Source-sink relations in Lolium perenne L. as reflected by carbohydrate concentrations in leaves and pseudo-stems during regrowth in a free air carbon dioxide enrichment (FACE) experiment*

The effect of an elevated partial pressure of CO₂ (p_CO2) on carbohydrate concentrations in source leaves and pseudo-stems (stubble) of Lolium perenne L. (perennial ryegrass) during regrowth was studied in a regularly defoliated grass sward in the field. The free air carbon dioxide enrichment (FACE) technology enabled natural environmental conditions to be provided. Two levels of nitrogen (N) supply were used to modulate potential plant growth. Carbohydrate concentrations in source leaves were increased at elevated p_CO2, particularly at low N supply. Elevated leaf carbohydrate concentrations were related to an increased structural carbon (C) to N ratio and thus reflected an increased C availability together with a N-dependent sink limitation. Immediately after defoliation, apparent assimilate export rates (differences in the carbohydrate concentrations of young source leaves measured in the evening and on the following morning) showed a greater increase at elevated p_CO2 than at ambient p_CO2; however, replenishment of carbohydrate reserves was not accelerated. Distinct, treatment-dependent carbohydrate concentrations in pseudo-stems suggested an increasing degree of C-sink limitation from the treatment at ambient p_CO2 with high N supply to that at elevated p_CO2 with low N supply. During two growing seasons, no evidence of a substantial change in the response of the carbohydrate source in L. perenne to elevated p_CO2 was found. Our results support the view that the response of L. perenne to elevated p_CO2 is restricted by a C-sink limitation, which is particularly severe at low N supply.     

Growth response of Trifolium repens L. and Lolium perenne L. as monocultures and bi-species mixture to free air CO2 enrichment and management

Trifolium repens L. and Lolium perenne L. were grown in monocultures and bi-species mixture in a Free Air Carbon Dioxide Enrichment (FACE) experiment at elevated (60 Pa) and ambient (35 Pa) CO₂ partial pressure (pCO₂) for three years. The effects of defoliation frequencies (4 and 7 cuts in 1993; 4 and 8 cuts in 1994/95) and nitrogen fertilization (10 and 42 g m^–2 y^–1 N in 1993; 14 and 56 g m^–2 y^–1 N in 1994/95) on the growth response to pCO₂ were investigated. There were significant interspecific differences in the CO₂ responses during the first two years, while in the third growing season, these interspecific differences disappeared. Yield of T. repens in monocultures increased in the first two years by 20% when grown at elevated pCO₂. This CO₂ response was independent of defoliation frequency and nitrogen fertilization. In the third year, the CO₂ response of T. repens declined to 11%. In contrast, yield of L. perenne monocultures increased by only 7% on average over three years at elevated pCO₂. The yield response of L. perenne to CO₂ changed according to defoliation frequency and nitrogen fertilization, mainly in the second and third year. The ratio of root/yield of L. perenne increased under elevated pCO₂, low N fertilizer rate, and frequent defoliation, but it remained unchanged in T. repens. We suggest that the more abundant root growth of L. perenne was related to increased N limitation under elevated pCO₂. The consequence of these interspecific differences in the CO₂ response was a higher proportion of T. repens in the mixed swards at elevated pCO₂. This was evident in all combinations of defoliation and nitrogen treatments. However, the proportion of the species was more strongly affected by N fertilization and defoliation frequency than by elevated pCO₂. Based on these results, we conclude that the species proportion in managed grassland may change as the CO₂ concentration increases. However, an adapted management could, at least partially, counteract such CO₂ induced changes in the proportion of the species. Since the availability of mineral N in the soil may be important for the species’ responses to elevated pCO₂, more long-term studies, particularly of processes in the soil, are required to predict the entire ecosystem response.     

Bottom‐up control in the benthos of ocean‐exposed sandy beaches?

Bottom‐up control is a fundamental structuring force in food webs. Food webs of ocean‐exposed sandy beaches are predicted to be bottom‐up controlled systems, underpinned by imported organic matter rather than in situ primary production. This ecological model of resource‐based regulation of biological assemblages is juxtaposed against a prevailing paradigm built around a dominance of physical drivers in sandy beach ecosystems. Surprisingly, given the apparently ubiquitous energetic subsidies of beach food webs, the central premise of bottom‐up control has not been tested. Here we experimentally manipulated in situ nutrient levels on a sandy beach to test food web responses at the levels of primary producers (benthic microalgae) and their grazers (meiofauna). The meiofauna community as a whole appeared most strongly influenced by the local physical environment, particularly changes in sediment grain size – this supports the traditional ‘environmental control paradigm’. We also detected a significant, positive response of two consumer groups of the meiofauna (nematodes, ostracods) to nutrient enrichment that supports a model of biological, bottom‐up control. Although the predicted response of elevated producer biomass following nutrient enrichment was not detected, intense grazing pressure on new, stimulated production may have masked positive responses by the primary producers. Multichannel regulation of food webs is likely for many exposed sandy beaches, albeit an often lower importance of in situ bottom‐up forces compared with stronger environmental control.     

Oxygen Supply Limits Nitrogenase Activity of Clover Nodules After Defoliation

The aim of this study was to test the effect of oxygen partialpressure as a possible limiting factor of nitrogen fixationfollowing defoliation. The response of nitrogenase activity(C₂H₂-reduction) of defoliated and undefoliated white and redclover plants (Trifolium repens L. and Trifolium pratense L.)to either 19 kPa oxygen or 55 kPa oxygen was investigated. Priorto defoliation, white clover plants were grown for five weeksin a growth chamber, and red clover plants for 7 or 11 weeksin a glasshouse. The results included measurements of ¹⁶N₂-uptake. Increasing oxygen partial pressure from 19 to 55 kPa severelyrestricted nitrogenase activity of undefoliated white cloverplants; however, 2 h after complete defoliation, the same treatmentcaused a significant increase. A fivefold increase in nitrogenaseactivity upon exposure to the elevated oxygen partial pressurewas found at the end of a 24 h period. This beneficial effectdecreased gradually from 1 to 5 d after defoliation. The responseof recently defoliated red clover plants to 55 kPa oxygen partialpressure was similar to that of white clover, independentlyof plant age. The gradual recovery of nitrogenase activity duringthree weeks of regrowth was associated with a simultaneous changein the response to increased oxygen partial pressure, leadingagain to the response of undefoliated plants. These data suggested that lack of oxygen at the site of nitrogenfixation, resulting from a dramatic increase in oxygen-diffusionresistance, is the main factor limiting nitrogenase activityfollowing defoliation. Trifolium repens L., Trifolium pratense L., white clover, red clover, defoliation, regrowth, nodules, nitrogen fixation, nitrogenase activity, oxygen limitation