Nutrient-limited Ecosystems Research Articles

Decomposition of litter produced under elevated CO2: Dependence on plant species and nutrient supply

We investigated the effect of CO2 concentration and soilnutrient availability during growth on the subsequent decomposition andnitrogen (N) release from litter of four annual grasses that differ inresource requirements and native habitat. Vulpia microstachys isa native grass found on California serpentine soils, whereas Avenafatua, Bromus hordaceus, and Lolium multiflorum areintroduced grasses restricted to more fertile sandstone soils (Hobbs & Mooney 1991). Growth in elevated CO2 altered litter C:N ratio,decomposition, and N release, but the direction and magnitude of thechanges differed among plant species and nutrient treatments. ElevatedCO2 had relatively modest effects on C:N ratio of litter,increasing this ratio in Lolium roots (and shoots at high nutrients),but decreasing C:N ratio in Avena shoots. Growth of plants underelevated CO2 decreased the decomposition rate of Vulpialitter, but increased decomposition of Avena litter from the high-nutrient treatment. The impact of elevated CO2 on N loss fromlitter also differed among species, with Vulpia litter from high-CO2 plants releasing N more slowly than ambient-CO2litter, whereas growth under elevated CO2 caused increased Nloss from Avena litter. CO2 effects on N release in Lolium and Bromus depended on the nutrient regime in whichplants were grown. There was no overall relationship between litter C:Nratio and decomposition rate or N release across species and treatments.Based on our study and the literature, we conclude that the effects ofelevated CO2 on decomposition and N release from litter arehighly species-specific. These results do not support the hypothesis thatCO2 effects on litter quality consistently lead to decreasednutrient availability in nutrient-limited ecosystems exposed to elevatedCO2.

Coexistence of multiple food chains in a heterogeneous environment: Interactions among community structure, ecosystem functioning, and nutrient dynamics

A model is developed and analyzed for a nutrient-limited ecosystem containing an arbitrary number of parallel plant-herbivore-detritus food chains in a heterogeneous environment, with a view to exploring interactions among community and ecosystem processes. Physical properties of the environment and functional properties of the entire ecosystem such as nutrient supply, transport, and recycling are shown to exert a profound influence on the composition, diversity, and assembly of the biological community. Community structure as well as the physical properties of the environment in turn affect ecosystem functions such as nutrient and energy flow. In particular, the rate of nutrient transport in the physical medium plays a critical role in competition and coexistence among food chains. The potential for coexistence decreases continuously as the nutrient transport rate increases in food chains with donor-controlled or no herbivory, while it increases continuously or is greatest at an intermediate value of the nutrient transport rate in herbivore-controlled food chains. On the other hand, energy flow generally increases with the rate of nutrient transport. Herbivory also exerts a significant, predictable influence on energy flow and coexistence among plants and food chains.

Consumers as Maximizers of Matter and Energy Flow in Ecosystems

This article investigates the conditions under which consumers maximize matter and energy flow in ecosystems, using a simple, general, abstract nutrient-limited ecosystem model. The model is analyzed for several functional forms of trophic interactions. The conditions for enhancement of energy flow by consumers are found to be qualitatively the same in all cases. The first and most important condition is that consumers act to accelerate the circulation of matter within the ecosystem; more precisely, the mean transit time of nutrients in the path consumers-decomposers-nutrient pool must be sufficiently smaller than their mean transit time in the path producers-decomposers-nutrient pool. The second condition is that the total quantity of nutrients in the ecosystem must be higher than some threshold value. The third condition is that consumption rate must be moderate. These conditions being usually met, it is likely that consumers as a whole do generally play a role of maximizers of matter and energy flow in stable natural ecosystems.

Is the concept of new production—regenerated production valid for the Southern Ocean?

The concepts developed for nutrient-limited areas are not readily applicable to the Southern Ocean, which is, paradoxically, an ‘oligotrophic’ but not nutrient-limited ecosystem. A synthesis of direct and indirect measurements clearly shows the uniqueness of the Southern Ocean: (1) The presence of a pycnocline allows the calculation of new production by measurement of the depletion of nitrate that results from winter mixing or from permanent nutrient supply. In the open ocean, the new production during the summer period averages 30 g C m −2. The complete depletion of nitrate (e.g. following fertilization with iron) would allow a new production during the summer of 200 g C m −2, which would be only slightly lower than the annual production. (2) The nitrate flux entering the euphotic layer cannot be used as an index of primary production. It is necessary to take into account the nutrient leaving the system by way of the water masses originating in the Southern Ocean: the Antarctic Bottom Water and Antarctic Intermediate Water. (3) The classical relationship between f and total production is not appropriate. The oceanic zone concurrently exhibits a low production (less than 50 g C m −2 year −1) and a high f ratio (at times more than 0.5 ). The ‘neritic’ region appears to be more productive than the oceanic region, but has a lower mean f ratio because of regeneration-dominated phases. Some recent results from the Seasonal Ice Zone (SIZ) also support this conclusion. (4) The fourth original aspect is the high regenerated production that occurs in a nitrate-rich environment with concentrations which frequently remain above 20 mM m −3. The extent to which high ammonium concentrations repress the utilization of nitrate is a basic but still poorly documented question. (5) Diatoms create the main organic flux, i.e. either during episodic events in the SIZ (more than 80% of the annual flux during a very short summer period), or regularly throughout the summer season in the ‘oceanic’ areas. A relatively high proportion of particulate organic matter produced in the surface layer reaches the sediment. It becomes clear that the accumulation of diatom frustules in the sediments does not require a high level of new production (there is no evidence of a real spring bloom in the open ocean); therefore, the richness of the sediments can be explained by a moderate but regular flux.

The influence of shrubs on some chemical and physical properties of an aridic soil in north-eastern Patagonia, Argentina

Soil profiles in mounds beneath shrub clumps and the intervening areas were examined in an arid ecosystem in north-eastern Patagonia. Ten profiles to 80 cm were sampled in the mounds beneath shrub clumps and 10 profiles to 40 cm in the adjacent open areas. Rainfall simulation at an intensity of 70 mm h−1 was used to determine infiltration rates for the two locations. Total N, organic C, NaHCO−3-extractable PO3−4-P, exchangeable K+ and Mg2+ were significantly higher in the soil beneath shrub clumps than in the adjacent areas. The electrical conductivity of the saturation extract (ECse) and the percentage of exchangeable sodium (ESP), although low for both locations, were significantly higher beneath shrub clumps than in the intervening areas. The soil beneath the shrubs presented a significantly lower bulk density and penetrometer resistance and a higher infiltration rate than that of the adjacent areas. In this water- and nutrient-limited ecosystem, the positive effect of the shrub clumps on the underlying soil with good hydrological conditions, high fertility and the greatest concentration of biomass, contrasted with the intervening areas which had poor hydrological conditions, low fertility and sparse vegetation cover.

Nutrient Scavenging of Rainfall by the Canopy of an Amazonian Rain Forest

Fluxes of nutrient elements entering the forest canopy by rainfall are compared with fluxes to the forest floor by throughfall, for two tropical rain forest types growing on soils of low nutrient content in southern Venezuela. In contrast with other forests, total yearly rainfall fluxes of calcium, sulfur, and phosphorus were greater than corresponding throughfall fluxes. For other elements, rainfall fluxes were occasionally greater than throughfall fluxes. We hypothesize that these nutrients are intercepted in the canopy by algae and lichens growing on leaf surfaces, resulting in nutrient conservation in this nutrient-limited ecosystem. THE RAIN FOREST WHICH COVERS much of the central and eastern portion of the Amazon Basin grows on soils low in essential nutrient elements (Sioli 1975, Klinge and Ohle 1964). Despite the low nutrient content of the soils, the forests appear large and vigorous. The question of how the forests survive in the nutrient-poor environment has interested ecologists for many years, and it has been hypothesized (Richards 1952) that nutrient-conserving mechanisms evolved in these ecosystems which have enabled these forests to survive and grow in this nutrient-poor environment. In order to study the structure and function of the Amazon rain forest, including nutrient-conserving mechanisms, an ecological project has been established near San Carlos de Rio Negro, Venezuela, lat. 20N, 67?W (Medina et al. 1977). The area is within the northern part of the drainage basin of the Amazon River. Some of the initial results of the project have shown that there is virtually no nutrient input to the ecosystem from the mineral soil and subsoil (Klinge et al. 1977, Stark and Jordan 1978, Stark and Spratt 1977). This finding means that atmospheric input must be a major source of nutrients for the central Amazon forests. In this paper we report that nutrients are filtered out of the rainwater as the water passes through the forest canopy, and we hypothesize that this interception is an important nutrient-conserving mechanism for the Amazonian

Effect of feeding legume grain supplements on ovulation rate in Border Leicester × Merino ewes

Invasive nitrogen-fixing plants drive vegetation dynamics and may cause irreversible changes in nutrient-limited ecosystems through increased soil resources. We studied how soil conditioning by the invasive alien Lupinus nootkatensis affected the seedling growth of co-occurring native plant species in coastal dunes, and whether responses to lupin-conditioned soil could be explained by fertilisation effects interacting with specific ecological strategies of the native dune species. Seedling performance of dune species was compared in a greenhouse experiment using field-collected soil from within or outside coastal lupin stands. In associated experiments, we quantified the response to nutrient supply of each species and tested how addition of specific nutrients affected growth of the native grass Festuca arundinacea in control and lupin-conditioned soil. We found that lupin-conditioned soil increased seedling biomass in 30 out of 32 native species; the conditioned soil also had a positive effect on seedling biomass of the invasive lupin itself. Increased phosphorus mobilisation by lupins was the major factor driving these positive seedling responses, based both on growth responses to addition of specific elements and analyses of plant available soil nutrients. There were large differences in growth responses to lupin-conditioned soil among species, but they were unrelated to selected autecological indicators or plant strategies. We conclude that Lupinus nootkatensis removes the phosphorus limitation for growth of native plants in coastal dunes, and that it increases cycling of other nutrients, promoting the growth of its own seedlings and a wide range of dune species. Finally, our study indicates that there are no negative soil legacies that prevent re-establishment of native plant species after removal of lupins.