Patterns Of Nutrient Cycling Research Articles

Long-term nutrient cycling patterns in Douglas-fir and red alder stands: a simulation study

Long-term patterns in nutrient cycling in regrowing Douglas-fir ( Pseudosuga menziesii Mirb. Franco) and red alder ( Alnus rubra Bong.) on native soils plus soils previously occupied by other species were simulated using the nutrient cycling model. Simulations of regrowing stands were also compared with observations of nutrient cycling in mature Douglas-fir and red alder. We hypothesized that (1) prolonged presence of red alder will cause a depletion in soil base cations due to increased nitrification and NO 3 − leaching; (2) lower base cation availability under red alder will ultimately cause biomass production to decline; (3) high N availability in red alder soils will favor regrowth of Douglas-fir; (4) higher base cation and P status of the Douglas-fir soils will favor growth of red alder both in the short- and long-term, since N is not limiting to red alder; and (5) in regrowing red alder, NO 3 − leaching will increase with time as a result of increased N fixation. All hypotheses were confirmed, but the effect of soil type on biomass production was minimal both for red alder and Douglas-fir. The higher soil organic matter content in the mature red alder stand most likely reflected previous occupation by old-growth Douglas-fir and also a large litter input from the understory vegetation. In general, the nutrient cycling model simulated differences in nutrient cycling patterns at least qualitatively between Douglas-fir and red alder and was helpful in identifying potential gaps in the understanding of biogeochemical cycling as well as uncertainties in the data. The nutrient cycling model did not fully elucidate differences in P cycling between Douglas-fir and red alder and overestimated weathering rates under Douglas-fir. Uncertainties in the data included: (1) temporal patterns in N fixation in the regrowing stands; (2) understory litterfall; and (3) site history and, consequently, presence of pre-existing differences in site conditions.

Dynamics of nutrient cycling and related benthic nutrient and oxygen fluxes during a spring phytoplankton bloom in South San Francisco Bay (USA)

Benthic oxygen uptake and nutrient releases of N, P and Si were measured weekly at 2 sites in South San Francisco Bay around the 1996 spring bloom. Exchanges across the sediment-water interface were estimated from whole core incubations performed in the laboratory at in situ temperature and in dark. Fluxes changed significantly on a weekly time scale. Over a period of 15 wk the fluxes of dissolved inorganic N, P and Si ranged from -40 to +200, 0 to 13 and from 30 to 400 µmol m-2 h-1 respectively. Sediment oxygen demand increased from 10 before to 64 mg O2 m-2 h-1 just after the bloom period. During the bloom, nutrient fluxes represented about 20, 16 and 9% of the Si, P and N requirements for primary production. Before and after the bloom period, Si fluxes contributed up to 30 and >100% of this requirement and P and N fluxes up to 15 and 50% respectively. Simple empirical models explain most of the spatial-temporal variability of benthic fluxes of Si, P and NH4 (but not NO3) from 3 predictor variables: sediment porosity, nutrient concentration in bottom waters and chlorophyll content of surficial sediments. These models show that algal blooms influence benthic-pelagic nutrient exchange through 2 processes: (1) depletion of nutrients from the water column (which enhances gradient-driven transports across the sediment-water interface) and (2) sedimentation of labile phytodetritus (which promotes remineralization in or on the surficial sediments). Rates and patterns of nutrient cycling were very different at the shallow and deep study sites, illustrating the challenge of extrapolating measurements of coupled algae-nutrient dynamics to whole ecosystems.

Plant Effects on Spatial and Temporal Patterns of Nitrogen Cycling in Shortgrass Steppe

Because of the water-limited nature and discontinuous plant cover of shortgrass steppe, spatial patterns in ecosystem properties are influenced more by the presence or absence of plants than by plant type. However, plant type may influence temporal patterns of nutrient cycling between plant and soil. Plants having the carbon-3 (C3) or carbon-4 (C4) photosynthetic pathway differ in phenology as well as other attributes that affect nitrogen (N) cycling. We estimated net N mineralization rates and traced nitrogen-15 (15N) additions among plant and soil components during May, July, and September of 1995 in native plots of C3 plants, C4 plants, or mixtures of C3 and C4. Net N mineralization was significantly greater in C3 plots than in C4 plots during both July and September. C3 plots retained significantly more 15N in May than did mixed and C4 plots; these differences in 15N retention were due to greater 15N uptake by C3 plants than by C4 plants during May. There were no significant differences in total 15N retention among plant communities for July and September. Soil 15N was influenced more by presence or absence of plants than by type of plant; greater quantities of 15N remained in soil interspaces between plants than in soil directly under plants for July and September. Our results indicate that plant functional type (C3 versus C4) can affect both the spatial and the temporal patterns of N cycling in shortgrass steppe. Further research is necessary to determine how these intraseasonal differences translate to longer-term and coarser-scale effects of plants on N cycling, retention, and storage.

Convective cloud downdrafts as the cause of large blowdowns in the Amazon rainforest

Nelson et al. (1994) report a non-uniform distribution of large (0.33 to 33 km2) blowdowns in the rainforests of Amazonia. Occurrence of such large disturbances in the wet, low wind speed regime of the Amazon Basin cannot be explained by conventional mechanisms such as fire or hurricanes. While downdrafts from deep convective clouds are likely candidates, this paper draws upon meteorological observations from the central Amazon Basin to show that the requisite magnitudes of wind speed can be reached but only under conditions which would explain the observed horizontal distributions of the large blowdowns. Outflow velocities due to density driven downdrafts in the convective clouds are shown from observations to reach 15 m/s and correspond to the propagation velocities of longlived Amazon squall lines. Maintenance over 48 h of these squall lines depends upon the correspondence between the outflow velocity and the propagation velocity. The storms propagate within a basic current moving at 5 m/s, increasing the outflow velocities to 21 m/s. The development of a storm-generated pressure field must be called upon to increase the magnitude of the density and basic currents to more than 30 m/s. Such conditions occur only when the convection is embedded in squall lines which reach their diurnal maximum at locations nearly coincident with the observed locations of the maxima in large blowdowns. The characteristics of the observed blowdowns in terms of orientation, size and shape are consistent with winds derived from the postulated downward moving density currents being deflected and accelerated orthogonally at canopy top and forest floor. The causes and distribution of large blowdowns in the wet, low wind regime of the Amazon rainforest are shown to be important in explaining the disturbance regime of that system. Such disturbances are necessary in maintaining diversity of the system by locally disrupting the canopy, altering levels of incident radiation received at the surface and changing patterns of nutrient cycling, carbon uptake and storage.

Forestry Trees as Invasive Aliens

Some alien tree species used in commercial forestry and agroforestry cause major problems as invaders of natural and seminatural ecosystems. The magnitude of the problem has increased significantly over the past few decades, with a rapid increase in afforestation and changes in land use. Trends can be explained by analyzing natural experiments created by the widespread planting of a small number of species in different parts of the world. The species that cause the greatest problems are generally those that have been planted most widely and for the longest time. The most affected areas have the longest histories of intensive planting. Pinus spp. are especially problematic, and at least 19 species are invasive over large areas in the southern hemisphere, where some species cause major problems. The most invasive Pinus species have a predictable set of life‐history attributes, including low seed mass, short juvenile period, and short interval between large seed crops. Pine invasions have severely impacted large areas of grassland and scrub‐brushland in the southern hemisphere by causing shifts in life‐form dominance, reduced structural diversity, increased biomass, disruption of prevailing vegetation dynamics, and changing nutrient cycling patterns. The (unavoidable) negative impacts of forestry with alien species are thus spilling over into areas set aside for conservation or water production. There is an urgent need to integrate the various means available for reducing the negative impacts of current invaders and to implement protocols to regulate the translocation of species that are known to be invasive.Arboles Forestales como InvasoresAlgunas especies de árboles introducidas por actividades forestales y agroforestales causan problemas mayores como invasores de ecosistemas naturales y semi‐naturales. La magnitud del problema se ha incrementado significativamente durante las últimas décadas, con un incremento rápido en la deforestación y cambios rápidos del uso del suelo. Algunos patrones pueden ser explicados al analizar experimentos naturales creados por la amplia dispersión de la siembra de un número pequeño de especies en diferentes partes del mundo. Las especies que causan los problemas mas graves son generalmente aquellas que han sido ampliamente sembradas y por mayor tiempo. Las áreas mas afectadas tienen los historiales mas largos de siembra intensiva. Pinus spp son especialmente problemáticos y al menos 19 especies son invasivas en áreas grandes del hemisferio sur, donde algunas especies causan mayores problemas. La especie mas invasiva de Pinus tiene una serie de atributos predecibles de historia de vida (masa de semillas pequeña, período juvenil corto y cortos intervalos entre cosechas grandes dde semillas). Las invasiones de pinos han afectado severamente grandes áreas de pastizales y zonas arbustivas en el hemisferio sur, causando desvíos en las formas de vida dominantes, reduciendo la diversidad estructural, incrementando la biomasa, perturbando las dinámicas de la vegetación prevaleciente y cambiando los patrones de los ciclos de nutrientes. Los impactos negativos (inevitables) de la forestería con especies extrañas estan por lo tanto diseminándose en áreas separadas para conservación o producción de agua. Hay una necesidad urgente de integrar los medios posibles para reducir los impactos negativos de los invasores actuales, además de implementar protocolos para regular la translocación de especies que se conoce son invasivas.

Biogeochemical Cycling following Planting to Red Pine on a Sandy Prairie Soil

Native prairies in the Lower Wisconsin River Valley provide a means of comparing nutrient cycling patterns across undisturbed and managed ecosystems. Bulk precipitation, throughfall, and soil solutions at three depths were collected from August 1991 to December 1993 at three study sites (native prairie and native prairie planted to 30 and 40-year-old red pine, Pinus resinosa Ait.). Elemental input-output balances were calculated from water chemistry data and from water balances using chloride as a tracer. Concentrations of cations and dissolved organic carbon (DOC) in soil solution were greater in the conifer plantations than in the prairie ecosystem. Inorganic acid concentrations were only slightly higher under pine than under prairie, suggesting that the greater cation leaching under pine vegetation was dependent on DOC contributed by throughfall and leaching from the forest floor. There was a small net gain in Mg and losses in Ca, K, and S in the prairie ecosystem. There was a net loss in all elements from the pine ecosystem, and losses were greater than from the prairie. The greater losses in Mg, K, and S from the pine ecosystem are attributed to interception of dry deposition by the tree canopy. Sulfur contributed by dry deposition to the pine ecosystem was mainly returned to the forest floor as organic S and oxidized to SO2−4 as solutions passed through the soil profile. In contrast, greater Ca losses under pine than under prairie are attributed to increased weathering due to afforestation.

Nutrient Cycling and Nutrient Use Efficiency in Short Rotation, High Density Central Himalayan Tarai Poplar Plantations

This paper deals with the pattern of nutrient cycling and nutrient use efficiency in four (1–4 years old) poplar ( Populus deltoides Marsh) plantations previously investigated for dry matter dynamics. The present plantations were planted at 3 × 5 m spacing after clear felling of natural sal ( Shorea robusta Dipterocarpaceae) mixed broad-leaved forests in central Himalayan Tarai. The nutrient concentrations (N, P and K) in different layers of vegetation were in the order: tree> shrub> herb; whereas the standing state of nutrients were in the order: tree> herb> shrub. Soil, litter and vegetation, respectively accounted for 81–96, 2–4 and 2–15% of the total nutrients in the system. Considerable reductions (trees, 50–68; shrubs, 35–40; and herbs 18–26 %) in the concentration of nutrients in leaves occurred during senescence. The uptake of nutrients by the vegetation, and also by the different components, with and without adjustment for internal recycling, was calculated separately. Annual transfer of litter nutrients to the soil by vegetation was 91–148 N, 8–15 P and 70–99 K kg ha -1 yr -1 . The turnover rate for different nutrients ranged between 0·83 and 0·92 yr -1 .The nutrient use efficiency of poplar plantations ranged from lSI to 174 kg ha -1 ye1 for N, 1338 to IS66 kg ha -1 yr -1 for P, and 313 to 318 kg ha -1 yr -1 for K. Compared with low density eucalypt and older poplar stands, there was a higher proportion of nutrient retranslocation in present poplars, largely because of higher tissue nutrient concentrations. This indicates lower nutrient use efficiency as compared to eucalypt plantations. Compartment models for nutrient dynamics have been developed to represent the distribution of nutrient pools and net annual fluxes within the system.

Nutrient Cycling and Nutrient Use Efficiency in Short Rotation, High Density Central Himalayan Tarai Poplar Plantations

This paper deals with the pattern of nutrient cycling and nutrient use efficiency in four (1–4 years old) poplar (Populus deltoidesMarsh) plantations previously investigated for dry matter dynamics. The present plantations were planted at 3×5 m spacing after clear felling of natural sal (Shorea robustaDipterocarpaceae) mixed broad-leaved forests in central Himalayan Tarai. The nutrient concentrations (N, P and K) in different layers of vegetation were in the order: tree>shrub>herb; whereas the standing state of nutrients were in the order: tree>herb>shrub. Soil, litter and vegetation, respectively accounted for 81–96, 2–4 and 2–15% of the total nutrients in the system. Considerable reductions (trees, 50–68; shrubs, 35–40; and herbs 18–26%) in the concentration of nutrients in leaves occurred during senescence. The uptake of nutrients by the vegetation, and also by the different components, with and without adjustment for internal recycling, was calculated separately. Annual transfer of litter nutrients to the soil by vegetation was 91–148 N, 8–15 P and 70–99 K kg ha−1yr−1. The turnover rate for different nutrients ranged between 0.83 and 0.92 yr−1. The nutrient use efficiency of poplar plantations ranged from 151 to 174 kg ha−1yr−1for N, 1338 to 1566 kg ha−1yr−1for P, and 313 to 318 kg ha−1yr−1for K. Compared with low density eucalypt and older poplar stands, there was a higher proportion of nutrient retranslocation in present poplars, largely because of higher tissue nutrient concentrations. This indicates lower nutrient use efficiency as compared to eucalypt plantations. Compartment models for nutrient dynamics have been developed to represent the distribution of nutrient pools and net annual fluxes within the system.

Spatial Characteristics of Soil Properties along an Elevational Gradient in a Carolina Bay Wetland

AbstractSoil chemistry gradients exist in wetlands but information on the spatial variability of wetland soil properties is limited. Soils were collected from a small, depressional wetland (Carolina bay) in South Carolina to determine spatial patterns along an elevational gradient. The bay was divided into four quadrants and a transect was established in each quadrant. Plots were sampled at every 10‐cm rise in elevation along the four transects. Samples from the A and B horizons were analyzed for soil texture, organic carbon (OC), pH, cation‐exchange capacity (CEC), and exchangeable Ca, Mg, and K. Significant difference were noted for OC, clay content, CEC, and base saturation for the A horizon across the elevational gradient. The OC content of the A horizon decreased from 4.7% in the center of the bay to 2.7% near the rim. The CEC decreased from 26.0 cmolc kg−1 in the center to 13.9 cmolc kg−1 near the rim. Significant differences also were observed for pH, OC, CEC, sand, silt, clay, exchangeable Mg, Ca, and base saturation among the four transects. In the B horizon, there were no significant differences for any parameter across the elevational gradient, or among the four transects. The clay content of the B horizon was about 20%, compared with <10% in the A horizon. The differences in soil parameters in the A horizon may be a reflection of vegetation patterns or hydrology of the bay. Sampling protocol for soils of depressional wetlands may require an evaluation of elevation, hydrology, and vegetation patterns to determine spatial patterns of nutrient cycling.

Nitrogen and Phosphorus Cycling in Grazed and Ungrazed Plots in a Temperate Subhumid Grassland in Argentina

The distribution and dynamics of N and P were studied for two adjacent grazed and ungrazed areas of native grassland in the Flooding Pampa, Argentina, over the winter-spring season of maximum primary production. We evaluated how grazing effects on vegetation structure altered patterns of nutrient cycling at the ecosystem level. Nutrient transfers were estimated from element concentrations measured in plant and soil pools at minimum and peak standing crop and from previous work in the study area. Total nutrient amounts in vegetation (9.4-12.6 g m -2 N and 0. 50-0.76 g m -2 P) did not differ consistently between grazed and ungrazed grassland, but nutrient location reflected the effect of cattle on phytomass structure and species composition. Plant pools of N and P were mainly located in below-ground organs, to the extent of 80-90% in grazed and 63-75% in ungrazed vegetation. Nutrients in above-ground green material from grazed prairie were 50% of the content for ungrazed prairie. At peak standing crop the grazed grassland contained more than 50% N and P in forb biomass, whereas protected vegetation had most nutrients in graminoids. Nitrogen concentration in live shoots declined through the season while it was increasing in roots, indicating preferential below-ground allocation. Seasonal patterns of plant N were amplified by continuous grazing. Phosphorus allocation appeared to be closely regulated in this grassland. Although changes in shoot P concentrations were subtle, P allocation to shoots seemed higher in ungrazed grassland. Shoot N:P ratios suggested that vegetation was relatively more P-limited in early winter, but more N-limited in late spring. In general, N and P contents tended to increase in living vegetation and to decrease in dead plant compartments over the season studied. Nutrient uptake by roots was 30-50% higher in the grazed plot, in correspondence with enhanced mineralization rates. Concentrations of N and P in graminoid live shoots were also higher under grazing, although grazing decreased the overall translocation of N to shoots. Livestock consumption represented more than 40% of above-ground nutrient translocation, and thus grazing increased nutrient flow through the below-ground plant pathway. Strong demand from underground sinks in grazed vegetation probably contributed to increased root uptake. Grazing did not affect soil-available nutrients, although it did accelerate nutrient cycling rates, mostly of P. Ammonia volatilization and N exports through cattle removal were of similar magnitude (0.22 g m -2 year -1 N), and were 60% of wet deposition. Therefore, N tended to accumulate in this grassland. Annual P outputs in animal biomass produced a small net loss of P from the managed ecosystem.

The characteristics of the nutrient cycling in birch stands with different age stages

The pattern of nutrient cycling in a succession series from birch (Betulu platyphylla) stands to Korean pine (Pinus koraiensis) stands was revealed by studying the extant amount and nutrient contents of litter, biomass of shrubs and herbs and so on. In this succession series, the litter and its mineral elements increased with the succession developing, but the maximum was in birch stand rather than in Korean pine stand stage. With the age increased, the biomass of the shrubs and herbs decreased but the nutrient contents increased with; and the total biomass and its mineral elements of the shrubs and herbs showed fluctuation property.

Impacts of invading N2-fixing Acacia species on patterns of nutrient cycling in two Cape ecosystems: evidence from soil incubation studies and 15N natural abundance values.

This study examines the impacts of woody, N2-fixing invasive Acacia spp. on the patterns of nutrient cycling in two invaded ecosystems of differing nutrient status in the Cape floristic region. Patterns of soil nutrient mineralization were measured by a field incubation method while the significance of the fixation process in altering nutrient cycling was assessed by the δ15N natural abundance technique. The results confirm earlier reports that invasion by woody shrubs results in organic matter and nutrient enrichment of surface soils of both ecosystems. However, patterns of nutrient availability (phosphorus and nitrogen) were not necessarily enhanced. In the more fertile strandveld both phosphorus and nitrogen (significant at P<0.10) showed trends towards enhanced annual mineralization rates upon invasion, while in the low nutrient fynbos system only phosphorus followed this trend. It is unclear whether this differential response is a consequence of plant- or soil-derived feedbacks on the decomposition processes in each system. The δ15N values of the soils from the invaded sites of both ecosystems indicated a strong influence of the alien species on the soil nitrogen component. However, as with other studies of natural ecosystems, the contribution of nitrogen from fixation could not be readily quantified with the δ15N natural abundance method because of problems in selecting suitable non-N2-fixing reference plants. A technique of disrupting nodule structure and function, by fumigation with O2, to obtain the δ15N value of a non-N2-fixing speciment of the study species was tried and found to overcome some of the problems associated with the lack of suitable reference plants. With this technique it was possible to detect the almost total dependence of A. saligna on N2-fixation in the fynbos soils with their low nitrogen mineralization rates. In the strandveld ecosystem with much higher soil nitrogen release rates A. cyclops was only partly dependent on fixation (about half) for its nitrogen. The nutrient enrichment of both ecosystems and trends towards enhanced rates of nutrient mineralization could have profound implications on the long-term success of alien invader clearing operations and the restoration of the indigenous flora at these sites.

Successional patterns and gap phase dynamics of a humid tropical forest of the Western Ghats of Kerala, India: ground vegetation, biomass, productivity and nutrient cycling

The population dynamics of the ground vegetation and its energetics such as biomass accumulation and net primary productivity, and the nutrient cycling patterns in the humid tropical forest of the Western Ghats in India are largely determined by gap age and by whether gaps are formed naturally or through selection felling. Responses of plant categories such as herbs, shrubs, tree seedlings and saplings also vary depending upon gap type and age. An exotic species such as Chromolaena odorata occurred only in selection-felled gaps ((9 ± 3)−(49 ± 4) individuals (100 m) −2). Nilgirianthus ciliatus, a dominant shrub, plays a key role in the gaps in determining population dynamics of others. The net primary productivity of the ground vegetation, which is about 31.17 ± 4.26 kg (100 m) −2 year −1 in an undisturbed site, increased a year after gap formation to 102.82 ± 6.46 kg (100 m) −2 year −1 in natural gaps and to 71.82 ± 2.36 kg (100 m) −2 year −1 in selection-felled gaps. Five years after gap formation, net primary productivity of the ground vegetation declined considerably, this being related to decline in fast-growing shrub and secondary tree species in the vegetation and gap closure. A similar trend was also recorded for the rates of nutrient uptake and nutrient accumulation in the vegetation. In natural gaps the soil nutrient level increased gradually with gap age. This could be attributed to slow release of nutrients from the fallen trunks and nutrient storage in the rapidly recovering vegetation. In contrast, in selection-felled gaps, the quantities of soil nutrients such as nitrogen, phosphorus and magnesium were higher in 1-year-old gaps than in undisturbed sites, owing to the release of these nutrients from leaf litter and wood debris which were deposited in larger quantities within the gap itself, and owing to sparse ground vegetation resulting from the greater disturbance of the soil, in the first 1 or 2 years. The fractional annual turnover rates of elements of the ground vegetation and the soil were higher in 1-year-old gaps and declined with gap age. The significance of these results for forest management is discussed.

Landscape-level processes and wetland conservation in the Southern Appalachian Mountains

The function of wetland ecosystems is not independent of the landscapes in which they are embedded. They have strong physical and biotic linkages to the surrounding landscape. Therefore, incorporating a broad-scale perspective in our study of wetland ecology will promote our understanding of these habitats in the Southern Appalachians. Changes in the surrounding landscape will likely affect wetlands. Broad-scale changes that are likely to affect wetlands include: 1) climate change, 2) land use and land cover change, 3) water and air-borne pollution, 4) a shift in disturbance/recovery regimes, and 5) habitat loss and fragmentation. Changes in climate and land cover can affect the hydrology of the landscape and, therefore, the water balance of wetlands. Excessive nutrients and toxin transported by air and water to wetlands can disrupt natural patterns of nutrient cycling. Periodic disturbances, like flooding in riparian zones, is required to maintain some wetlands. A change in disturbance regimes, such as an increase in fire frequency, could alter species composition and nutrient cycles in certain wetlands. Many plant and animal species that found in small, isolated wetlands have populations that are dependent on complementary habitats found in the surrounding landscape. Loss or fragmentation of these complementary habitats could result in the collapse of wetland populations.

The effect of detrital addition on the development of nanoflagellates and bacteria in Lake Kinneret

The effect of adding dissolved substrates derived from algal cells on the patterns of nutrient cycling and growth of bacteria, heterotrophic nanoflagellates (HNAN) and photoautotrophs was determined in samples of near-surface waters from Lake Kinneret. Supplementation of substrates always resulted in an increased peak of HNAN numbers and had little effect on bacterial numbers. HNAN-mediated nutrient remineralization of nitrogen and phosphorus was also stimulated. In light-incubated samples the remineralized nutrients were taken up by photoautotrophic cells. Maximum growth rates observed for HNAN ranged from 0.03 to 0.11 h −1 , clearance rates for bacteria 1.1–7.3 nl HNAN −1 h −1 and remineralization rates 6.4–8.4 μg N mg dry wt −1 h −1 and 0.37–0.99 μg P mg dry wt −1 h −1 .

Nutrient Cycling in Forests of Walker Branch Watershed, Tennessee: Roles of Uptake and Leaching in Causing Soil Changes

AbstractPrevious studies showed that subsoil exchangeable Ca2+ and Mg2+ decreased over the period 1971 to 1982 in several plots on Walker Branch Watershed. It was hypothesized that wood accumulation (i.e., uptake and sequestering in both living and dead tree biomass) was the dominant cause of the exchangeable Ca2+ decreases, whereas atmospheric deposition‐induced leaching was the dominant cause of the exchangeable Mg2+ decreases. As hypothesized, wood accumulation far exceeded leaching of Ca2+ in those plots where soil exchangeable Ca2+ decreased. The hypothesis regarding causes of exchangeable Mg2+ decrease was neither accepted nor rejected from the data on hand. Leaching exceeded wood accumulation of Mg2+ in three of the four plots studied (pine, Pinus; yellow‐poplar, Liriodendron tulipifera; and chestnut oak Quercus prinus). Cation leaching in these three plots was balanced by SO2−4, and input‐output balances indicated that soil solution SO2−4 originated primarily from atmospheric deposition. Thus, the trends toward decreased subsoil exchangeable Mg2+ in those particular plots (while not all statistically significant at the 95% level), are attributable primarily to atmospheric S deposition. However, leaching was minimal in the fourth plot (oak hickory, Quercus‐Carya spp.) due to very low soil solution SO2−4 concentrations, yet subsoil Mg2+ decreases were significant at the 95% level. In the latter case, wood uptake was the dominant mechanism of Mg2+ removal from the soil. The importance of slope position in determining nutrient cycling patterns and the long‐term changes in nutrient status is also discussed.

Annual cycles of nutrients in a shallow inshore area, Kiel Bight. — variability and trends

Abstract The annual cycle of nutrients in relation to biological processes and seasonality of the physical environment of a coastal ecosystem, Kiel Bight, is presented. Distinct patterns of nutrient cycling succeed each other in the course of the annual cycle in the ecosystem; five such phases are identified as recurring features of the system, although there is interannual variability in temporal scales of these phases. During spring, autumn and winter pelagic regeneration is low. Whereas pelagic uptake exceeds sedimentary nutrient release in spring, the reverse pattern prevails in autumn. In the transition from spring to summer nutrients released by the benthos are efficiently retained by the pelagic food web. In summer, pelagic uptake is balanced by pelagic and benthic regeneration in sediments above the thermocline. In the subthermocline environment nutrient release from the sediments is increasingly influenced by the declining oxygen content and this situation prevails throughout the autumn phase. Winter nutrient equilibrium is achieved 4–6 weeks after the end of the growth season. Thereafter concentrations are maintained at constant levels largely by non-biological mechanisms till the onset of spring growth. Winter concentrations of dissolved inorganic nitrogen have been remarkably constant since 1969, whereas for silicate a decline is indicated. In contrast to these nutrients, phosphate winter concentrations exhibit an increase, which has led to almost a doubling of winter levels during the last two decades.

NUTRIENT CYCLING IN MOIST TROPICAL FOREST

Early studies of nutrient cycling in moist tropical forests described productive forests rich in nutrients (98, 114, 176) in which rates of primary production and the amounts of nutrients cycled clearly exceeded those in temperate zone forests. Reviews of global-scale patterns in biomass, production, and nutrient cycling reported these results as representative of tropical forests (130, 175). At the same time, tropical forest soils were described as acid, infertile clays that harden irreversibly to laterite when cleared (106), or as bleached quartz sands low in mineral nutrients (88). This apparent paradox was crystallized by Whittaker (174) in the statement The tropical rain forest thus has a relatively rich nutrient economy perched on a nutrient-poor substrate (p. 271). Reviews of more recent research on overall patterns of mineral cycling in the tropics (78, 124), and of important components such as biomass (20, 21), litterfall nutrients (123, 164), and decomposition (5), clearly show that patterns of nutrient cycling in tropical forests are diverse. It makes no more sense to describe a 'typical' tropical forest than a 'typical' temperate forest (33, 151). Variations in mineral cycling nonetheless follow coherent, explicable patterns in tropical forests. Our purposes in undertaking this review are: (a) to illustrate the patterns of nutrient cycling in moist tropical forests; (b) to identify the mechanisms which regulate those patterns; and (c) to show how those patterns affect the productivity, physiology, and population biology of tropical forests and their large-scale linkages with aquatic ecosystems and the atmosphere. We emphasize the cycling of nitrogen, phosphorus, potassium, calcium, and magnesium; these are the best-studied soil-derived nutrients, and they are the nutrients most likely to limit primary production and other ecosystem functions.

WETLANDS AS ACCRETING SYSTEMS: NUTRIENT CYCLING IN WETLANDS

SUMMARY The major concepts involved in nutrient cycling in wetlands are discussed using phosphorus and nitrogen as examples. The differences in nutrient cycling patterns between hydrologically contrasting wetlands, and the significance of fluctuating water levels in wetlands, are stressed. In South Africa, some research into nutrient cycling in submerged aquatic macrophyte communities has been undertaken, but very little information is available on these processes in wetlands dominated by emergent vegetation forms. Generally, the information available on nutrient cycling processes in South African wetlands is of limited applicability to the assessment of nutrient cycling in whole wetland systems. Therefore, rational wetland management strategies require further research on nutrient cycling.

Robert H. Whittaker (1920?1980): The man and his work

R. H. Whittaker enlivened many fields within ecology, systematics and evolution with his insights. Perhaps his most significant contributions to ecology lie in the development of the theories and methods of gradient analysis. Through the verification of the individualistic hypothesis with field data from many regions, and the subsequent development and dissemination of methods for studying species distributions along continua, he helped replace the Clementsian paradigm with a Gleasonian one. His extensive field data on primary production, nutrient cycling patterns and species diversity established new standards for documentation in synecology and helped clarify the basis for site-to-site variation in these variables. Through his broad command of the ecological literature, his writings and his contact with ecologists throughout the world he fostered international understanding of the diversity of approaches to vegetation study.