J. D. Graves

Root production, turnover and respiration under two grassland types along an altitudinal gradient: influence of temperature and solar radiation

Abstract We have measured the rates of root production and death and of root respiration in situ under two grasslands along an altitudinal gradient in the northern Pennines, UK, represented by a lowland site at 171 m in an agricultural setting, and three upland sites between 480 and 845 m. One grassland was dominated by Festuca ovina and was on a brown earth soil; the other was dominated by Juncus squarrosus and Nardus stricta and occurred on a peaty gley. The natural altitudinal gradient was extended by transplantation. Although root biomass and root production (estimated using minirhizotrons) both showed pronounced seasonal peaks, there was no simple altitudinal gradient in either variable, and neither root production nor root death rate was a simple function of altitude. Increased root accumulation in summer was a function of change in the length of the growing season, not of soil temperature. Root populations in winter were similar at all sites, showing that increased production at some sites was accompanied by increased turnover, a conclusion confirmed by cohort analyses. Respiration rate, measured in the field by extracting roots and measuring respiration at field temperature in an incubator, was unrelated to temperature. The temperature sensitivity of respiration (expressed as the slope of a plot of log respiration rate against temperature) showed no simple seasonal or altitudinal pattern. Both root growth (under Festuca) and respiration rate were, however, closely related to radiation fluxes, averaged over the previous 10 days for growth and 2 days for respiration. The temperature sensitivity of respiration was a function of soil temperature at the time of measurement. These results show that root growth and the consequent input of carbon to soil in these communities is controlled by radiation flux not temperature, and that plants growing in these upland environments may acclimate strongly to low temperatures. Most carbon cycle models assume that carbon fluxes to soil are powerfully influenced by temperature, but that assumption is based largely on short-term studies and must be reassessed.

Root production and turnover in an upland grassland subjected to artificial soil warming respond to radiation flux and nutrients, not temperature

Abstract Root demographic processes (birth and death) were measured using minirhizotrons in the soil warming experiments at the summit of Great Dun Fell, United Kingdom (845 m). The soil warming treatment raised soil temperature at 2 cm depth by nearly 3°C. The first experiment ran for 6 months (1994), the second for 18 (1995–1996). In both experiments, heating increased death rates for roots, but birth rates were not significantly increased in the first experiment. The lack of stimulation of death rate in 1996 is probably an artefact, caused by completion of measurements in late summer of 1996, before the seasonal demography was concluded: root death continued over the winter of 1995–1996. Measurements of instantaneous death rates confirmed this: they were accelerated by warming in the second experiment. In the one complete year (1995–1996) in which measurements were taken, net root numbers by the end of the year were not affected by soil warming. The best explanatory environmental variable for root birth rate in both experiments was photosynthetically active radiation (PAR) flux, averaged over the previous 5 (first experiment) or 10 days (second experiment). In the second experiment, the relationship between birth rate and PAR flux was steeper and stronger in heated than in unheated plots. Death rate was best explained by vegetation temperature. These results provide further evidence that root production acclimates to temperature and is driven by the availability of photosynthate. The stimulation of root growth due to soil warming was almost certainly the result of changes in nutrient availability following enhanced decomposition.

Root production and mortality under elevated atmospheric carbon dioxide

An essential component of an understanding of carbon flux is the quantification of movement through the root carbon pool. Although estimates have been made using radiocarbon, the use of minirhizotrons provides a direct measurement of rates of root birth and death. We have measured root demographic parameters under a semi-natural grassland and for wheat. The grassland was studied along a natural altitudinal gradient in northern England, and similar turf from the site was grown in elevated CO2 in solardomes. Root biomass was enhanced under elevated CO2. Root birth and death rates were both increased to a similar extent in elevated CO2, so that the throughput of carbon was greater than in ambient CO2, but root half-lives were shorter under elevated CO2 only under a Juncus/Nardus sward on a peaty gley soil, and not under a Festuca turf on a brown earth soil. In a separate experiment, wheat also responded to elevated CO2 by increased root production, and there was a marked shift towards surface rooting: root development at a depth of 80–85 cm was both reduced and delayed. In conjunction with published results for trees, these data suggest that the impact of elevated CO2 will be system-dependent, affecting the spatio-temporal pattern of root growth in some ecosystems and the rate of turnover in others. Turrnover is also sensitive to temperature, soil fertility and other environmental variables, all of which are likely to change in tandem with atmospheric CO2 concentrations. Differences in turnover and time and location of rhizodeposition may have a large effect on rates of carbon cycling.

Carbon transfer between C3 and C4 plants linked by a common mycorrhizal network, quantified using stable carbon isotopes

Abstract Changes in the natural abundance of 13C were used to quantify carbon transfer between C3 and C4 plants in a common mycorrhizal network. Experiments using two mesh sizes to either prevent (0.45 μm) or allow (20 μm) mycorrhizal connections between Plantago lanceolata (C3) and Cynodon dactylon (C4) plants were run. Root and shoot samples were taken for δ13C determinations. It could not be assumed that all the pairs of plants were linked; therefore non-parametric statistical tests were used. In order to measure transfer between pairs of plants the shoot δ13C value was used as a reference and the deviation of the root δ13 C value from this as a measure of carbon transfer. As the C. dactylon root δ13 C value became more negative with the 20 μm mesh present, the root-shoot difference increased. In P. lanceolata both the root and shoot δ13C values became less negative but the root-shoot difference did increase gradually. When the root-shoot difference in C. dactylon was plotted against root δ13C value the relationship was ≈unity, suggesting that transferred carbon remained in the roots and fungal material did not move into the shoots. In P. lanceolata the results showed that the slope of the relationship between root-shoot difference and root δ13C value was ≈0.5. This suggests any transferred carbon moved into both the root and shoot material. The root-shoot difference can be used to estimate % carbon transfer. For individual C. dactylon plants the results varied from 0 to 41% with most values falling at 10% or below. It was not possible to calculate % transfer amounts for P. lanceolata. This range of variation could have important implications for plant interactions in communities.

Intraspecific transfer of carbon between plants linked by a common mycorrhizal network

To quantify the involvement of arbuscular mycorrhiza (AM) fungi in the intraspecific transport of carbon (C) between plants we fumigated established Festuca ovina turf for one week with air containing depleted 13C. This labelled current assimilate in a section of mycorrhizal or non-mycorrhizal turf. Changes in the 13/12C ratio of adjacent, unfumigated plants, therefore, allowed the movement of C between labelled and unlabelled plants to be estimated. In mycorrhizal turves, 41% of the C exported to the roots from the leaves was transported to neighbouring plants. The most likely explanation of this is was the transport of C via a common hyphal network connecting the roots of different plants. No inter-plant transport of C was detected in non-mycorrhizal turves. There was no evidence that the C left fungal structures and entered the roots of receiver plants. Mycorrhizal colonisation increased carbon transport from leaves to root from 10% of fixed carbon when non-mycorrhizal to 36% in mycorrhizal turves. These results suggest that AM fungi impose a significantly greater C drain on host plants than was previously thought.

The impact of phosphorus on interactions of the hemiparasitic angiosperm Rhinanthus minor and its host Lolium perenne

Abstract The effects of phosphorus supply on the outcome of interactions between the hemiparasitic angiosperm Rhinanthus minor L. with its host species Lolium perenne L. were investigated in a glasshouse experiment. Host plants were grown in 3-l pots in the presence and absence of R. minor at limiting (0.13 mm P) and optimal (0.65 mm P) concentrations of phosphorus for the growth of the host species. Phosphorus was supplied at 2-day intervals in the form of half-strength Long Ashton nitrate-based solution with phosphorus concentrations adjusted accordingly. Parasitism by R. minor significantly suppressed host growth, with final biomass losses ranging between 32% and 44%. Phosphorus supply had a marked impact on the outcome of the host-parasite interaction. By the end of the growing period, parasite biomass at 0.65 mm P was 90% lower than that achieved at 0.13 mm P. In contrast, host biomass at 0.65 mm P was 74% higher than achieved at 0.13 mm P, indicting that the negative impact of parasitism on the host species was reduced when phosphorus supply was increased. The effects of phosphorus on the host-parasite association appeared to be mediated by changes in both the morphological characteristics of the host roots and the relative sink strengths of the host and parasite.

Spatial distribution and population dynamics of Striga hermonthica seeds in naturally infested farm soils

Densities and spatial distribution in soil of seeds of Striga hermonthica were analysed for four naturally infested farm fields in Western Kenya. A revised method for extraction of Striga seeds from soil was used, combining centrifugation with existing techniques based on flotation. Tests showed that 85% of Striga seeds were retrieved from soil samples. In all fields the majority of seeds were found in the plough layer (0 – 20 cm). New seeds entering the soil from the surface after seed shedding created a strong gradient with depth. Downward penetration from the soil surface was larger in sandy soil than in clay soil. In tilled soils no significant vertical density gradient was found within the plough layer. At a fine scale (0.2 m) seed densities showed little horizontal variation, but significant differences in seed densities in the horizontal plane were found at larger scale distances (several m) between locations in all fields. At 125 days after sowing the estimated average number of seeds produced per emerged Striga shoot was 4,827, excluding an approximately similar amount of seeds present in maturing capsules. The estimated average number of seeds produced per mature Striga seed capsule was 1188. Large seasonal fluctuations in the Striga seedbank were measured. An average net increase of 88,825 Striga seeds m-2 (equivalent to 340%) was calculated from seedbank analyses in 16 sorghum plots. The level of Striga infestation in one field had decreased by 62% from 34,250 seeds m-2 to 13,125 seeds m-2 after keeping it fallow for a year. A sharp decline in Striga seed density was found in samples taken at increasing distances from highly infested fields, irrespective of wind direction or slope, suggesting very limited dispersal of Striga seeds by wind or water. Parasite emergence was non-linearly related to initial Striga seed densities in the soil, but this relationship was only observable at the scale of individual plant holes. Seed production was also non-linearly related to numbers of emerged parasites, when measured at plot scale (25 m2), but not at the scale of individual plant holes. In the fields we studied, seed densities below levels of 13,000 Striga seeds m-2 could be considered to suppress the number of emerging parasites. However, if two or three emerged Striga plants per m-2 were left to seed, enough seeds would be produced to keep the seedbank in balance.

Striga seed avoidance by deep planting and no-tillage in sorghum and maize

The possibility of reducing Striga hermonthica (Del.) Benth. parasitism in severely infested fields, by means of deep planting - thereby reducing the root length in the upper layers of the soil where Striga seeds are predominantly found - was tested in field trials with maize and sorghum in western Kenya. Sorghum seeds were planted in Striga-infested fields approximately 2.5 cm deep in the soil or at the bottom of conically-shaped plant holes (15-20 cm deep). Depth of plant holes for maize varied from 0 to 30 cm, in un-tilled soil. Deep planting in un-tilled soil gave higher (up to double) grain yields, compared with standard planting in tilled soil. Parasite emergence was related negatively to planting depth of maize (p< 0.05). Deep planting in tilled soil gave 74% more sorghumgrain yield relative to standard planting. In this treatment Striga seed production was not reduced but in un-tilled fields with deeply planted sorghum Striga seed production was completely suppressed. Therefore, a combination of zero-tillage and deep planting seems to be the most effective treatment. The probable mechanism causing these results is avoidance of Striga seed by the host root system, resulting in a delay in the onset of Striga attachment and the formation of smaller numbers of attachments.