Soil-plant Cores Research Articles

Intraspecific Root Trait Variability Along Environmental Gradients Affects Salt Marsh Resistance to Lateral Erosion

Recent studies in salt marshes have demonstrated the role of plant roots in sediment stabilisation, and hence the importance of marshes in providing coastal protection. However, the relative role of root traits and environmental factors in controlling sediment stability, and how intraspecific variability of root traits vary within and among marshes, remain poorly understood. In this study, we investigated which root trait(s) drive sediment stability (resistance to lateral erosion) in two marsh species with an important role in coastal protection (Spartina anglica and Atriplex portulacoides) and how the environment affects the expression of these traits. We sampled three marshes along salinity gradients in each of two estuaries in Wales (UK), establishing replicate plots in the respective dominant zones of each species. In all plots we sampled abiotic variables (sand, redox potential, pH, salinity) and root traits (root density, specific root density, root volume, root length density); in a subset of these plots (three per species in each marsh/species/marsh) we extracted soil-plant cores and assessed their erosion resistance in a flume. Sediment stability was enhanced by increases in root density and reductions in sand content. Abiotic variables affected root density in different ways depending on species: in S. anglica, redox was the only significant factor, with a positive, linear effect on root density; in A. portulacoides, redox had a non-linear (U-shaped) effect on root density, while sand had a negative effect. Collectively, these results show that i) intraspecific variability in root density can influence sediment stability in salt marshes, and ii) sediment properties not only influence sediment stability directly, but also indirectly via root density. These results shed light on spatial variability in the stability of salt marshes to lateral erosion and suggest that root density should be incorporated into coastal vegetation monitoring programs as an easy-to-measure root trait that links the environment to sediment stability and hence to the function and services provided by marshes.

Role of N 2-fixation in the sustainability of the ponded grass pasture system

Ponded pastures in northern Australia produce green fodder in the seasonally dry winter period and may be employed to reduce grazing pressure on dryland pastures. The soils under ponded pasture, currently 26,000 ha in Queensland, are inherently infertile. This study was conducted to determine if a non-symbiotic association between bacteria and grass roots was responsible for the supply of N to ponded grasses. Intact soil-plant cores were obtained from a new ponded pasture of Aleman ( Echinochloa polystachya) and an 8-year-old pasture of Hymenachne ( Hymenachne amplexicaulis). Nitrogenase (N 2-ase) activity was measured using the acetylene reduction assay and bacteria were selectively isolated to N-free malate medium from root segments of the most active plants. N 2-ase activity of the intact soil-plant cores ranged from 76 to 380 g N ha −1 d −1 for the Aleman pasture and from 5 to 179 g N ha −1 d −1 for the Hymenachne pasture. Assays on excised roots showed the greatest activity on adventitious roots formed on the submerged nodes of Hymenachne stems. No major differences in colony morphology were detected in N 2-fixing bacteria isolated from the roots of the two grasses. An association appeared to exist between bacteria and the roots of both grasses with most of the N for the young Aleman pasture being fixed N, whereas the fixed N supply for the older Hymenachne pasture was supplemented by the mineralization of organic N.

Environmental regulation of nitrogen fixation in a high arctic lowland ecosystem

This study examined spatial and temporal variation in cyanobacterial nitrogen fixation and the environmental regulation of this variation at Truelove Lowland, Devon Island, N.W.T. Acetylene reduction rates of soil–plant cores from a variety of plant communities were measured under uniform conditions of light and temperature during the 1987 and 1988 growing seasons. Concurrent measurements of soil moisture and extractable nitrogen and phosphorus were also made. Effects of temperature, moisture, and phosphorus were examined in manipulative experiments. Acetylene reduction rates were highest in brackish environments, intermediate in mesic to hydric terrestrial and in aquatic communities, and lowest on xeric beach ridges. Rates generally increased during early season, then decreased through mid to late season. Among the three parameters examined, rates were most highly correlated to soil moisture. The temperature optimum for fixation was near 20 °C. There was a strong, but reversable, depression in acetylene reduction in response to experimental desiccation, and weekly phosphorus fertilization had a strong positive effect on fixation rates. The highest fixation rates along the marine shoreline were associated with high phosphorus input from marine algae and greater biomass of cyanobacteria. Because of the overriding importance of moisture, changes in nitrogen and phosphorus levels accompanying ecosystem development do not appear to strongly control nitrogen fixation in terrestrial, nonbrackish sites in this polar desert oasis. Key words: nitrogen fixation, environmental regulation, Devon Island, High Arctic, Nostoc commune, cyanobacteria.

Some observations on the oxygen sensitivity and seasonal variations of the nitrogenase activity associated with excised soil-plant cores of Phragmites australis (Cav.) Trin.

The sensitivities of Phragmites soil-plant core and litter nitrogenase activity were assayed by the acetylene reduction assay method. Acetylene reduction activity (ARA) of soil-plant cores incubated under ambient O 2 concentration, increased with increasing pC 2H 2 up to 0.20 atm. A Km of 1.05 atm and V ax of 234 nmol C 2H 2g −1h −1 were calculated. The highest rates of ARA overall occurred in the 8–16 cm zone. Within the core profile, mean rates varied with depths: 0–8, 8–16 and 16–20 cm, and incubation pO 2. In the 0–8 cm zone, ARA at pO 2 0.0, 0.10 and 0.20 atm did not differ significantly, but in the 8–16 and 16–20 cm zones, the highest rates were obtained at pO 2 0.0 atm. A lag of approximately 5 h preceded linear rates of litter ARA in time course assays. Change from aerobic to anaerobic atmospheres and the reverse substantially reduced core and excised root ARA for a period. Core ARA increased with incubation temperature between 0 and 20°C; mean rates were the same at 20 and 25°C. ARA, numbers of diazotrophs, soil temperature, pH and Eh showed a seasonal pattern of variability. The highest rates of ARA were obtained between mid May and June; peak numbers of diazotrophs were counted between June and mid July. Variations in soil pH were slight, but the most obvious change (6.3–7.5) occurred when the soil was warmest and most reduced.

Factors affecting nitrogenase activity associated with marsh grasses and their soils from eutrophic lakes

The acetylene reduction activity (ARA) of soil-plant cores and intact, soil-free plant-root systems was used to study the influence of soil moisture content, diurnal cycles of temperature and light, and inorganic N and P on the nitrogenase activity (AR) associated with Phalaris arundinacea L. and Phragmites australis (Cav.) Trin. from two eutrophic lakes (lochs), Balgavies and Forfar. A positive correlation ( r = 0.81, n = 26) was established between AR and soil moisture content in individual soil-plant cores of Phalaris from Forfar Loch. Nitrogenase activity, soil moisture and NO 2 −-N increased with decreasing distance from lake water in Balgavies Loch. Diurnal fluctuations in AR, probably attributable to a combined effect of soil temperature and illumination changes, were observed under field conditions for Phalaris and Phragmites. Under laboratory conditions, the shading and cutting of Phalaris shoots did not inhibit ARA, which suggested that new photosynthates did not necessarily supply substrate for activity in the short term. Partial and temporary inhibition of ARA was obtained in dissected soil-plant cores after a single application of NH 4 +-N and NO 3 −-N (350 μg N g −1 fresh weight). At concentrations equivalent to 300 μg P g −1 fresh weight of Phalaris cores, PO 4 3−-P also caused partial and temporary inhibition of ARA of soil-free plant-root systems, but stimulated activity in intact simulated in situ systems.

Variation of 14 C‐labeled Photosynthate Recovery from Roots and Rooting Media of Warm Season Grasses 1

Variation of photosynthate supply to the grass rhizosphere has been hypothesized to explain variation of acetylene reduction rates among soilplant cores of grasses. Kleingrass (Panicum coloratum L.), pearl millet (Pennisetum americanum (L.) K. Schum.), and bahlagrass (Paspalum notatum Flugge) were grown hydroponically at concentrations of 56, 112, and 224 ppm to determine the proportion of photosynthate translocated to roots and recovered in nutrient solution or leachates from rooting media of 60-day-old plants. The proportion of photosynthate washed from roots in solution, associated with a fritted clay medium, and respired by roots and bacteria was also determined for hydroponically grown tetraploid and diploid bahiagrass genotypes. Photosynthate was radioactively pulse-labeled via photosynthetic assimilation of 14CO2 to quantify the proportion of photosynthate in bacteria and organic compounds separated from the roots. Bahiagrass translocated 7% of the 14C-labeled photosynthate to roots, compared to 12% for kleingrass and pearl millet. Of the 14C-labeled photosynthate translocated to roots, nearly 3 times more (1.1%) appeared in nutrient solution or leachates from the rooting medium of bahiagrass compared to kleingrass (0.4%) and pearl millet (0.4 %). The total of 14C washed from roots, associated with fritted clay, and respired comprised an average of 5.7 % of the 14C-labeled photosynthate recovered per plant from the two bahiagrass genotypes. Diazotrophlc colonization and N2 fixation appeared more probable for the bahiagrass than the other species because in bahiagrass a larger proportion of the photosynthate translocated to roots appeared in bacteria and organic compounds washed from roots at low N levels. However, the low proportion of photosynthate released from roots of the tetraploid and diploid genotypes indicated only a small amount of substrate was available in the rooting medium and rhizosphere during juvenility.

Nitrogen assimilation and dissimilation in five genotypes of Brachiaria spp.

Five genotypes of Brachiaria spp. were planted in a field experiment with and without nitrate fertilization. Nitrogen metabolism was evaluated by measurements of nitrate reductase (NR) activity in leaves, nitrate accumulation in stems, and nitrogenase activity and dentrification in intact soil–plant cores. There were differences between genotypes in all parameters and a tendency was observed for genotypes with high NR activity and nitrate accumulation to have low nitrogenase activity and vice versa. Brachiaria radicans (Tanner grass) was representative for the first type and B. ruziziensis (CPI 30623) for the second. Denitrification reached 7% of the applied N within 63 h and was lowest in Tanner grass and highest in B. brizantha (FL 902-4). Brachiaria ruziziensis (CPI 30623) plants were able to withstand N stress better than B. radicans as a consequence, possibly, of differences in nitrogenase activity.

Nitrogen assimilation and dissimilation in five genotypes of Brachiaria spp.

Five genotypes of Brachiaria spp. were planted in a field experiment with and without nitrate fertilization. Nitrogen metabolism was evaluated by measurements of nitrate reductase (NR) activity in leaves, nitrate accumulation in stems, and nitrogenase activity and dentrification in intact soil–plant cores. There were differences between genotypes in all parameters and a tendency was observed for genotypes with high NR activity and nitrate accumulation to have low nitrogenase activity and vice versa. Brachiaria radicans (Tanner grass) was representative for the first type and B. ruziziensis (CPI 30623) for the second. Denitrification reached 7% of the applied N within 63 h and was lowest in Tanner grass and highest in B. brizantha (FL 902-4). Brachiaria ruziziensis (CPI 30623) plants were able to withstand N stress better than B. radicans as a consequence, possibly, of differences in nitrogenase activity.

Seasonal variation in the nitrogenase activity of aPanicum maximum var.trichoglume pasture and identification of associated bacteria

The influence of seasonal variation on nitrogenase (N2-ase) activity of undisturbed soil-plant cores ofPanicum maximum var.trichoglume was measured using the C2H2 reduction assay. The largest N2-ase activity in the field, 14.7 g N ha−1 day−1, occurred in spring when soil moisture was high, soil temperature was low and nitrogenous fertiliser influence was at a minimum. The potential N2-ase activity of the cores, measured under controlled conditions, reached a maximum of 27.2 g N ha−1 day−1 and averaged 26.3 g N ha−1 day−1 over the 14 month sampling period. N2-ase activity was positively correlated (P=0.05) with field soil moisture and negatively correlated with field soil temperature (r=0.59 and −0.78 respectively). Multiple regression showed that 69% of the variation of N2-ase activity in the field was associated with the combined effects of soil moisture and soil temperature. Nitrogen fixing bacteria were isolated from the roots ofP. maximum and based upon morphology, biochemical tests and fluorescent antibody reaction, were found to be closely related toAzospirillum lipoferum.

Nitrogenase activity associated with three tropical grasses growing in undisturbed soil cores

Nitrogenase activity associated with the root system of three tropical grasses Axonopus compressus, Digitaria decumbens var. pangolaand Paspalum notatum was measured by C 2H 2 reduction assay of soil-plant cores. The cores were incubated in perspex chambers in which 10% of the air was replaced with C 2H 2. Gas samples were taken at 7, 24, 31, 48, 55 and 72 h. No lag before onset of C 2H 4 production was evident and good agreement was obtained between replicates. Cumulative C 2H 4 production maintained a linear trend during the 72 h incubation. The largest increase in N 2-ase activity was detected in the A. compressus—gleyed podzolic system while D. decumbens (lateritic podzolic) and P. notatum (sandy yellow podzolic) had smaller activities. Variation between sampling sites in the second year of sampling of the experiment was associated with large variations in soil moisture. N 2 fixation estimated from N 2-ase activity in soil-plant cores was similar to the amount of N accumulated in the above-ground herbage in the field during 12 weeks. Response curves relating N 2-ase activity to soil moisture and soil temperature were established for all species. P. notatum and D. decumbens responded similarly to changes in both soil temperature and soil moisture while A. compressus contrasted sharply to the other two species in its reaction to both.

Nitrogenase activity on the roots of tropical forage grasses

Nitrogen fixation in the rhizospheres of field grown tropical forage grasses was studied by the acetylene reduction method. Values varied considerably between sites but indicate the possible economic importance of several of the species studied. Maximal nitrogenase activity measured (nmoles C 2H 4g −1 dry roots h −1) was 754 for Pennisetum purpureum, 750 for Brachiaria mutica, 341 for Digitaria decumbens, 299 for Panicum maximum, 283 for Paspalum notatum, 269 for Cynodon dactylon, 41 for Melinis minutiflora and 29 for Hyparrhenia rufa. Nitrogenase activity varied considerably with season and was maximal during active vegetative growth of two of the grasses. Significant differences between Paspalum notatum ecotypes and cultivars. in Azotohacter paspali occurrence and nitrogen fixation, indicate the possibility of plant breeding to enhance nitrogen fixation in grass rhizospherc associations. Other research lines of agronomic importance are fertilizer effects. In intact soil plant cores with the Paspalum system 10 parts/10 6 NH 4 + J-N inhibited nitrogenase activity within 2 h and 10 parts/10 6 NO − 3-N within 4 h. but after 1 week these effects were negligible. In the field, nitrogenase activity on roots of P. purpureum and D. decumbens, assayed 2 weeks after top dressings of 20 kg N ha −1 as NH 4NO 3. was not affected even after eight such dressings.

Nitrogenase Activity and Oxygen Sensitivity of the Paspalum notatum-Azotobacter paspali Association

SUMMARY: Nitrogenase activity in the rhizosphere of a grass, Paspalum notatum, and its associated soil was measured by the reduction of acetylene. Roots of the cultivar ‘batatais’ colonized by Azotobacter paspali, when taken from the soil, produced 1 to 32 nmol C2H4/g dry wt/h, whereas the cultivar ‘pensacola’, which is not colonized by A. paspali, produced less than 0.5 nmol/g/h. There was a lag of 12 to 24 h before maximum, linear, rates of acetylene reduction were reached. Activity was almost completely inhibited in air or in the absence of O2 and was greatest at around pO2 0.04 atm. Activity of soil cores containing plants with leaves attached was little affected by pO2 and showed no lag. Soil-plant cores maintained in a 16 h day+8 h night showed no diurnal fluctuation in activity; as the dark period was extended, activity decreased but was restored on returning plants to the light. Roots and rhizomes had most activity, the soil very little and aerial parts none. Washing the roots removed less than half the activity. Disturbance of soil-plant cores decreased activity. The soil next to the root surface contained most A. paspali; more were associated with active plants than with less active plants. Sections of roots showed abundant bacteria adjacent to the root surface. Nitrogen fixation by the association was estimated to be up to 90 kg N/ha/annum.