Role of cell wall of groundnut roots in solubilizing sparingly soluble phosphorus in soil

Groundnut showed a superior ability to take up P from a soil with low P fertility compared with sorghum and soybean. This ability was not related to its better root development or production of root exudates capable of solubilizing iron-and aluminum-bound P. In efforts to determine the role of roots per se, we found that root cell walls from groundnut showed a higher P-solubilizing activity than those from soybean or sorghum. This finding corresponds well with observations in field and pot experiments using a soil with low P availability. The reaction site of P-solubilizing activity is stable against heating and enzyme digestion by cellulase and pectinase. This is probably the first evidence to demonstrate that cell walls of plant roots are involved in P-solubilizing activity. ei]Section editor: H Marschner (deceased 21 September 1996)     

Predictive models for phosphorus retention in wetlands

The potential of wetlands to efficiently remove (i.e., act as a nutrient sink) or to transform nutrients like phosphorus under high nutrient loading has resulted in their consideration as a cost-effective means of treating wastewater on the landscape. Few predictive models exist which can accurately assess P retention capacity. An analysis of the north American data base (NADB) allowed us to develop a mass loading model that can be used to predict P storage and effluent concentrations from wetlands. Phosphorus storage in wetlands is proportional to P loadings but the output total phosphorus (TP) concentrations increase exponentially after a P loading threshold is reached. The threshold P assimilative capacity based on the NADB and a test site in the Everglades is approximately 1 g m^–2 yr^–1. We hypothesize that once loadings exceed 1 g m^–2 yr^–1 and short-term mechanisms are saturated, that the mechanisms controlling the uptake and storage of P in wetlands are exceeded and effluent concentrations of TP rise exponentially. We propose a One Gram Rule for freshwater wetlands and contend that this loading is near the assimilative capacity of wetlands. Our analysis further suggests that P loadings must be reduced to 1 g m^–2 yr^–1 or lower within the wetland if maintaining long-term low P output concentrations from the wetlands is the central goal. A carbon based phosphorus retention model developed for peatlands and tested in the Everglades of Florida provided further evidence of the proposed One Gram Rule for wetlands. This model is based on data from the Everglades areas impacted by agricultural runoff during the past 30 years. Preliminary estimates indicate that these wetlands store P primarily as humic organic-P, insoluble P, and Ca bound P at 0.44 g m^–2 yr^–1 on average. Areas loaded with 4.0 g m^–2 yr^–1 (at water concentrations>150 g·L^–1 TP) stored 0.8 to 0.6 g m^–2 yr^–1 P, areas loaded with 3.3 g m^–2 yr^–1 P retained 0.6 to 0.4 g m^–2 yr^–1 P, and areas receiving 0.6 g m^–2 yr^–1 P retained 0.3 to 0.2 g m^–2 yr^–1. The TP water concentrations in the wetland did not drop below 50 g·L^–1 until loadings were below 1 g m² yr^–1 P.     

Models to predict lake annual mean total phosphorus

A lake is a product of processes in its watershed, and these relationships should be empirically quantifiable. Yet few studies have made that attempt. This study quantifies and ranks variables of significance to predict annual mean values of total phosphorus (TP) in small glacial lakes. Several new empirical models based on water chemistry variables, on map parameters of the lake and its catchment, and combinations of such variables are presented. Each variable provides only a limited (statistical) explanation of the variation in annual mean values of TP among lakes. The models are markedly improved by accounting for the distribution of the characteristics (e.g., the mires) in the watershed. The most important map parameters were the proportion of the watershed lying close to the lake covered by rocks and open land (as determined with the drainage area zonation method), relief of the drainage area, lake area and mean depth. These empirical models can be used to predict annual mean TP but only for lakes of the same type. The model based on map parameters (r ²=0.56) appears stable. The effects of other factors/variables not accounted for in the model (like redox-induced internal loading and anthropogenic sources) on the variation in annual mean TP may then be estimated quantitatively by residual analysis. A new mixed model (which combines a dynamic mass-balance approach with empirical knowledge) was also developed. The basic objective was to put the empirical results into a dynamic framework, thereby increasing predictive accuracy. Sensitivity tests of the mixed model indicate that it works as intended. However, comparisons against independent data for annual mean TP show that the predictive power of the mixed model is low, likely because crucial model variables, like sedimentation rate, runoff rate, diffusion rate and precipitation factor, cannot be accurately predicted. These model variables vary among lakes, but this mixed model, like most dynamic models, assumed that they are constants.     

Vertical canopy structure of Dutch chalk grasslands in relation to their management

Analysis of leaf canopy overtopping relationships was carried out using a non-destructive point quadrat method followed by a destructive stratified harvest of the above-ground phytomass in Dutch chalk grasslands with different management: summer sheep grazing and annual autumn mowing. The two methods of analysis are compared: e.g. relative leaf overtopping can be recorded by the point quadrat method but it is obscured in vertical vegetation profiles based on stratified phytomass distribution. However the stratified harvest method describes the relationship between canopy phytomass and light microclimate, recorded by measuring Photosynthetically Active Radiation (PAR) at different heights in the vegetation. Plant growth form during peak standing crop is of greater significance than Raunkiaerian life form in determining structure of chalk grassland vegetation. In annually mown grasslands, the tall graminoid growth form, shown by Brachypodium pinnatum, reduces PAR in the lower canopy and suppresses most other growth forms except those which can reach the higher canopy levels, e.g. clumped herbs such as Origanum vulgare. In contrast, grazing can result in a reduction of dominance from the tall graminoids and reduction of the abundance of taller, grazing-sensitive herbs, e.g. clumped and climbing herbs such as Origanum and Lathyrus pratensis, and an increase in grazing-tolerant species, e.g. smaller rosette herb growth forms, e.g. Leontodon hispidus and shorter rhizomatous or stoloniferous graminoids, e.g. Carex flacca and Briza media. Changes in both the overtopping hierarchy at the peak of the growing season and the intensity of overtopping in the course of a growing season are of conclusive importance in determining the relative abundance of species in the vegetation.     

Recent palaeolimnology of three shallow Danish lakes

Recent eutrophication histories of three shallow lakes (mean depths <3 m) were studied using palaeolimnological methods. Freeze-cores were dated using ²¹⁰Pb, ¹³⁷Cs, ¹³⁴Cs and ²⁴¹Am. Resultant chronologies were problematical at two sites (Væng Sø and Vesterborg Sø) due to sediment-water interface mixing, indicated by uniform ¹³⁷Cs profiles over the surface 20–30 cm. Sediments at Langesø and Vesterborg Sø have a high carbonate content, which together with the high mixing rate have resulted in diatom dissolution below 30 cm at Vesterborg Sø. Diatom stratigraphy indicates relatively small biological changes at both Væng Sø and Langesø both lakes have been eutrophic for at least the last 150–200 years. Væng Sø is dominated by planktonic diatoms together with high percentages of benthic Fragilaria spp., and Langesø by planktonic diatoms, especially Cyclostephanos dubius and Stephanodiscus parvus.Epilimnetic phosphorus concentrations were inferred using weighted averaging; at Væng Sø the diatom-inferred TP (DI-TP) concentrations were close to observed values in the early 1980s, but failed to record the post-biomanipulation decrease to 55 g l^-1 after 1988, presumably due to the smoothing of the sediment record by resuspension and mixing. At Langesø two increases in DI-TP suggest a two-phase enrichment of the lake, initial eutrophication 1880, and hypertrophy after 1950. The recent DI-TP values are lower than the annual observed values, and reasons for this are discussed. Despite problems associated with sediment mixing, the sediment records of these shallow systems can still be used for monitoring and for environmental reconstructions.     

Nutrient additions by waterfowl to lakes and reservoirs: predicting their effects on productivity and water quality

Lakes and reservoirs provide water for human needs and habitat for aquatic birds. Managers of such waters may ask whether nutrients added by waterfowl degrade water quality. For lakes and reservoirs where primary productivity is limited by phosphorus (P), we developed a procedure that integrates annual P loads from waterfowl and other external sources, applies a nutrient load-response model, and determines whether waterfowl that used the lake or reservoir degraded water quality. Annual P loading by waterfowl can be derived from a figure in this report, using the days per year that each kind spent on any lake or reservoir. In our example, over 6500 Canada geese (Branta canadensis) and 4200 ducks (mostly mallards, Anas platyrhynchos) added 4462 kg of carbon (C), 280 kg of nitrogen (N), and 88 kg of P y^–1 to Wintergreen Lake in southwestern Michigan, mostly during their migration. These amounts were 69% of all C, 27% of all N, and 70% of all P that entered the lake from external sources. Loads from all external sources totaled 840 mg P m^–2 y^–1. Application of a nutrient load-response model to this concentration, the hydraulic load (0.25 m y^–1), and the water residence time (9.7 y) of Wintergreen Lake yielded an average annual concentration of total P in the lake of 818 mg m^–3 that classified the lake as hypertrophic. This trophic classification agreed with independent measures of primary productivity, chlorophyll-a, total P, total N, and Secchi disk transparency made in Wintergreen Lake. Our procedure showed that waterfowl caused low water quality in Wintergreen Lake.Contribution 824 of the National Fisheries Research Center-Great Lakes, 1451 Green Road, Ann Arbor, Michigan 48105, U.S.A. and 722 of the Kellogg Biological Station, Michigan State University.     

Bioavailability of phosphorus in agriculturally loaded rivers in southern Finland

The potential bioavailability of phosphorus in agriculturally loaded rivers of southern Finland was determined by an algal bioassay and the release of the potentially bioavailable particulate P was estimated by sorption studies. According to the bioassay 0 to 13.2 per cent (mean 5.1%) of the particulate P in river water samples was potentially bioavailable. Dissolved reactive P (DRP) in river waters appeared to be totally bioavailable whereas the dissolved unreactive P appeared not to be utilized by algae. In addition to river waters two lake sediment samples were also assayed. In these samples 0 and 2.6% of the P was bioavailable. The potential bioavailability of particulate P in agriculturally loaded rivers obtained in this study was lower than that reported in studies from other countries. The difference was assumed to arise partly from methodological factors and partly from the nature of the Finnish soils. The EPC (equilibrium phosphate concentration) values indicated that during the period when most of the agricultural loading enters the lakes in Finland, potentially bioavailable P is not released from the particles because of the relatively high DRP concentration in the receiving waters. However, during the algal production period the DRP concentration in lakes decreases below the EPC and potentially bioavailable particulate P is desorbed. The increase in pH during this period may further enhance the desorption of P.     

Nutrient balances and phytoplankton dynamics in two agriculturally loaded shallow lakes

The impact of agriculture was estimated on two shallow, eutrophic lakes, Lake Kotojärvi and Lake Villikkalanjärvi in southern Finland. The main emphasis was on phosphorus and nitrogen budgets and on the phytoplankton dynamics. Special attention was paid to internal P loading and blue-green algal blooms. The mean Tot-P load from agricultural land was 1.2 kg ha^-1 a^-1 in both basins and Tot-N loads were 19 kg ha^-1 a^-1 in L. Villikkalanjärvi and 12 kg ha^-1 a^-1 in L. Kotojärvi. The Tot-P input to L. Kotojärvi was on an average 0.62 g m^-2 a^-1 (per lake surface area), and the Tot-N input 9.1 g m^-2 a^-1. The corresponding inputs to L. Villikkalanjärvi were 3.1 and 57 g m^-2 a^-1, respectively. The annual variation followed the runoff volumes. About half of the Tot-P and one third of the Tot-N load was retained in L. Kotojärvi. In L. Villikkalanjärvi the retention was only 24% for Tot-P and 19% for Tot-N. The difference was very probably due to a longer theoretical retention time in L. Kotojärvi. In L. Villikkalanjärvi the mean concentration of Tot-P was 120 µg 1^-1 and that of Tot-N 1700 µg 1^-1 and the corresponding figures in L. Kotojärvi 67 and 990 µg 1^-1, respectively. The mean chlorophyll a concentration was, however, higher in L. Kotojärvi (26 µg 1^-1) than in L. Villikkalanjärvi (20 µg 1^-1). This was probably due to an internal P load in L. Kotojärvi: in 1988 the internal load of dissolved P was estimated to be as much as twofold the external load. In L. Villikkalanjärvi the internal dissolved P load was only up to 50% of the external input. In L. Kotojärvi the high internal P load coupled with a low DIN:DIP ratio resulted in a strong blue-green algal bloom in the summer of 1988. In L. Villikkalanjärvi blue-green algae were observed only in small amounts. Even in August 1990, when the DIN:DIP ratio was low enough to favor the occurrence of blue-green algae, they contributed only up to 10–15% of the total phytoplankton biomass.     

Different responses to changes in phosphorus,P, among lakes. A study of slopes in chl-a = f(P) graphs

The least squares estimator of a linear regression coefficient _L will give an overall expression for the change in with x. In fresh water ecology, however, subgroups, % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiuaSGaci% 4Aaaaa!37BE!\[P\operatorname{k}\], of a parent population may have slopes which differ from the overall slope, _L. By constructing frequency histograms for the set of angles: Arctang % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaci4uaSGaam% yAaiaadQgaaaa!38AE!\[\operatorname{S} ij\],% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaci4uaSGaam% yAaiaadQgaaaa!38AE!\[\operatorname{S} ij\]= para sa y and x% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaiikaiaadM% faliaadMgakiabgkHiTiaadMfaliaadQgakiaacMcacaGGVaGaaiik% aiaadIhaliaadMgakiabgkHiTiaadIhaliaadQgakiaacMcaaaa!42F0!\[(Yi - Yj)/(xi - xj)\], i < j, % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiEaSGaam% yAaOGaeyiyIKRaamiEaSGaamOAaaaa!3BAB!\[xi \ne xj\], peaks in the distribution may be identified and related to ecological phenomenon. To identify peaks we fit Gaussian distributions to the frequency histograms. For a set consisting of 142 observations of chlorophyll-a and total phosphorus (nutrient) concentrations (TP) from 16 lakes we found four Gaussian peaks corresponding to four subgroups, % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiuaSGaci% 4Aaaaa!37BE!\[P\operatorname{k}\]k = 1,4. One group identified a response of chl-a to changes in TP which correspond approximately to the average slope found by least square regression (the slope was 0.49). The second group consisted of steeper response than the average (1.28). A third group showed that there is an enhanced proportion of cases where chl-a does not respond to TP (zero slope, all the three deep lakes > 10 m, included in the date set contributed to this group). The size of the last group, spanning a wide range of slopes, suggested that about 30% of the inter annual changes in chl-a is unrelated to TP. The results are compared to result obtained by simple least squares regression and to the Theil non-parametric slope estimator.     

EFFECTS OF FLOWING WATER ON NITROGEN- AND PHOSPHORUS-LIMITED PHOTOSYNTHESIS AND OPTIMUM N:P RATIOS BY SPIROGYRA FLUVIATILIS (CHAROPHYCEAE)1,2

The effects of flowing water on net photosynthesis, dark respiration, specific growth rate, and optimum N:P ratios by Spirogyra fluviatilis Hilse were assessed. The alga was cultivated under nitrogen or phosphorus limitation in laboratory streams at three flow velocities: 3, 12, and 30 cm·s^?1. The Droop equation adequately described respiration and photosynthesis (PS_net) as a function of N or P cell quota (Q^N or Q^p). The data show that for N- or P-limited Spirogyra fluviatilis, flowing water is physiologically costly. Generally, flowing water had little effect on respiration rates; however, the proportion of gross photosynthesis devoted to dark respiration did increase with flow velocity. For photosynthesis, the minimum N and P cell quotas increased with velocity, and the theoretical PS_net maxima for N and P both appeared greatest at 12 cm·s^?1. The Droop models showed that for any given Q^N or Q^p, PS_net, was reduced by the 30-cm·s^?1 treatment. Consistent with this finding, independent estimates of specific growth rates for P-limited S. fluviatilis in the laboratory streams were inversely related to flow velocity when ambient PO₄^?3 was undetectable. However, growth was not diminished at the fastest velocity when PO₄^?3 was available for uptake. Thus, the increase in cellular phosphorus demand can be offset by flow-enhanced P uptake when conditions permit; otherwise, growth will be impaired. The optimum N:P ratios for S. fluviatilis at 3, 12, and 30 cm·s^?1 were 50, 58, and 52 by atoms, respectively, when calculated for PS_net= 0. The optimum ratios were inversely related to PS_net and decreased to approximately 20 when PS_net was near maximum. The potential for flowing water to mediate nutrient partitioning among lotic algae by altering growth rates and optimum nutrient ratios is discussed.