No abstract

Arbuscular mycorrhizal (AM) fungi have a major influence on the structure, responses and below-ground C allocation of plant communities. Our lack of understanding of the response of AM fungi to factors such as light and temperature is an obstacle to accurate prediction of the impact of global climate change on ecosystem functioning. In order to investigate this response, we divided a grassland site into 24 plots, each either unshaded or partly shaded with soil either unheated or heated by 3 1C at 2 cm depth. In both shortterm studies in spring and autumn, and in a 1-year-long study, we measured root length colonization (L R C) by AM and non-AM fungi. For selected root samples, DNA sequences were amplified by PCR with fungal-specific primers for part of the small sub-unit (SSU) rRNA gene. In spring, the total L R C increased over 6 weeks from 12% to 25%. Shading significantly reduced AM but increased non-AM fungal colonization, while soil warming had no effect. In the year-long study, colonization by AM fungi peaked in summer, whereas non-AM colonization peaked in autumn, when there was an additive effect of shading and soil warming that reduced AM but increased non-AM fungi. Stepwise regression revealed that light received within the 7 days prior to sampling was the most significant factor in determining AM L R C and that mean temperature was the most important influence on non-AM L R C. Loglinear analysis confirmed that there were no seasonal or treatment effects on the host plant community. Ten AM fungal sequence types were identified that clustered into two families of the Glomales, Glomaceae and Gigasporaceae. Three other sequence types were of non-AM fungi, all Ascomycotina. AM sequence types showed seasonal variation and shading impacts: loglinear regression analysis revealed changes in the AM fungal community with time, and a reduction of one Glomus sp. under shade, which corresponded to a decrease in the abundance of Trifolium repens. We suggest that further research investigating any impacts of climate change on ecosystem functioning must not only incorporate their natural AM fungal communities but should also focus on niche separation and community dynamics of AM fungi.Total number of counts (N 5 72) was reduced to 54 if grouping was carried out according to cluster analysis, which separated only Glo 4 and Glo 1 (both N 5 18, giving N 5 36). z Chi-square values (Chi-sq.). § Degrees of freedom (df). z Corresponding P-values of loglinear analysis for the saturated model and models of subsequent removal of individual factors ((A) AM sequence types (4), (H) heating (2), (S) shading (3) treatments and (T) sampling time (3)) and their interaction (for details see Helgason et al., 1999) with significance: *P o 0.05; **P o 0.01.

Accurate knowledge of the response of root turnover to a changing climate is needed to predict growth and produce carbon cycle models. A soil warming system and shading were used to vary soil temperature and received radiation independently in a temperate grassland dominated by Holcus lanatus L. Minirhizotrons allowed root growth and turnover to be examined non‐destructively. In two short‐term (8 week) experiments, root responses to temperature were seasonally distinct. Root number increased when heating was applied during spring, but root death increased during autumnal heating. An experiment lasting 12 months demonstrated that any positive response to temperature was short‐lived and that over a full growing season, soil warming led to a reduction in root number and mass due to increased root death during autumn and winter. Root respiration was also insensitive to soil temperature over much of the year. In contrast, root growth was strongly affected by incident radiation. Root biomass, length, birth rate, number and turnover were all reduced by shading. Photosynthesis in H. lanatus exhibited some acclimation to shading, but assimilation rates at growth irradiance were still lower in shaded plants. The negative effects of shading and soil warming on roots were additive. Comparison of root data with environmental measurements demonstrated a number of positive relationships with photosynthetically active radiation, but not with soil temperature. This was true both across the entire data set and within a shade treatment. These results demonstrate that root growth is unlikely to be directly affected by increased soil temperatures as a result of global warming, at least in temperate areas, and that predictions of net primary productivity should not be based on a positive root growth response to temperature.

The impact of climate change on N leaching from hill land plant/soil systems was investigated using a transplant technique involving the movement of intact lysimeter cores of three contrasting soil types down an altitudinal gradient at Great Dun Fell, Cumbria. Air and soil temperatures and precipitation were monitored at four elevations down an altitudinal transect using automatic weather stations for a period of two years. The altitudinal sequence of air temperature followed the anticipated pattern, providing mean annual temperatures at the four locations of 3.4, 5.0, 6.3 and 8.1 °C. Lapse rates of both mean air and soil temperatures over the altitudinal range 171–845 m were 6.6 (1993) and 7.0 °C km–1 (1994). Soil monthly temperature gradients for a particular soil type for each of the two years showed a seasonal range of 6.0 and 7.4 °C km–1, respectively, and for air temperature of 4.3 and 3.1 °C km–1. Precipitation gradients showed the expected general increase with altitude, but were less predictable. Inorganic nitrogen leaching was studied in lysimeter leachates with climatic amelioration resulting in dramatic reductions in leachate nitrate concentrations and associated total concentrations of inorganic nitrogen. Decreases in leachate nitrate concentrations were observed for all three soil types studied. Soils receiving supplemented rainfall also showed decreased N concentrations, suggesting that temperature was the main controlling factor responsible for the observed reductions. Increased N uptake by the vegetation, in response to the increases in temperature, is considered to be critical in controlling soil solution chemistry at these sites.

A mobile laboratory was developed to administer a controlled flow of 13C labelled CO2 at ambient concentrations (∼350 ppm) in the field. The stable isotope delivery (SID) system consists of an isotope‐mixing unit with flow control to a series of 12 independent labelling chambers. In‐line CPU controlled infrared gas analysers allow automated measurement of chamber CO2 concentrations and gas flow management. A preliminary experiment was established on an upland pasture located at the NERC Soil Biodiversity experimental site, Sourhope, UK, in August 1999. The objective of this investigation was to determine the magnitude of pulse‐derived C incorporation into a typical upland plant community. To achieve this, the SID system was deployed to pulse‐label vegetation with CO2 enriched with 13C (50 atom %) at ambient concentrations (∼350 ppm) on two consecutive days in August 1999. Samples of headspace CO2, shoot and root were taken on four occasions over a period of 28 days after 13C labelling. These materials were then prepared for 13C/12C ratio determination by continuous‐flow/combustion/isotope ratio mass spectrometry (CF‐C‐IRMS). Results showed that pulse derived CO2‐C was assimilated at a rate of 128 ± 32 µg g shoot‐C hour−1. Dynamic samplings showed that pulse‐derived 13C concentrations in the labelled plant tissues declined by 77.4 ± 6% after 48 hours. The rapid decline in 13C concentrations in plant matter was the result of C loss from the plant in the form of respired CO2 and root exudates, and dilution by subsequent unlabelled C assimilates. This novel system offers considerable potential for in situ tracer investigations. Copyright © 2000 John Wiley & Sons, Ltd.