Aboveground Plant Material Research Articles

Goat Urine and Limestone Affect Nitrogen and Cation Distributions in an Acidic Grassland

Prior surface application of limestone may influence soil changes resulting from high rates of urine deposition occurring where goats (Capra aegagrus hircus L.) congregate. A quantity of limestone equivalent to 0 and 6720 kg ha−1 was surface applied to vegetated 45‐cm‐deep columns of a Typic Hapludult soil collected from an abandoned Fescue spp. grassland in southern West Virginia. Eighteen weeks after lime application, one, two, or three applications of urine supplying a total of 36, 98, or 177 g m−2 of N, respectively, were made. One and two applications of urine increased vegetative growth and decreased the amount of water leaching through the column, but the third addition damaged plants. The three‐addition treatment reduced the amount of N taken up by the plants, decreased transpiration, increased leachate volume, and resulted in a 12‐fold increase in the amount of N leached from the columns compared with the one‐application treatment. Leachate Ca, Mg, and K were increased by urine. Soil pH and extractable Ca and Mg were decreased by urine and increased by limestone, and extractable Al was increased by urine and decreased by limestone applications. Net amounts of N recovered as NH3 gas released to the atmosphere, N taken up into aboveground plant material, NH4+ and NO3− extracted from soil, and NH4+ and NO3− in drainage water ranged from 49 to 77% of the amount added. Surface application of limestone alleviated some of the detrimental effects of high rates of urine addition by increasing levels of pH, Ca, and Mg and reducing Al as deep as 28 cm in the profile.

Reproductive strategy of an invasive thistle: effects of adults on seedling survival

Invasive species are known for their ability to form monocultures that exclude native species, yet intraspecific interactions among invasives have not been well studied. Cynara cardunculus (L.) is an invasive perennial thistle that establishes high-density populations in coastal California grasslands. We examined the natural distribution of C. cardunculus seedlings in an established population and found that nearly 100% of seedlings grew within 2 m of adults despite an expected distribution peak at 3 m from source plants based on measured dispersal distances. We then investigated the role of mature plants in seedling survival and establishment with regard to live vegetation, litter, and seedling distance by planting seedlings at increasing distances around adults and applying removal treatments to the focal adult rosettes. We applied control (no removal), adult rosette removal (live leaves), litter removal (dead leaves), and adult rosette plus litter removal (all aboveground plant material) treatments. Seedlings experienced a higher rate of survival, measured by senescence date, and establishment, measured by return rate the following year, with all adult rosette removal treatments. Inhibition by adult rosettes was reduced with distance to 60–80 cm from the rosette, and there was little effect of adult plants between 80 and 200 cm. These results suggest that adult rosettes may inhibit conspecific seedlings at very close distances but provide a favorable environment for seedlings within nearby interspaces. This pattern may contribute to the creation and maintenance of high-density populations in C. cardunculus. Land managers seeking to control this species may improve long-term effectiveness by expanding management efforts to include a 2 m radius around adult plants and treating within 5 months of seedling emergence to prevent recruitment rather than treating adults alone.

Islands of fertility induce co-occurring negative and positive plant-soil feedbacks promoting coexistence

Positive plant-soil feedback by “ecosystem engineers” is an important driver for the structuring and organization of resource-limited ecosystems. Although ample evidence demonstrates that plant-soil feedbacks can range from positive to strongly negative, their co-occurrence in plant communities have not yet been investigated. We test the hypothesis that the plant-soil feedback generated by the nitrogen-fixer shrub Medicago marina during primary succession in a sand dune community has a positive effect on the coexisting grass Lophochloa pubescens and a negative effect on the shrub species itself. We conducted field measurements and laboratory bioassays to evaluate (1) the effects of islands of fertility on the recruitment and growth of its ecosystem engineer and on the performance of a coexisting species and (2) the mechanisms involved that can explain the opposite effects of islands of fertility on coexisting species. Islands of fertility were present under Medicago crowns evidenced by higher available nitrogen, extractable phosphorus and potassium, organic matter, microbial activity, water holding capacity, soil humidity, and lower salt concentrations. The effects of these islands of fertility were clearly species-specific, with a facilitative impact on Lophochloa and a negative effect on Medicago recruitment. Lophochloa was denser and produced more biomass when rooted inside as compared to outside the crown area of the shrub. Contrarily, the number of seedlings of Medicago was lower inside, despite the higher seed abundance, and higher outside the crown area of adult shrubs as compared to predictions based on random distribution, thus showing a Janzen-Connell distribution. Laboratory experiments demonstrate the occurrence of Medicago negative plant-soil feedback, and that the auto-toxicity of the aboveground senescent plant material is a potentially important underlying mechanism explaining this negative feedback and the resulting Janzen-Connell distribution in the field.

Split nitrogen application strategies in seed production of two contrasting cultivars of timothy (Phleum pratense L.)

Split nitrogen (N) application to seed crops of timothy (Phleum pratense L.) was evaluated in 20 field trials, 11 in the south-Norwegian cultivar ‘Grindstad’ (origin 59°N) and nine in the north Norwegian cultivar ‘Vega’ (origin 67°N) in southeast (58–61°N) and central (63–64°N) Norway during 1998–2000. Three N rates (25, 50 or 75 kg N ha−1) at the start of growth were combined with four N rates (0, 25, 50 or 75 kg N ha−1) at the start of tiller elongation (Z 31). On average for all trials, the highest seed yield was obtained when a total rate of 75 kg N ha−1 was split into a main application of 50 kg N ha−1 at the start of growth and a supplemental application of 25 kg N ha−1 at Z 31. In both cultivars, the optimal N rate at Z 31 was uncorrelated with soil mineral N (NO3+NH4) in early spring and only weakly correlated (r 2=0.16–0.17, p% = 2–5) with the N concentration of aboveground plant material at Z 31. In ‘Grindstad’, a better correlation (r 2=0.46, p% < 0.1) was obtained between the optimal N rate at Z 31 and measurements by the hand-held chlorophyll meter Hydro N-Tester™ on the last fully developed leaf. The equation (HTN model) Optimal N input (kg N ha −1 ) at Z 31 = − 0.37×measured HNT value at Z 31+156 was further evaluated in ten on-farm trials during 2001–2002. Compared to the fixed N levels 0, 25 or 50 kg N ha−1, the HNT model predicted optimal N rates at Z 31 in 2002, but not in 2001, when less rainfall caused less lodging than in 1998–2000 and 2002. Nevertheless, we conclude: 1) that the spring N rates to timothy seed crops should be split into an early and a late application, and 2) that the HTN model will help in adjusting the second N rate at Z 31 in ‘Grindstad’.

Effects of Oxalis pes-caprae L. invasion in olive groves

The present work aimed at studying the effect of Oxalis pes-caprae invasion in the herbaceous understory of olive groves in the island of Lesvos, Greece. The number of species, their biomass and dead plant material production was followed throughout the vegetative period in invaded and non-invaded plots. Species richness in the area invaded by O. pes-caprae gradually declined and was significantly smaller in comparison with the natural vegetation. Although biomass was equal in the invaded and non-invaded areas 1 month after germination, annual net primary production (NPP) was three times smaller in the invaded than the non-invaded area. Moreover, aboveground dead plant material of O. pes-caprae decomposed faster than a mixture of litter species of the natural vegetation. Lower biomass production in combination with increased decomposition rates of O. pes-caprae is expected to lead to reduced soil C sequestration in invaded areas. It was concluded that although O. pes-caprae does not add a new life form or functional type to the invaded herbaceous system it has considerable impacts on the diversity and ecosystem functions in olive groves.

Winter losses of nitrogen and phosphorus from Italian ryegrass, meadow fescue and white clover in a northern temperate climate

Abstract We have studied to what degree Italian ryegrass (Lolium multiflorum Lam.), white clover (Trifolium repens L.) and meadow fescue (Festuca pratensis L.) are able to preserve nitrogen (N) and phosphorous (P) in shoots and roots from one growing season to the next in a northern temperate climate. Field experiments were performed during four consecutive winters in central southeast Norway (60°42′N, 10°51′E), and N and P in plant biomass were measured in the autumn and in the spring. We also measured the contents of total N, total P and organic carbon (C) in seepage water that percolated through the aboveground plant material. Uptake of N and P in Italian ryegrass and white clover was substantially larger than in meadow fescue. The winter losses varied greatly from year to year, depending on the winter climate. On the average for all three of the plant species, the winter losses of N from aboveground biomass were 6, 35, 68 and 10% in the four experimental years, respectively. The corresponding P losses were 11, 36, 60 and 22%. On the average for all plant species and experimental years, 43 (±12)% (S.E., n = 12) of the N, 34 (±9)% of the P and 4 (±1)% of the C that was lost from the aboveground plant biomass during the winter, was recovered in seepage water, basically as a nutrient pulse in melt water in early spring. The very low C recovery rate in seepage water suggested a considerable microbial growth on lost plant C. Assuming that all un-recovered plant C was consumed by microorganisms not included in measurements of the seepage water, modelling showed that microbial immobilisation theoretically might explain the unexpectedly low recovery rates of N and P. The study was not designed to investigate the possible effects of psychrophilic microbes on N and P cycling. Therefore, it is inconclusive and underlines the need for more knowledge on this matter.

Loss of forb diversity in relation to nitrogen deposition in the UK: regional trends and potential controls

AbstractIn this study we investigate the impact of nitrogen (N) deposition on the diversity of three different vegetation functional groups – forbs, grasses and mosses – using a field survey of acid grasslands across Great Britain. Our aim is to identify the vegetation types that are most vulnerable to enhanced N deposition, and to shed light on the mechanisms that may be driving N‐initiated species changes in the UK. Sixty‐eight randomly selected grasslands belonging to the UK National Vegetation Classification group U4 (Festuca ovina–Agrostis capillaris–Galium saxatile grassland) were studied along a gradient of atmospheric N deposition ranging from 6 to 36 kg N ha−1 yr−1. At each site, vegetation was surveyed and samples were taken from the topsoil and subsoil. Aboveground plant material was collected from three species: a forb, grass and moss. Both the species richness and cover of forbs declined strongly with increasing N deposition, from greater than eight species/20% cover per m2 quadrat at low levels of N to fewer than two species/5% cover at the highest N deposition levels. Grasses showed a weak but significant decline in species richness, and a trend toward increasing cover with increasing N input. Mosses showed no trends in either species richness or cover. Most of the decline in plant species richness could be accounted for by the level of ammonium deposition. Soil KCl‐extractable ammonium concentration showed a significant positive correlation with N input, but there was no relationship between N deposition and extractable nitrate. In the soil O/A horizon, there was no relationship between N deposition and %N, and only a very weak positive relationship between the level of N deposition and the C : N ratio. Finally, in the vegetation, there was no relationship between N deposition and either shoot tissue N concentration or N : P ratio for any of the three reference species. Combining our regional survey with the results of published N‐addition experiments provides compelling evidence that there has been a significant decline in the species richness and cover of forbs across Great Britain, and that the primary cause is competition due to an increase in the cover of grasses in response to enhanced deposition of reactive N, primarily NH4+.

INVASIBILITY OF ROADLESS GRASSLANDS: AN EXPERIMENTAL STUDY OF YELLOW STARTHISTLE

Roadless habitats are commonly found to be less invaded than habitats near roads, but few studies have tested whether this pattern is due to propagule limitation or to greater invasion resistance of roadless sites. We examined reasons for the lower frequency and cover of yellow starthistle (Centaurea solstitialis) in grassland sites >1000 m vs. 10 m from roads in an inland California, USA, foothill landscape. During winter 2001 and 2002, we planted 100 Centaurea seeds in 64 pairs of 30 x 30 cm plots (uncleared and cleared of aboveground plant material) at sites stratified by distance from roads (10 m and >1000 m), soil type (nonserpentine and serpentine), and aspect (cool, warm, and neutral slopes). In nonserpentine grasslands, Centaurea survival was greater in uncleared (but not cleared) near (10 m) plots than in distant (>1000 m) plots. These findings suggest that the effect of distance from roads on survival corresponds with higher aboveground biotic resistance in distant sites than in near sites. Centaurea biomass was greater in near than in distant plots (both uncleared and cleared) on nonserpentine soil, suggesting that either abiotic resistance or belowground biotic resistance could limit its growth in distant sites. These distance effects were no longer significant in models that included two biotic co- variates: native grass cover (which was higher in distant sites and negatively correlated with Centaurea performance) and bare ground (which was lower in distant sites and pos- itively correlated with Centaurea performance). On serpentine soil, there was no effect of road proximity on the performance of planted Centaurea. Our results suggest that Centaurea seeds can germinate in nonserpentine and serpentine grasslands regardless of distance from roads. Beyond the seedling stage, however, biotic resistance associated with higher native grass cover and lower levels of disturbance may inhibit Centaurea invasion of nonserpentine grassland sites that are distant from roads.

Plant Functional Group Diversity as a Mechanism for Invasion Resistance

Abstract A commonly cited mechanism for invasion resistance is more complete resource use by diverse plant assemblages with maximum niche complementarity. We investigated the invasion resistance of several plant functional groups against the nonindigenous forb Spotted knapweed (Centaurea maculosa). The study consisted of a factorial combination of seven functional group removals (groups singularly or in combination) and two C. maculosa treatments (addition vs. no addition) applied in a randomized complete block design replicated four times at each of two sites. We quantified aboveground plant material nutrient concentration and uptake (concentration × biomass) by indigenous functional groups: grasses, shallow‐rooted forbs, deep‐rooted forbs, spikemoss, and the nonindigenous invader C. maculosa. In 2001, C. maculosa density depended upon which functional groups were removed. The highest C. maculosa densities occurred where all vegetation or all forbs were removed. Centaurea maculosa densities were the lowest in plots where nothing, shallow‐rooted forbs, deep‐rooted forbs, grasses, or spikemoss were removed. Functional group biomass was also collected and analyzed for nitrogen, phosphorus, potassium, and sulphur. Based on covariate analyses, postremoval indigenous plot biomass did not relate to invasion by C. maculosa. Analysis of variance indicated that C. maculosa tissue nutrient percentage and net nutrient uptake were most similar to indigenous forb functional groups. Our study suggests that establishing and maintaining a diversity of plant functional groups within the plant community enhances resistance to invasion. Indigenous plants of functionally similar groups as an invader may be particularly important in invasion resistance.

The impact of recent increases in atmospheric CO2 on biomass production and vegetative retention of Cheatgrass (Bromus tectorum): implications for fire disturbance

AbstractCheatgrass (Bromus tectorum) is a recognized, invasive annual weed of the western United States that reduces fire return times from decades to less than 5 years. To determine the interaction between rising carbon dioxide concentration ([CO2]) and fuel load, we characterized potential changes in biomass accumulation, C : N ratio and digestibility of three cheatgrass populations from different elevations to recent and near‐term projections in atmospheric [CO2]. The experimental CO2 values (270, 320, 370, 420 μmol mol−1) corresponded roughly to the CO2 concentrations that existed at the beginning of the 19th century, that during the 1960s, the current [CO2], and the near‐term [CO2] projection for 2020, respectively. From 25 until 87 days after sowing (DAS), aboveground biomass for these different populations increased 1.5–2.7 g per plant for every 10 μmol mol−1 increase above the 270 μmol mol−1 preindustrial baseline. CO2 sensitivity among populations varied with elevational origin with populations from the lowest elevation showing the greatest productivity. Among all populations, the undigestible portion of aboveground plant material (acid detergent fiber ADF, mostly cellulose and lignin) increased with increasing [CO2]. In addition, the ratio of C : N increased with leaf age, with [CO2] and was highest for the lower elevational population. These CO2‐induced qualitative changes could, in turn, result in potential decreases in herbivory and decomposition with subsequent effects on the aboveground retention of cheatgrass biomass. Overall, these data suggest that increasing atmospheric [CO2] above preambient levels may have contributed significantly to cheatgrass productivity and fuel load with subsequent effects on fire frequency and intensity.

Use of near infrared reflectance spectroscopy to predict nitrogen uptake by winter wheat within fields with high variability in organic matter

In this study, the ability to predict N-uptake in winter wheat crops using NIR-spectroscopy on soil samples was evaluated. Soil samples were taken from unfertilized plots in one winter wheat field for three years (1997–1999) and in another winter wheat field nearby for one year (2000). Soil samples were analyzed for organic C content and their NIR-spectra. N-uptake was measured as total N-content in aboveground plant materials at harvest. Models calibrated to predict N-uptake were internally cross-validated and validated across years and across fields. Cross-validated calibrations predicted N-uptake with an average error of 12.1 to 15.4 kg N ha−1. The standard deviation divided by this error (RPD) ranged between 1.9 and 2.5. In comparison, the corresponding calibrations based on organic C alone had an error from 11.7 to 28.2 kg N ha−1 and RPDs from 1.3 to 2.5. In three of four annual calibrations within a field, the NIR based calibrations worked better than the organic C based calibrations. The prediction of N-uptake across years, but within a field, worked slightly better with an organic C based calibration than with a NIR based one, RPD = 1.9 and 1.7, respectively. Across fields, the corresponding difference was large in favour of the NIR-calibration, RPD = 2.5 for the NIR-calibration and 1.5 for the organic C calibration. It was concluded that NIR-spectroscopy integrates information about organic C with other relevant soil components and therefore has a good potential to predict complex functions of soils such as N-mineralization. A relatively good agreement of spectral relationships to parameters related to the N-mineralization of datasets across the world suggests that more general models can be calibrated.

Short‐term N2O, CO2, NH3 fluxes, and N/C transfers in a Danish grass‐clover pasture after simulated urine deposition in autumn

AbstractUrine patches are significant hot‐spots of C and N transformations. To investigate the effects of urine composition on C and N turnover and gaseous emissions from a Danish pasture soil, a field plot study was carried out in September 2001. Cattle urine was amended with two levels of 13C‐ and 15N‐labeled urea, corresponding to 5.58 and 9.54 g urea‐N l–1, to reflect two levels of protein intake. Urine was then added to a sandy‐loam pasture soil equivalent to a rate of 23.3 or 39.8 g urea‐N m–2. Pools and isotopic labeling of nitrous oxide (N2O) and CO2 emissions, extractable urea, ammonium (NH4+), and nitrate (NO3–), and plant uptake were monitored during a 14 d period, while ammonia (NH3) losses were estimated in separate plots amended with unlabeled urine. Ammonia volatilization was estimated to account for 14% and 12% of the urea‐N applied in the low (UL) and high (UH) urea treatment, respectively. The recovery of urea‐derived N as NH4+ increased during the first several days, but isotopic dilution was significant, possibly as a result of stress‐induced microbial metabolism. After a 2 d lag phase, nitrification proceeded at similar rates in UL and UH despite a significant difference in NH4+ availability. Nitrous oxide fluxes were low, but generally increased during the 14 d period, as did the proportion derived from urea‐N. On day 14, the contribution from urea was 23% (UL) and 13% (UH treatment), respectively. Cumulative total losses of N2O during the 14 d period corresponded to 0.021% (UL) and 0.015% (UH) of applied urea‐N. Nitrification was probably the source of N2O. Emission of urea‐derived C as CO2 was only detectable within the first 24 h. Urea‐derived C and N in above‐ground plant material was only significant at the first sampling, indicating that uptake of urine‐C and N via the leaves was small. Urine composition did not influence the potential for N2O emissions from urine patches under the experimental conditions, but the importance of site conditions and season should be investigated further.

Cropping frequency, wheat classes and flexible rotations: Effects on production, nitrogen economy, and water use in a Brown Chernozem

Producers in the semiarid Canadian prairies rely on frequent summerfallowing (F) to conserve water, control weed infestations, and maximize soil mineral N reserves, but this practice often results in soil degradation. A crop rotation experiment was initiated in 1987 on a medium-textured, Orthic Brown Chernozem at Swift Current, Saskatchewan, to determine the most ideal cropping frequency for wheat in this region and whether a fixed rotation such as fallow-wheat (Triticum aestivum L.) - wheat (F-W-W) or F-W-W-W would be more effective than flexible rotations in which fallowing is decided each spring based on criteria such as available soil water (if water), or the need to control perennial weed infestations (if weeds). The study also compared the production of traditional Canada Western Red Spring (CWRS) wheat class with the newer higher-yielding (Hy), Canada Prairie Spring (CPS) wheat class. We analyzed results of six rotations over the first 12 yr of the study. The rotations included F-W-W, F-W-W-W, F-Hy-Hy, Continuous wheat (Cont W), Cont W (if weeds), and Cont W (if water). Reduced tillage management was used and stubble was cut tall to enhance snowtrap. Fertilizer N was applied based on soil tests and fertilizer P was applied based on the general recommendations for the region. Over the 1988–1999 period, weather conditions were generally favourable and yields were above average for this region. Canada Prairie Spring wheat outyielded CWRS by 32% when grown on fallow and by 17% when grown on stubble; however, straw yields of the two wheat classes were similar on fallow and CPS was 11% less than CWRS on stubble. Harvest index (HI) averaged 44% for CPS and 37% for CWRS wheat. Water use efficiency for CWRS wheat grown on fallow averaged 7.2 kg ha-1 mm-1 and for CPS 9.4; when grown on stubble the respective values were 6.3 and 7.5 kg ha-1 mm-1. Grain N concentration for CWRS was slightly higher for wheat grown on fallow (25.7 g kg-1) than on stubble (24.5 g kg-1), but was similar for CPS wheat on grown on fallow and stubble (21.9 g kg-1). Straw N concentration averaged 3.8 g kg-1 for CWRS and 4.4 g kg-1 for CPS. Nitrogen yield for grain from CPS was 9% greater than from CWRS when grown on fallow, but there was no effect of wheat class when grown on stubble. Nitrogen yield of CPS straw was 15% greater than for CWRS when grown on fallow, but on stubble N yield was generally not affected by wheat class. Nitrogen harvest index (NHI) averaged about 80% for both wheat classes, whether grown on fallow or stubble. On a rotation basis, grain produced with F-W-W was 1502 kg ha-1 yr-1. The F-W-W-W and Cont W (if weeds) rotations produced 9% more grain than F-W-W, while Cont W (if water) produced 24% more, F-Hy-Hy produced 26% more, and Cont W produced 30% more than F-W-W. Nitrogen production in the grain, straw and aboveground plant material was lowest in F-W-W, highest in Cont W, and intermediate for other rotations. Although the economic and soil quality assessments have yet to be completed, a preliminary conclusion based on crop production characteristics alone suggests that a flexible cropping system in which available soil water in spring is used as the determining criterion is superior to a fixed F-W-W or F-W-W-W rotation. Key words: Yield, N concentration, N yield, water deficit, wheat classes, regressions

Floating row covers affect Pb and Cd accumulation and antioxidant status in Chinese cabbage

In 1993 and 1994, crop covering treatments [perforated polyethylene (PO, 500 holes/m 2) and a polypropylene (AO, Agryl-P17)] were used to produce different environmental conditions for the growth of Brassica pekinensis (Lour) Rupr cv. Nagaoka 50. Plant material was sampled five times from transplanting to harvest, and the amounts of chlorophylls, peroxidase activity, biomass, and Pb and Cd accumulation in the above-ground plant material were measured. Uncovered control plants, received lower air and root temperatures as well as lower relative humidity, and showed the highest chlorophyll a, chlorophyll b, carotene and peroxidase activity values. Under floating row covers (PO and AO) the improved thermal conditions resulted in increased fresh and dry mass and accumulation of Pb and Cd. It is also important to consider that because of this accumulation these plants may pose a risk for human health through Pb and Cd contamination of food supplies.

Chemically-induced uptake of gold by root crops: Its significance for phytomining

Five root crops (carrot, red beet, onion and two cultivars of radish) were grown in an artificial substrate consisting of 3.8 mg/kg (ppm) of elemental gold dispersed in sand. The possibility of using these crops for phytomining for gold was determined by separately adding the chelating agents ammonium thiocyanate and ammonium thiosulfate at respective rates of 1 and 2 g/kg to the substrate after a growing period of 9 weeks. The plants were harvested 10 days later and analysed for gold by atomic absorption spectrometry. In most cases, gold concentrations were higher in the roots than in the above-ground plant material. The highest mean gold concentrations were found in carrot roots (48.3 mg/kg dry weight), and roots of two radish cultivars (113 and 102 mg/kg dry weight). Because of the very high biomass yield of carrot (16 t/ha — dry weight), this plant showed the best potential for phytomining for gold. After allowing for reduced biomass yields over mine tailings, and taking into account the cost of the chelates as well as agrarian expenses and the cost of burning the crop, it was concluded that there was some potential of use of carrot to grow an economic crop of gold from mine tailings. None of the other four plants had potential for this purpose.

Effect of nitrogen application on concentration of cadmium and nutrient ions in soil solution and in durum wheat

A growth chamber experiment was conducted to study the effect of nitrogen fertilizer on the chemical composition of the soil solution over time, and to determine Cd uptake as a function of rates of nitrogen fertilizer application and transpiration. Sceptre durum wheat was grown in a fine sandy loam soil, in pots with treatments of 0, 50, 100, 200, 400, and 800 µg N g−1 as urea. The soil solution was removed by water displacement and analyzed for Cd and other nutrient ions at time of seeding, 10, 20, 30, and 40 d after seeding, and at the time of crop maturity. Soil samples were analyzed at each sampling time, and aboveground plant material was also harvested at these times and analyzed for Cd and other nutrient ions. Behaviour of Cd was compared to that of the nutrient ions to gain a better understanding of patterns of ion behaviour. Conductivity measurements were taken as estimators of ionic strength. Both solution Cd concentration and DTPA-extractable soil Cd increased significantly with increasing nitrogen rate. The increases in Cd concentration with N fertilization were greatest immediately after fertilization and appeared to be related to an increase in soil and solution conductivity, with pH also having an influence on DTPA-extractable Cd. The soil solution concentration of all nutrients, with the exception of phosphate, increased with N rate. The effects of N fertilization on the amounts of extractable nutrients in the soil were more variable. Plant Cd concentrations increased with increasing N application rate to 800 µg g−1, but dry matter yield and transpiration only increased with N rate to 200 µg g−1. Cadmium was the element most affected by increasing the N rate, and there were minor changes in uptake of other elements such as N, P, K, Ca, Mg, Mn, Zn, and Cu with N application rate and time. This study clearly illustrated the effect of nitrogen fertilization on the concentration of Cd in the soil solution and its uptake by durum wheat. Key words: Cadmium, nitrogen, urea, soil solution, ionic strength, durum, uptake

Photosynthesis and carbon balance of a Sahelian fallow savanna

AbstractEddy‐covariance measurements of CO2 exchange above a Sahelian savanna consisting of small shrubs over a near‐continuous herb layer were made during the HAPEX‐Sahel experiment in Niger, West Africa. The measurements were made near‐continuously during an 8‐week period, covering the main part of the rainy season and three weeks at the beginning of the dry season. The measurements were corrected for in‐canopy storage of CO2 and the night‐time measurements used to derive respiration functions for the soil, roots and above‐ground plant material. Photosynthetic CO2 uptake was estimated and compared to simulations using a biochemical photosynthesis model in a simple, ‘big‐leaf’, implementation. The model satisfactorily reproduced the measurements (coefficient of determination 0.80) using parameters defined from the literature and based on soil nutrient concentrations. When the quantum yield (α) and rubisco capacity (Vmr) were fitted to the data with allowance for physiological changes through the season, an excellent agreement between model and measurements was obtained (coefficient of determination 0.93, RMS error 1.46 μmol m–2 s–1).The fitted photosynthesis and respiration model was used to estimate the carbon balance of the savanna site during the growing season of 1992 and for the complete calendar year. Harvest estimates of net plant biomass accumulation during the growing season and annual wood accumulation agreed well with modelled net photosynthesis and annual net carbon accumulation, respectively. Peak instantaneous ecosystem CO2 uptake was comparable to peak values observed in other biomes, but annual photosynthesis and carbon sequestration were considerably lower than observed elsewhere.

Impacts and fate of experimentally enhanced nitrogen deposition on a British lowland heath

Between 1989 and 1996, nitrogen, in the form of (NH 4) 2SO 4 (7.7 or 15.4 kg ha −1 year −1), was added to experimental plots of Calluna at a lowland dry heath in the south of England. Background deposition at this site was estimated at between 13–18 kg N ha −1 year −1, with experimental additions therefore taking total deposition slightly above the proposed critical nitrogen load for lowland heaths. A destructive harvest, carried out in December 1996, revealed large increases in above-ground growth and litter production in response to nitrogen treatments. Plant, litter and soil nitrogen concentrations were also significantly increased. Higher levels of soil decomposer activity, together with faster turnover of litter in nitrogen-treated plots suggest an effect of treatment additions on rates of internal nitrogen cycling. A nitrogen budget for the site revealed that only a relatively small proportion (14–18%) of the nitrogen additions had been accumulated in plant biomass. A similarly small proportion (10–15%) of the extra nitrogen was stored in the litter layer. Since leaching and denitrification losses were minimal, the bulk of the experimental additions were therefore accumulated within the soil compartment. Although no changes in species composition were seen during this experiment, effects on internal nutrient cycling and soil nitrogen status could be expected to have an impact on the outcome of competition between regenerating Calluna and grass seedlings. The results indicate that use of the standard management practice at this site, involving cutting and removal of only above-ground plant material, whilst essential for the maintenance of a mixed-age Calluna sward, will have only a relatively small impact on nitrogen accumulation within this system. The importance of management in modifying the impact of enhanced nitrogen deposition is discussed in the context of critical loads for healthland systems in the UK.

Inactivation of mercury in contaminated soils using natural zeolites

The application of zeoliferous rocks, from Metaxades region (Thrace, Greece) to soils contaminated with mercury significantly reduced the concentrations of mercury in the shoots and roots of alfalfa ( Medicago sativa L.) and ryegrass ( Lolium perenne). Use of natural zeolites at application rates of 1%, 2% and 5% by soil weight caused reductions in mercury concentrations of up to 86.0% in shoots and 58.2% in roots, compared with controls with no added zeolites. The reductions are more pronounced in above-ground plant material than in roots of the two plant species. The use of natural zeolites, as soil additives, to reduce the uptake of mercury by plants and the restriction of the entry of mercury into the food chain, is noted.

Does nitrogen nutrition restrict the CO2 response of fertile grassland lacking legumes?

The extent of the response of plant growth to atmospheric CO2 enrichment depends on the availability of resources other than CO2. An important growth-limiting resource under field conditions is nitrogen (N). N may, therefore, influence the CO2 response of plants. The effect of elevated CO2 (60 Pa) partial pressure (pCO2) on the N nutrition of field-grown Lolium perenne swards, cultivated alone or in association with Trifolium repens, was investigated using free air carbon dioxide enrichment (FACE) technology over 3 years. The established grassland ecosystems were treated with two N fertilization levels and were defoliated at two frequencies. Under elevated pCO2, the above-ground plant material of the L. perenne monoculture showed a consistent and significant decline in N concentration which, in general, led to a lower total annual N yield. Despite the decline in the critical N concentration (minimum N concentration required for non-N-limited biomass production) under elevated pCO2, the index of N nutrition (ratio of actual N concentration and critical N concentration) was lower under elevated pCO2 than under ambient pCO2 in frequently defoliated L. perenne monocultures. Thus, we suggest that reduced N yield under elevated pCO2 was evoked indirectly by a reduction of plant-available N. For L. perenne grown in association with T. repens and exposed to elevated pCO2, there was an increase in the contribution of symbiotically fixed N to the total N yield of the grass. This can be explained by an increased apparent transfer of N from the associated N2-fixing legume species to the non-fixing grass. The total annual N yield of the mixed grass/legume swards increased under elevated pCO2. All the additional N yielded was due to symbiotically fixed N. Through the presence of an N2-fixing plant species more symbiotically fixed N was introduced into the system and consequently helped to overcome N limitation under elevated pCO2.