were investigated in a laboratory experiment lasting 20 weeks. Using soil from a fallow site which had been set aside for about 15 years, we expected that nitrogen would be of limited supply to plants and hypothesized that the soil animals studied, particularly earthworms, would increase nutrient availability to plants and thereby also modify aphid reproduction and development. Plant growth was modified strongly by the presence of soil animals. Earthworms caused a more than twofold increase in shoot and root mass of P. annua but increased that of T. repens by only 18% and 6%, respectively. However, earthworms neither affected plant shoot/root ratio nor the nitrogen concentration in plant tissue. In contrast, the presence of Collembola caused a reduction in plant biomass particularly that of P. annua roots, but plant tissue nitrogen concentration was increased, although only slightly. Aphid reproduction on T. repens was lowered in the presence of Collembola on average by 45% but on P. annua increased by a factor of about 3. It is concluded that Collembola decrease aphid reproduction on more palatable host plants like T. repens but increase that on less palatable ones like P. annua. Earthworm presence also affected aphid reproduction but the effect was less consistent than that of Collembola. In the presence of earthworms, aphid reproduction was in one experimental period increased by some 70%. Earthworms also modified the numbers of Collembola and their vertical distribution in experimental chambers. Exploitation of deeper soil layers by H. nitidus was increased but, generally, O. scotarius numbers were reduced whereas those of H. nitidus increased in earthworm treatments. The presence of Collembola also influenced earthworm body mass during the experiment. In general it declined, but in the presence of Collembola loss of body mass of A. caliginosa was more pronounced. We conclude that inhibiting effects between Collembola and earthworms resulted from the use of a common resource, litter material rich in nitrogen. This is supported by the higher C/N ratio of the litter material in the presence of earthworms and Collembola by the end of the experiment. Effects of soil invertebrates like Collembola and earthworms on plant performance and aphid development are assumed to be modified by complex direct and indirect interactions among soil animal groups.

Dynamics of the evaporation zone in dry sandy soils

To evaluate the effect of soil moisture deficit on the restriction of evaporation from bare soil surfaces, the process of evaporation in dry soil was investigated from field observations at a fallow site and a sand dune site. Analysis of the surface energy balance showed that soil drying affects not only the inter-diurnal variation of energy fluxes, but also their diurnal variation. In addition, the relationship between the surface resistance ( r s) and soil water content ( θ) of surface soil was found to be different between inter-diurnal and diurnal time scales. Stable isotopic analysis of soil water showed that liquid water transport from deeper soil layers stops at the bottom boundary of the dry surface layer (DSL) and that vapor water transport is dominant in the DSL. In addition, diurnal variations in water content and soil air humidity suggest that evaporation and condensation occurs alternatively within the DSL with diurnal cycle due to cyclic change in solar radiation and soil temperature. From these observational results, a conceptual model of the dynamics of the evaporation zone in dry sandy soils is presented. In the model, the evaporation zone consists of two sub-zones: a narrow sub-zone around the bottom boundary of the DSL, and a relatively wide sub-zone within the DSL. The former is active through a day and moves vertically with the development/reduction of the DSL at both diurnal and inter-diurnal time scales. The latter acts as evaporation zone only in the morning and acts as condensation zone in the late afternoon and at night. The dynamic behavior of the evaporation zone and those related to the form of water content profile can explain the difference in the r s– θ relationship between inter-diurnal and diurnal time scales.

Benthic community re-adjustment following dredging of a muddy-maerl matrix

Changes in the benthic infaunal community structure at a location in south-west Ireland subject to dredging impacts were studied. Dredging of a muddy-maerl matrix takes place with a seasonally varying frequency of 2–5 times per month. The benthic infauna at a currently dredged site was compared to a fallowed portion of the area, at which extraction ceased 6 months prior to the study. As virtually the entire licensed area is dredged in rotation, longer term recovery is not possible at the study site. In contrast to numerous other study, the benthic infauna at the dredged site was not depleted, but did exhibit higher levels of stress and a higher diversity than the fallowed site. A clear shift from the abundance of omniverous crustaceans in the dredged area to filter-feeding bivalves in the fallowing area was observed. These observations are thought to be linked to sediment mobility, the creation of spatial patchiness and the mobilisation of food resources.