Summary1. Plant diversity has profound effects on primary production. Plant diversity has been shown to correlate with increased primary production in nutrient-limited grassland ecosystems. This overyielding has been attributed to vertical niche differentiation among species below-ground, allowing for complementarity in resource capture. However, a rigorous test of this longstanding hypothesis is lacking because roots of different species could not be distinguished in diverse communities. 2. Here, we present the first application of a DNA-based technique that quantifies species abundances in multispecies root samples. We were thus able to compare root distributions in monocultures of two grasses and two forbs with root distributions in four-species mixtures. In order to investigate if vertical niche differentiation is driven by soil nutrient depletion, the topsoil layer of the communities were either nutrient-rich or -poor. 3. Immediately in the first year, 40% more root biomass was produced in mixtures than expected from the monocultures, together with significant below-ground complementarity effects, probably preceding above-ground overyielding. This below-ground overyielding appeared not to be the result of vertical niche differentiation, as rooting depth of the community tended to decrease, rather than increase in mixtures compared to monocultures. Roots thus tended to clump in the very dense topsoil layer rather than segregate over the whole profile in mixtures. The below-ground overyielding was mainly driven by enhanced root investments of one species, Anthoxanthum odoratum, in the densely rooted topsoil layer without retarding the growth of the other species. 4. Synthesis. Conventional ecological mechanisms, such as competition for nutrients, do not seem to be able to explain the increased root investments of A. odoratum in mixtures compared to monocultures, with apparently little effect on the root growth of the other species. Instead, the observed root responses are consistent with species-specific root recognition responses. From a community perspective, the observed early below-ground overyielding may initiate the recently reported increased soil organic matter, mineralization and N availability and thus may ultimately be responsible for the higher productivity at high plant species diversity.

We studied the effects of experimentally induced variation in leaf litter chemistry on leaf litter decomposition and leaf litter nutrient release of Carex species from habitats that differ in nutrient availability. Carex diandra, C. rostrata, and C. lasiocarpa are dominant in less productive mesotrophic fens, whereas C. acutiformis is dominant in more productive eutrophic fens. For each species, three types of litter were used: litter collected in the field (FLD); and litter from experimental populations grown at low (LN: 3.3 g N·m Ϫ2 ·yr Ϫ1 ) and high N supply (HN: 20.0 g N·m Ϫ2 ·yr Ϫ1 ). For all the litter chemistry parameters studied there were highly significant interactions between species and litter type. This implies that, due to differential plant-mediated controls on leaf litter chemistry, it is not always possible to predict changes in litter chemistry in response to increased nutrient supply.Litter decay was determined in a long-term (3 yr) field experiment using litter bags. The litter types of each species were incubated at their native growing sites. Contrary to what is generally found, the leaf litter decomposition rate of the species growing at the nutrient-poorest site (C. diandra) was higher than that of the species growing at the nutrientrichest site (C. acutiformis). Only for C. diandra and C. lasiocarpa was the decomposition rate of the litter from the HN treatment higher than that of the field litter. Thus, increased nutrient supply does not necessarily lead to higher litter decomposition rates. Nutrient controls on litter decay changed with time: initial litter decay (Յ3 mo) was strongly controlled (high r 2 values) by all P-related litter quality parameters, whereas long-term litter decay (Ͼ1 yr) was most strongly related with the phenolics : N ratio, the phenolics : P ratio, the lignin : N ratio, and the C:N ratio. Our data suggest that high levels of atmospheric N deposition, such as in the study area (The Netherlands), may lead to a relative shortage of P in the plant-derived substrates for bacteria and fungi. As a result, P-related litter chemistry parameters exert a strong influence on litter decay.Our data did not support the hypothesis that high-nutrient species increase nutrient cycling due to the production of easily decomposable litter with high rates of nutrient release. The leaf litter from Carex acutiformis, the species from the high-productivity fens, decomposed more slowly than that of the other species, immobilized more N and P and had a longer period of net N and P immobilization. However, this species has a higher litter production than the other species and thereby increases the rate of nutrient cycling. At the intraspecific level, increased nutrient supply led to lower amounts of immobilized N and P and faster N and P release from litter in most species, and thereby to a higher rate of nutrient cycling. This positive feedback between nutrient supply rate and the rate of nutrient cycling is reinforced by the increase in litter production in response to increased nutrient...

Summary 1.Recent studies have shown that the positive relationship between plant diversity and plant biomass ('overyielding') can be explained by soil pathogens depressing productivity more in low than in high diverse plant communities. However, tests of such soil effects in field studies were constrained by experimental limitations to manipulate soil community composition independent of plant community composition. Here, we report of an experiment where feedback effects to plants were tested for both plant and soil monocultures and mixtures.2. Our results demonstrate that overyielding is the result of plant species in mixture being more growth-limited by 'own' soil biota than by soil biota of other plant species. This effect disappeared when the soils had been sterilized by gamma-irradiation. Mixing plants themselves did not result in overyielding except when grown in the soil of one of the species (Leucanthemum vulgare), where growth of one species disproportionally increased in mixture compared to monoculture.3. Soil nutrient availability could not explain differences in growth between the non-sterilized soils. Therefore, our results suggest that plant species-specific soil biota rather than the plants have contributed to the plant community overyielding.4. Species biomass ranking in mixtures highly differed between non-sterilized soils of different histories of soil conditioning, whilst the ranking was more consistent in sterilized soil. Sterilized soils of different origin differed significantly in nutrient availability. These results suggest that shifts in competitive hierarchies depend on plant species-specific interactions influenced by soil biota and cannot be induced by mineral nitrogen. Synthesis.Our results show that overyielding in four plant species mixtures can be due to species-specific interactions between plants and their specific soil biota. Neither mixing the plant species alone nor the differential responses of species to mineral nitrogen influenced community productivity, but mixing soil biota did.

Riparian buffer zones are known to reduce diffuse N pollution of streams by removing and modifying N from agricultural runoff. Denitrification, often identified as the key N removal process, is also considered as a major source of the greenhouse gas nitrous oxide (N2O). The risks of high N2O emissions during nitrate mitigation and the environmental controls of emissions have been examined in relatively few riparian zones and the interactions between controls and emissions are still poorly understood. Our objectives were to assess the rates of N2O emission from riparian buffer zones that receive large loads of nitrate, and to evaluate various factors that are purported to control N emissions. Denitrification, nitrification, and N2O emissions were measured seasonally in grassland and forested buffer zones along first-order streams in The Netherlands. Lateral nitrate loading rates were high, up to 470 g N m(-2) yr(-1). Nitrogen process rates were determined using flux chamber measurements and incubation experiments. Nitrous oxide emissions were found to be significantly higher in the forested (20 kg N ha(-1) yr(-1)) compared with the grassland buffer zone (2-4 kg N ha(-1) yr(-1)), whereas denitrification rates were not significantly different. Higher rates of N2O emissions in the forested buffer zone were associated with higher nitrate concentrations in the ground water. We conclude that N transformation by nitrate-loaded buffer zones results in a significant increase of greenhouse gas emission. Considerable N2O fluxes measured in this study indicate that Intergovernmental Panel on Climate Change methodologies for quantifying indirect N2O emissions have to distinguish between agricultural uplands and riparian buffer zones in landscapes receiving large N inputs.

Root systems are highly plastic as they express a range of responses to acquire patchily distributed nutrients. However, the ecological significance of placing roots selectively in nutrient hotspots is still unclear. Here, we investigate under what conditions selective root placement may be a significant functional trait that determines belowground competitive ability. We studied two grasses differing in root foraging behaviour, Festuca rubra and Anthoxanthum odoratum. The plants were grown in stable and more dynamic heterogeneous environments, by switching nutrient patches halfway through the experiment. A. odoratum was a factor of two less selective in placing its roots into nutrient-rich patches than F. rubra. A. odoratum produced overall higher root length densities with higher specific root length than F. rubra and acquired more nutrients. A. odoratum appeared to be the superior competitor, irrespective of the nutrient dynamics. Our results suggest that root behaviour consisting of producing high root length densities at relatively low biomass investments can be a more effective foraging strategy than placing roots selectively in nutrient hotspots. When understanding the functionality of root traits among different species, specific root length may play a key role.