Plant–herbivore interactions and ecological stoichiometry: when do herbivores determine plant nutrient limitation?

Recent studies on plant–herbivore indirect interactions via nutrient recycling have led to the hypothesis that herbivores with a low nitrogen: phosphorus ratio, feeding on plants with a higher nitrogen: phosphorus ratio, recycle relatively more nitrogen, driving plants into phosphorus limitation. We demonstrate in this paper that such a hypothesis is valid only under restricted conditions, i.e. the nitrogen: phosphorus ratio of inorganic nutrients supplied to the system must be neither too high nor too low compared with the nitrogen: phosphorus ratio of the whole plant + herbivore biomass. If plants have a greater affinity for phosphorus than for nitrogen, low herbivore nitrogen: phosphorus ratio can even promote nitrogen limitation. These results are qualitatively robust, whether grazing functions are donor-controlled or recipient-controlled. We present a graphical analysis of these conditions based on the Zero Net Growth Isocline method.     

The effects of quantity and duration of simulated pollutant nitrogen deposition on root-surface phosphatase activities in calcareous and acid grasslands: a bioassay approach

A field and laboratory based bioassay has been developed to investigate the effects of the quantity and duration of simulated pollutant nitrogen (N) deposition on root-surface phosphomonoesterase (PME) activities in calcareous and acid grasslands. Seedlings of Plantago lanceolata were transplanted to a calcareous grassland and Agrostis capillaris seedlings were grown in microcosms containing soil from an acid grassland that had received either 7 yr (long-term) N additions or 18 months (short-term) N and phosphorus (P) additions. The bioassay revealed that short-term N treatments had little effect on the enzyme activity, whereas long-term N additions significantly increased PME activity within 7 d of transplanting into the field plots. Root-surface PME activity of A. capillaris was significantly reduced in soil that received additions of P. In the plots receiving long-term additions of N, a strong relationship was observed between extractable soil ammonium and root-surface PME activity. Soil ammonium concentrations accounted for 67% of the variation in PME activity of P. lanceolata in the calcareous grassland, and 86% of the variation in PME activity of A. capillaris in the acid grassland. These results provide evidence that N deposition may have considerable effects on the demand and turnover of P in ecosystems that are approaching or have reached N saturation.     

Silicon supply modifies C:N:P stoichiometry and growth of Phragmites australis

Silicon is a non-essential element for plant growth. Nevertheless, it affects plant stress resistance and in some plants, such as grasses, it may substitute carbon (C) compounds in cell walls, thereby influencing C allocation patterns and biomass production. How variation in silicon supply over a narrow range affects nitrogen (N) and phosphorus (P) uptake by plants has also been investigated in some detail. However, little is known about effects on the stoichiometric relationships between C, N and P when silicon supply varies over a broader range. Here, we assessed the effect of silicon on aboveground biomass production and C:N:P stoichiometry of common reed, Phragmites australis, in a pot experiment in which three widely differing levels of silicon were supplied. Scanning electron microscopy (SEM) showed that elevated silicon supply promoted silica deposition in the epidermis of Phragmites leaves. This resulted in altered N:P ratios, whereas C:N ratios changed only slightly. Plant growth was slightly (but not significantly) enhanced at intermediate silicon supply levels but significantly decreased at high levels. These findings point to the potential of silicon to impact plant growth and elemental stoichiometry and, by extension, to affect biogeochemical cycles in ecosystems dominated by Phragmites and other grasses and sedges.     

黄土丘陵区不同类型生物结皮下的土壤生态化学计量特征

生物结皮在土壤养分累积和循环中起着重要作用.本研究以黄土丘陵区浅色藻结皮、深色藻结皮、藻藓混合结皮、藓结皮、地衣结皮和普通念珠藻结皮6类典型生物结皮为对象,分析不同类型生物结皮土壤碳(C)、氮(N)、磷(P)含量及其化学计量学特征,研究不同类型生物结皮对土壤养分的影响.结果表明:不同类型生物结皮土壤C、N、P、C/N、C/P、N/P差异显著;生物结皮层C、N、P、C/N、C/P、N/P均显著高于0~10 cm土层土壤.6类生物结皮土壤C、N含量均随土层加深而下降,P含量受土层深度影响较小.对于生物结皮层,藓结皮C、N、P含量分别为27.07、2.42、0.67 g·kg^-1,显著高于其他类型生物结皮.念珠藻结皮的0~2 cm土层土壤C、N、P、C/N、C/P、N/P显著高于其他类型生物结皮.     

Alfalfa N2-fixation enhances the phosphorus uptake of walnut in interplantings

Walnut tree seedlings exhibited greater phosphorus (P) uptake from³²P-labelled hydroxyapatite when interplanted with alfalfa than with other walnuts, black locust, or orchard grass. Three mechanisms are proposed as possible explanations of this enhnaced P uptake by walnut. In this study, diffusion of solubilized apatite-P to the roots of walnut at points of walnut-alfalfa root intersection is believed to be the operative mechanism. Phosphorus is solubilized due to rhizosphere acidification of alfalfa during nitrogen fixation. These results underscore the interdependence of nutrient cycles. Enhancement of the phosphorus cycle through manipulation of the nitrogen cycle has important implications for world food and fiber production.     

Empirical models for predicting the excretion of nutrients (N and P) by aquatic metazoans: taxonomic differences in rates and element ratios

1. We developed empirical models for predicting the release of nutrients [nitrogen (N) and phosphorus (P)] by aquatic metazoans (zooplankton, mussels, benthic macroinvertebrates and fish). 2. The number of species represented in each model ranged from 9 to 74 (n = 40 – 1122), organism dry mass from 1 × 10^?5 to 8 × 10⁴ mg and water temperature from ?1.8 to 32 °C for all models. Organisms were from marine and freshwater (both lotic and lentic) environments. 3. Rates and ratios of nutrient excretion were modelled and intra‐ and intertaxon differences in excretion were examined. Rates of N and P excretion were not significantly different between marine and freshwater species within the same taxon (e.g. zooplankton). However, rates of excretion (as a function of organism dry mass and water temperature) were significantly different among different orders of zooplankton, mussels and fish. However, excretion of N was similar among different orders of benthic macroinvertebrates. 4. Detritivorous fish excreted both N and P at rates greater than all other taxa; whereas mussels excreted N and P generally at rates less than other taxa. There were no significant differences in the rate of N and P excretion between zooplankton and fish (i.e. the allometry of N and P excretion was similar between zooplankton and fish). 5. Molar N : P ratios of nutrients excreted increased with increasing organism dry mass for each group of metazoans, except for zooplankton and detritivorous fish (where N : P ratios declined with increasing organism dry mass). Molar N : P ratios in the excretions of aquatic metazoans were generally below the Redfield ratio of 16:1. 6. We examined the influence of variable abundance of zooplankton, benthic macroinvertebrates and fish on assemblage excretion rates. Rates of N and P excretion were calculated by applying our models to metazoan biomass and abundance data over seven consecutive years in two oligotrophic lakes. Rates of N and P excretion (g ha^?1 day^?1) increased linearly with increasing assemblage biomass (kg ha^?1). However, rates of N and P excretion were significantly and negatively correlated with the relative abundance of fish and positively correlated with the relative abundance of zooplankton.     

Changes in Production and Nutrient Cycling across a Wetness Gradient within a Floodplain Forest

Floodplain forest ecosystems are highly valuable to society because of their potential for water quality improvement and vegetation productivity, among many other functions. Previous studies have indicated that hydrology influences productivity but that the relationship between hydroperiod and productivity is a complex one. Consequently, we compared multiple indexes of productivity, nutrient circulation, and hydroperiod among three communities on the Flint River floodplain, Georgia, that differed in terms of inundation frequency. We hypothesized that (a) the wettest community would have the lowest total net primary production (NPP) values because of saturated soil conditions; (b) as wetness increases, nutrient circulation in litterfall would decrease because of the hypothesized lower productivity in the wetter community; and (c) as wetness increases, internal translocation would become more efficient. The study site was partitioned into three wetness types—somewhat poorly drained (SPD), intermediate (I) and poorly drained (PD). We found that belowground biomass was greatest on the SPD, litterfall was similar for all three sites, and that woody biomass current annual increment (CAI) was greatest in the PD community. However, when the three variables were totaled for each site, the PD had the greatest NPP, thus disproving hypothesis (a). For hypothesis (b), we observed that P content in litterfall, although not significant, followed the predicted trend; nitrogen (N) content displayed the opposite pattern (PD > I > SPD). As wetness increased, internal translocation became more efficient for phosphorus (support for hypothesis [c]), but the SPD community was more efficient at retranslocating N (contradiction of hypothesis [c]). Received 19 June 2000; accepted 19 October 2000.     

Dissolved reactive phosphorus release from sediments beneath a freshwater cage aquaculture development in West Scotland

Sediments below Atlantic salmon (Salmo salar) smolt cage rearing units in Loch Tralaig, a small freshwater loch in Western Scotland, were studied to assess the potential for waste products, such as fish faeces, uneaten or egested feed pellets, to release dissolved reactive phosphorus (DRP) from sediments.Replicate cores were incubated in the dark at 10 °C for 10 days, with the overlying water column constantly aerated. In a parallel experiment, cores were incubated under the same conditions, but with a 5 % formalin solution added to the overlying water to inhibit microbial activity.Results indicated that DRP release rates from unsterilised aerobic sediments beneath cages is an order of magnitude higher than those at a control site. Biotic activity is therefore seen to be of little influence in controlling aerobic release of DRP from undercage sediments.     

Epilimnetic nutrient loading by metalimnetic erosion and resultant algal responses in Lake Waramaug,Connecticut

Phosphorus regeneration from lake sediments, and subsequent migration to trophogenic surface water, significantly contributes to the lake nutrient budgets and algal bloom conditions in some lake types. Decomposition of organic matter in deep water and sediments results in the accumulation of regenerated nutrients, alternate electron acceptors (reduced products of anaerobic respiration = COD), carbon dioxide, and depletion of dissolved oxygen (electron acceptor in aerobic respiration). Thermal stratification creates spatial segregation of trophogenic and tropholytic environments in the lake, resulting in gradients between sediments, hypolimnion, and the epilimnion. Exchange of oxygen, nutrients, and reduced alternate electron acceptors between the hypolimnion and epilimnion affects the productivity of a lake. Secchi depth, temperature, and dissolved oxygen profiles were determined twice each week from May 1980 to October 1980 at each of five lake stations. Nutrient concentration profiles, including total soluble and total phosphorus, ammonium-N, nitrate, soluble Kjeldahl, and total Kjeldahl nitrogen were determined twice each month. Epilimnetic algal samples were collected twice each week using Kemmerer and water column ‘straw’ amplers. Cell counts of total, green, bluegreen, and diatom algae groups were made. Three methods were used to describe hypolimnetic-epilimnetic exchange, including coefficients of eddy diffusion (based on lake heat budget), a graphical method of defining thermocline location, and relative thermal resistance to mixing (RTRM, based on density differences). All three methods yeilded comparable estimates of net seasonal transport. The graphical and RTRM methods described events occurring at shorter intervals (greater resolution). We find general agreement between the three methods of describing hypolimnetic-epilimnetic transport. The frequency of sampling resulted in increased resolution of thermal profiles (in time), allowing accurate estimation of short-term nutrient flux into epilimnetic waters. An algal bloom event occurred 5 to 12 days following erosion of the top of the metalimnion to below the aerobic-anaerobic interface. The lag time to peak algal concentration, following such events, decreased through the summer (June = 12 days, September = 5 days)