Performance In Monoculture Research Articles

Performance of non-conventional yeasts in co-culture with brewers' yeast for steering ethanol and aroma production.

SummaryIncreasing interest in new beer types has stimulated the search for approaches to extend the metabolic variation of brewers’ yeast. Therefore, we tested two approaches using non‐conventional yeast to create a beer with lower ethanol content and a complex aroma bouquet. First, the mono‐culture performance was monitored of 49 wild yeast isolates of Saccharomyces cerevisiae (16 strains), Cyberlindnera fabianii (9 strains) and Pichia kudriavzevii (24 strains). Interestingly, both C. fabianii and P. kudriavzevii isolates produced relatively more esters compared with S. cerevisiae isolates, despite their limited fermentation capacity. Next, one representative strain of each species (Sc131, Cf65 and Pk129) was applied as co‐culture with brewers’ yeast (ratio 1:1). Co‐cultures with Cf65 and Pk129 resulted in a beer with lower alcohol content (3.5, 3.8 compared with 4.2% v/v) and relatively more esters. At higher inoculum ratios of Cf65 over brewers’ yeast, growth inhibition of brewers’ yeast was observed, most likely caused by competition for oxygen between brewers’ yeast and Cf65 resulting in a reduced level of ethanol and altered aroma profiles. With this study, we demonstrate the feasibility of using non‐conventional yeast species in co‐cultivation with traditional brewers’ yeast to tailor aroma profiles as well as the final ethanol content of beer.

Biodiversity effects on ecosystem functioning in a 15-year grassland experiment: Patterns, mechanisms, and open questions

Biodiversity effects on ecosystem functioning in a 15-year grassland experiment: Patterns, mechanisms, and open questions

Performance of lucerne genotypes for biomass production and nitrogen content differs in monoculture and in mixture with grasses and is partly predicted from traits recorded on isolated plants

Grass–legume mixtures are key crops to improve agricultural sustainability. Despite their significant use in mixture, lucerne (Medicago sativa L.) varieties are mostly tested and bred for monocultures. This study was performed to evaluate whether the ranking of lucerne genotypes for biomass components and quality was similar in monoculture and mixture, and whether traits recorded on isolated plants could help to predict performance in monoculture and mixture. For 46 genotypes planted in the three competition conditions, plant biomass, shoot height, shoot number and protein content were recorded. In addition, maximum leaf size, internode length, stem diameter, growth habit, specific leaf area and leaf dry-matter content were measured on isolated plants. A general positive correlation was observed between the performance of genotypes in monoculture and in mixture. However, significant changes in genotype ranking indicated that the species of its neighbours could modify the relative performance of a genotype. The traits that minimised competition intensity also changed according to the neighbour species. In mixture, competition intensity was highest for plants with long internodes, a high shoot number and a non-erect growth habit. In monoculture, plants with long internodes and larges leaves suffered less from competition. The agronomic performance of lucerne varieties differing in their architecture should be compared in monoculture and mixture to finally identify the traits to be phenotyped to improve lucerne variety performance in both cultivation modes.

Positive effects of plant diversity on soil microbial biomass and activity are associated with more root biomass production

ABSTRACTThis study aims to explore relationships between plant diversity and soil microbial function and the factors that mediate the relationships. Artificial plant communities (1, 2, 4 and 8 species) were established filled with natural and mine tailing soils, respectively. After 12 months, the plant species richness positively affected the soil microbial functional diversity in both soil environments but negatively affected microbial biomass and soil basal respiration in the natural soil. The root biomass positively correlated with the microbial biomass, cultural bacterial activity and soil basal respiration in both soil environments. Moreover, the Di (deviations between observed performances and expected performances from the monoculture performance of each species of mixture) of microbial biomass, cultural bacterial activity and soil basal respiration positively correlated with the Di of root biomass in both soil environments. Consistent with stress-gradient hypothesis, the Dmix (over-function index) of aboveground biomass positively correlated plant species richness in the mine tailing soil. Results suggest that the root biomass production is an important mechanism that affects the effects of plant diversity on soil microbial functions. Different responses of soil microbial function to increasing plant diversity may be due to root biomass production mediated by other factors.

An evolutionary game theoretical model shows the limitations of the additive partitioning method for interpreting biodiversity experiments

Summary The relationship between diversity and ecosystem functioning is often analysed by partitioning the change in species performance in mixtures into a complementarity effect (CE) and a selection effect (SE). There is continuing ambiguity in the literature on the interpretation of these effects, mainly in their relationship to ecological mechanisms and processes. Here, we present the emergence of complementarity and selection effects in the results of an evolutionary game theoretical model for plant competition which is exclusively based on competition for light. Eight plant strategies, differing only in the time of onset of flowering, were played against one another to determine the relationship between plant trait differences and CE vs. SE. We show that competitive exclusion may occur even in the presence of a positive CE, i.e. when CE > 0. CE was highest at intermediate differences in flowering time. Increasing trait differences may, therefore, increase CE without leading to coexistence. SE was strongly dependent on which of the strategies was paired. SE was mostly positive if one of the players was early flowering (with low seed yield) and the other strategy had higher monoculture yields. SE was mostly negative if one of the players was late flowering, leading to low monoculture yield for this strategy, but to high gains when competing with strategies that produce less leaf area. The average SE was negative over all pairs. Synthesis. These results show that CE may be positive if the species interaction is beneficial for only one of the two competitors, and the sign of SE depends critically on the underlying mechanism for performance in monoculture. Positive complementarity may go hand in hand with competitive exclusion, while widely different outcomes in SE are possible within a single mechanistic background. Consequently, the way SE changes with increasing richness are not only related to a sampling/selection effect but also by the way the interacting species affect the competitive environment. Interpreting the additive partitioning method with a set notion in mind about the driving mechanisms could lead to incorrect conclusions on how species richness effects drive ecosystem functioning.

Effects of spatial plant–soil feedback heterogeneity on plant performance in monocultures

Summary Plant–soil feedback (PSF) effects have almost exclusively been quantified on homogeneous soils, but as different plant species will influence their local soil differently, in reality PSF effects will be spatially heterogeneous. Whether plant performance in soils with spatially heterogeneous PSF can be predicted from pot experiments with homogeneous soils is unclear. In a glasshouse experiment, we tested the response of monocultures of six grassland species (two grasses, two legumes and two forbs) to three spatially explicit treatments (fine‐grained heterogeneity, coarse‐grained heterogeneity and homogeneous). Sixteen patches of conditioned soil (˜6 × 6 cm) were placed within each container. For homogeneous treatments, all patches contained the same conditioned soil within a container. The fine‐grained heterogeneous treatment contained four differently conditioned soils that were applied following a Latin square design, while for the coarse‐grained heterogeneous treatment, four contiguous square blocks of four cells each were created in each container. In general, species grew worse on soil conditioned by conspecifics. However, when the biomass production on all homogeneous soil treatments (own and foreign soils) was averaged and compared to the heterogeneous treatments, we found that biomass production was lower than expected in the heterogeneous soils. This effect of heterogeneity depended on both the conditioning and test species, but most heterogeneity effects were negative. The grain of the heterogeneity (coarse vs. fine: at the chosen spatial scale) did not affect plant performance. We hypothesize that a more diverse soil community is present in spatially heterogeneous soils. This increases (i) the chance of plants to encounter its antagonists, which may then rapidly increase in numbers; and (ii) the scope for synergistic co‐infections. Together, this may lead to non‐additive responses of plants to spatial heterogeneity in PSF. Synthesis. Plant performance was lower in spatially heterogeneous soils than predicted by spatially homogeneous soils. In natural grasslands that have mixed plant communities conditioning the soil, plant–soil feedback (PSF) effects on plant performance may therefore be more negative than what is predicted from pot experiments. Our results emphasize the need to incorporate the spatial dynamics of PSF both in empirical and modelling studies if we are to understand the role of PSF in plant–plant interactions and plant community dynamics.

From pots to plots: hierarchical trait‐based prediction of plant performance in a mesic grassland

Summary Traits are powerful predictors of ecosystem functions pointing to underlying physiological and ecological processes. Plant individual performance results from the coordinated operation of many processes, ranging from nutrient uptake over organ turnover to photosynthesis, thus requiring a large set of traits for its prediction. For plant performance on higher hierarchical levels, e.g. populations, additional traits important for plant‐plant and trophic interactions may be required which should even enlarge the spectrum of relevant predictor traits. The goal of this study was to assess the importance of plant functional traits to predict individual and population performance of grassland species with particular focus on the significance of root traits. We tested this for 59 grassland species using 35 traits divided into three trait clusters: leaf traits (16), stature traits (8) and root traits (11), using individual biomass of mesocosm plants as a measure of individual performance and population biomass of monocultures as a measure of population performance. We applied structural equation models to disentangle direct effects of single traits on population biomass and indirect effects via individual plant biomass or shoot density.We tested multivariate trait effects on individual and population biomass to analyse whether the importance of different trait clusters shifts with increasing hierarchical integration from individuals to populations. Traits of all three clusters significantly correlated with individual and population biomass. However, in spite of a number of significant correlations, above‐below‐ground linkages were generally weak, with few exceptions like N content. Stature traits exclusively affected population biomass indirectly via their effect on individual biomass, whereas root and leaf traits showed also direct effects and partly indirect effects via density. The inclusion of root traits in multiple regression models improved the prediction of individual biomass when compared with models with only above‐ground information only slightly (95% vs. 93% of variance prediction with and without root traits, respectively) but was crucial for the prediction of population biomass (77% and 49%, respectively). Root traits were more important for plant performance than leaf traits and were even the most important predictors at the population level Synthesis. Upscaling from the individual to the population level reflects an increasing number of processes requiring traits from different trait clusters for their prediction. Our results emphasize the importance of root traits for trait‐based studies especially at higher organizational levels. Our approach provides a comprehensive framework acknowledging the hierarchical nature of trait influences. This is one step towards a more process‐oriented assessment of trait‐based approaches.

Cultivar × binary mixture interaction effect on agronomic traits in orchardgrass

AbstractA study was conducted from 2009 to 2012 to evaluate and characterize the agronomic value of seven agronomic traits, including dry matter yield and forage quality of 25 orchardgrass cultivars grown in monoculture and binary mixtures with alfalfa. The irrigated Millville, UT, USA (41°41′24″N, 111°49′48″W) field site was characterized by a semiarid climate and elevation over 1300 m. The cultivar × mixture interaction was significant for all traits but flag leaf height, and contained a substantial crossover component among the cultivars. Differences among cultivar means were significant for all traits in both the monoculture and mixture settings. Yet, concordance for high cultivar performance in both monoculture and mixture was low. Thus, the strong underlying cultivar × mixture interaction found in this study complicates the identification of best cultivars in both settings. Yet, for each trait cultivars were identified that possessed statistically high phenotypic performance under the monoculture and mixed sward conditions. Thus, despite the substantial cultivar × environment effect, it should be possible to develop orchardgrass cultivars with improved agronomic performance under both management conditions.

Genotypic diversity and trait variance interact to affect marsh plant performance

SummaryIntraspecific diversity can have important effects on population, community and ecosystem processes, yet we have little understanding of the relative importance of genetic‐ versus trait‐based measures of intraspecific diversity.I conducted a manipulative field experiment of plant (Spartina alterniflora) genotypic diversity and trait diversity to examine their independent and interactive effects on plant performance and community structure. I focused on variation within and among genotypes in plant stem height, a trait that varies substantially across environmental gradients and can be an important predictor of plant competition intensity.Trait and genotypic diversity interactively affected multiple metrics of plant performance. Both stem density and spatial spread increased with genotypic diversity in the low trait diversity combinations, yet there were negligible to weak negative effects in the high trait diversity treatments.Spartina alterniflorapercentage cover also varied with genotypic and trait diversity, but not in a clear linear pattern.There were no effects of trait or genotypic diversity on associated macrofauna above‐ground, yet they interactively affected below‐ground measures. Infaunal abundance and sediment oxygen availability mirrored the idiosyncratic response of plant percentage cover.Despite the interactive effects of genotypic and trait diversity, high trait diversity consistently increased plant performance in genotypic monoculture.Synthesis. The effects of intraspecific plant trait diversity on a range of plant and community responses in this study reinforce the premise that functional differences underlie ecological effects of genetic diversity and suggest that readily measured trait variance may serve as a valuable predictor of plant performance.

Plot size matters: interference from intergenotypic competition in plant phenotyping studies.

Genetic and physiological studies often comprise genotypes diverse in vigour, size and flowering time. This can make the phenotyping of complex traits challenging, particularly those associated with canopy development, biomass and yield, as the environment of one genotype can be influenced by a neighbouring genotype. Limited seed and space may encourage field assessment in single, spaced rows or in small, unbordered plots, whereas the convenience of a controlled environment or greenhouse makes pot studies tempting. However, the relevance of such growing conditions to commercial field-grown crops is unclear and often doubtful. Competition for water, light and nutrients necessary for canopy growth will be variable where immediate neighbours are genetically different, particularly under stress conditions, where competition for resources and influence on productivity is greatest. Small hills and rod-rows maximise the potential for intergenotypic competition that is not relevant to a crop's performance in monocultures. Response to resource availability will typically vary among diverse genotypes to alter genotype ranking and reduce heritability for all growth-related traits, with the possible exception of harvest index. Validation of pot experiments to performance in canopies in the field is essential, whereas the planting of multirow plots and the simple exclusion of plot borders at harvest will increase experimental precision and confidence in genotype performance in target environments.

Belowground biotic complexity drives aboveground dynamics: a test of the soil community feedback model

Feedbacks between soil communities and plants may determine abundance and diversity in plant communities by influencing fitness and competitive outcomes. We tested the core hypotheses of soil community feedback theory: plant species culture distinct soil communities that alter plant performance and the outcome of interspecific competition. We applied this framework to inform the repeated dominance of Solidago canadensis in old-field communities. In glasshouse experiments, we examined the effects of soil communities on four plant species' performance in monoculture and outcomes of interspecific competition. We used terminal restriction fragment length polymorphism (TRFLP) analysis to infer differences in the soil communities associated with these plant species. Soil community origin had strong effects on plant performance, changed the intensity of interspecific competition and even reversed whether plant species were limited by conspecifics or heterospecifics. These plant-soil feedbacks are strong enough to upend winners and losers in classic competition models. Plant species cultured significantly different mycorrhizal fungal and bacterial soil communities, indicating that these feedbacks are likely microbiotic in nature. In old-fields and other plant communities, these soil feedbacks appear common, fundamentally alter the intensity and nature of plant competition and potentially maintain diversity while facilitating the dominance of So. canadensis.

Nitrogen uptake by grassland communities: contribution of N2 fixation, facilitation, complementarity, and species dominance

This study aimed to measure the effect of plant diversity on N uptake in grasslands and to assess the mechanisms contributing to diversity effects. Annual N uptake into above- and belowground organs and soil nitrate pools were measured in the Jena experiment on a floodplain soil with mixtures of 2–16 species and 1–4 functional groups, and monocultures. In mixtures, the deviation of measured data from data expected from monoculture performance was calculated to assess the contribution of complementarity/facilitation and selection. N uptake varied from <1 to 45 g N m−2 yr−1, and was higher in grasslands with than without legumes. On average, N uptake was higher in mixtures (21 ± 1 g N m−2 yr−1) than monocultures (13 ± 1 g N m−2 yr−1), and increased with species richness in mixtures. However, compared to N uptake expected from biomass proportions of species in mixtures, N uptake of mixtures was only slightly higher and a significant surplus N uptake was confined to mixtures containing legumes and non-legumes. In our study, high N uptake of species rich mixtures was mainly due to dominance of productive species and facilitation by legumes whereas complementarity among non-legumes was of minor relevance.

Density-dependent biodiversity effects on physical habitat modification by freshwater bivalves

Several decades of research have shown that biodiversity affects ecosystem processes associated with resource capture and the production of biomass within trophic levels. Although there are good reasons to expect that biodiversity influences non-trophic ecosystem processes, such as the physical creation or modification of habitat, studies investigating the role of biodiversity on physical processes are scarce. Here we report the results of a study using artificial streams to test the influence of freshwater mussel biodiversity on gravel erosion during high flows while manipulating mussel abundance. Mussel species vary in traits that should influence their effects on erosion, such as size, shell morphology, and burrowing behavior. We found that mussel species richness was associated with an increase in erosion at both low and high densities. Planned contrasts showed that the erosion observed in species mixtures was purely additive at low density, indicating that erosion in a species polyculture could routinely be predicted by the performance of monocultures. However, at high density certain combinations of species showed nonadditive effects on erosion, suggesting that organism abundance can fundamentally alter biodiversity effects. Although this may have been a result of altered species interactions at high density, our study design cannot confirm this.

Community and ecosystem effects of intraspecific genetic diversity in grassland microcosms of varying species diversity

Studies of whether plant community structure and ecosystem properties depend on the genetic diversity of component populations have been largely restricted to species monocultures and have involved levels of genetic differentiation that do not necessarily correspond to that exhibited by neighboring mature individuals in the field. We established experimental communities of varying intraspecific genetic diversity, using genotypes of eight species propagated from clonal material of individuals derived from a small (100-m2) limestone grassland community, and tested whether genetic diversity (one, four, and eight genotypes per species) influenced community composition and annual aboveground productivity across communities of one, four, and eight species. Eight-species communities were represented by common grass, sedge, and forb species, and four- and one-species communities were represented by four graminoids and the dominant grass Festuca ovina, respectively. After three years of community development, there was a marginal increase of species diversity with increased genetic diversity in four- and eight-species communities, and genetic diversity altered the performance of genotypes in monospecific communities of F. ovina. However, shifts in composition from genetic diversity were not sufficient to alter patterns of community productivity. Neighborhood models describing pairwise interactions between species indicated that genetic diversity decreased the intensity of competition between species in four-species mixtures, thereby promoting competitive equivalency and enhancing species equitability. In F. ovina monocultures, neighborhood models revealed both synergistic and antagonistic interactions between genotypes that were reduced in intensity on more stressful shallow soils. Although the dependence of F. ovina genotype performance on neighborhood genetic composition did not influence total productivity, such dependence was sufficient to uncouple genotype performance in genetic mixtures and monocultures. Our results point to an important connection between local genetic diversity and species diversity in this species-rich ecosystem but suggest that such community-level dependence on genetic diversity may not feedback to ecosystem productivity.

Aboveground overyielding in grassland mixtures is associated with reduced biomass partitioning to belowground organs

We investigated effects of plant species richness in experimental grassland plots on annual above- and belowground biomass production estimated from repeated harvests and ingrowth cores, respectively. Aboveground and total biomass production increased with increasing plant species richness while belowground production remained constant. Root to shoot biomass production ratios (R/S) in mixtures were lower than expected from monoculture performance of the species present in the mixtures, showing that interactions among species led to reduced biomass partitioning to belowground organs. This change in partitioning to belowground organs was not confined to mixtures with legumes, but also measured in mixtures without legumes, and correlated with aboveground overyielding in mixtures. It is suggested that species-rich communities invest less in belowground biomass than do monocultures to extract soil resources, thus leading to increased investment into aboveground organs and overyielding.

Mechanisms of invasion resistance: competition among intertidal mussels promotes establishment of invasive species and displacement of native species

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 383:187-197 (2009) - DOI: https://doi.org/10.3354/meps07982 Mechanisms of invasion resistance: competition among intertidal mussels promotes establishment of invasive species and displacement of native species J. S. Shinen1,*, S. G. Morgan2 1Estación Costera de Investigaciones Marinas, Departamento de Ecología, Pontificia Universidad Católica de Chile, Alameda 340, Santiago, Chile 2Bodega Marine Laboratory, Department of Environmental Science and Policy, University of California Davis, PO Box 247, Bodega Bay, California 94923, USA *Email: jlshinen@bio.puc.cl ABSTRACT: Understanding interactions between invasive species and recipient communities is essential to determining whether invasive species will become established and spread. In this study, we explored the role of competition and the specific mechanisms of interaction in limiting the spread of the Mediterranean bay mussel Mytilus galloprovincialis within a Pacific Northwest invasion front. We examined the role of direct (interference) and indirect (exploitation) mechanisms of competition among M. galloprovincialis and 2 native mussels (M. trossulus and M. californianus). As the fastest-growing organisms are often competitively dominant in space-limited systems, such as rocky intertidal communities, we used changes in relative performance (growth and survival) in monocultures and polycultures to assess interactions among mussels. Performance of M. galloprovincialis was always greater than that of the 2 native species of mussels in both field and laboratory manipulations of species composition and density, indicating that interspecific competition did not strongly limit the growth or survival of the invader. Moreover, the presence of M. galloprovincialis consistently led to both reduced growth and survival of M. trossulus. Laboratory studies of mussel feeding and behavior revealed M. galloprovincialis to be a robust interference competitor. The invader restricted movement, smothered and interfered with filter feeding of the 2 native species of mussels. Rather than limiting invasion, interference competition gave M. galloprovincialis a competitive advantage over the native mussels. Our results suggest M. galloprovincialis may have contributed to the displacement of M. trossulus along much of its historic southern range. KEY WORDS: Exploitation competition · Interference competition · Mytilus · Rocky intertidal communities Full text in pdf format PreviousNextCite this article as: Shinen JS, Morgan SG (2009) Mechanisms of invasion resistance: competition among intertidal mussels promotes establishment of invasive species and displacement of native species. Mar Ecol Prog Ser 383:187-197. https://doi.org/10.3354/meps07982 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 383. Online publication date: May 14, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

More plant biomass results in more offspring production in annuals, or does it?

Competitive ability in plants has been previously measured almost exclusively in terms of traits related to growth (biomass) or plant size. In this study, however, we used a multi‐species competition experiment with six annuals to measure relative competitive ability in terms of reproductive output, i.e. the number of offspring produced for the next generation. Under greenhouse conditions, plants of each species were started in pots from germinating seeds and were grown singly (free of competition) and at high density in both monocultures and in mixtures with all study species. Several traits traditionally regarded as determinants of competitive ability in plants were recorded for each species grown singly, including: seed mass, germination time, early growth rate and potential plant size (biomass and height). Under competition, several traits were recorded as indicators of relative performance in both monocultures and mixtures, including: biomass of survivors, total number of survivors, number of reproductive survivors, and reproductive output (total seed production) of the survivors. As expected, species that grew to a larger biomass in isolation had higher seed production in isolation. However, none of the traditional plant growth/size‐related traits, measured either in isolation or under competition, could predict between species variation in reproductive output under competition in either monocultures or mixtures. In mixtures, 97% of this variation in reproductive output could be explained by between‐species variation in the number of reproductive survivors. The results indicate that traits measured on plants grown singly may be poor predictors of reproductive output under competition, and that species’ rank order of competitive ability in terms of the biomass of survivors may bear no relationship to their rank order in terms of the number of offspring produced by these survivors. This has important implications for the interpretation of mechanisms of species coexistence and community assembly within vegetation.

Seeking a sound index of competitive intensity: Application to the study of biomass production under elevated CO2 along a nitrogen gradient

Abstract The aim of this paper is to evaluate (i) the relevance of currently proposed measures of competitive intensity to elevated CO2 studies by means of an example analysis, hypothesizing that competitive intensity is increased under elevated CO2; and (ii) an alternative method for predicting species performance in mixtures from monocultures. Relative competition intensity (RCI), relative physiological performance and normalized ecological performance were used to characterize the competitive ability of two grasses (Danthonia richardsonii Cashmore, Phalaris aquatica L.) and two legumes (Lotus pedunculatus Cav., Trifolium repens L.) grown in monocultures and mixtures of the four species along a N gradient under conditions of ambient and elevated CO2. Relative competition intensity could not be used to predict competitive outcomes in mixtures under conditions of elevated CO2 because it failed to account for changes in the size of interspecific differences along the N gradient and between CO2 concentrations. Relative physiological performance and relative ecological performance were more useful for investigating biomass production in mixtures and to predict species performance in mixtures from their performance in monocultures. Both indices of relative performance showed an increase in competitive intensity under elevated CO2 conditions. They also showed a decrease in competitive intensity with increasing N supply over most of the range of N supply, but a reversal of that trend at high levels of N supply. The merits and utility of these relative performance indices for elevated CO2 are discussed.

Evenness drives consistent diversity effects in intensive grassland systems across 28 European sites

1 Ecological and agronomic research suggests that increased crop diversity in species-poor intensive systems may improve their provision of ecosystem services. Such general predictions can have critical importance for worldwide food production and agricultural practice but are largely untested at higher levels of diversity. 2 We propose new methodology for the design and analysis of experiments to quantify diversity-function relationships. Our methodology can quantify the relative strength of inter-specific interactions that contribute to a functional response, and can disentangle the separate contributions of species richness and relative abundance. 3 Applying our methodology to data from a common experiment at 28 European sites, we show that the above-ground biomass of four-species mixtures (two legumes and two grasses) in intensive grassland systems was consistently greater than that expected from monoculture performance, even at high productivity levels. The magnitude of this effect generally resulted in transgressive overyielding. 4 A combined analysis of first-year results across sites showed that the additional performance of mixtures was driven by the number and strength of pairwise inter-specific interactions and the evenness of the community. In general, all pairwise interactions contributed equally to the additional performance of mixtures; the grass-grass and legume-legume interactions were as strong as those between grasses and legumes. 5 The combined analysis across geographical and temporal scales in our study provides a generality of interpretation of our results that would not have been possible from individual site analyses or experimentation at a single site. 6 Our four-species agricultural grassland communities have proved a simple yet relevant model system for experimentation and development of methodology in diversity-function research. Our study establishes that principles derived from biodiversity research in extensive, semi-natural grassland systems are applicable in intensively managed grasslands with agricultural plant species.

Synergistic effect of inorganic N and P fertilizers and organic inputs from Gliricidia sepium on productivity of intercropped maize in Southern Malawi

In Malawi, N and P deficiencies have been identified as major soil fertility constraints to maize (Zea mays, hybrid NSCM 41) productivity. In this study, we evaluated the effect of three rates of N and P fertilizers on maize performance in monoculture and maize intercropped with the nitrogen fixing legume gliricidia (Gliricidia sepium) in replicated field trials run for four years (2002/03-2005/06 seasons) at Makoka, in southern Malawi. Significant season-to-season variation was found in stand loss, ears per plant, stover yield, grain yield and thousand kernel weight (TKW), which was related to distribution of rainfall received during the growing season. All variables were significantly higher in the gliricidia/maize intercrop compared with monoculture maize. During the four consecutive cropping seasons, grain yields of maize increased by 343% (i.e. from 0.94 tons ha−1 in unfertilized sole maize to 4.17 tons ha−1 in gliricidia/maize intercropping). Optimum synergistic effect on grain yield (38% increase over unfertilized gliricidia/maize) was obtained when half recommended N and P rates were combined with gliricidia indicating interspecific facilitation. Response surface modelling showed that the optimum combination of factors for maximum grain yield (4.2 t ha-1) in monoculture maize was 80 kg N ha-1, 31 kg P ha-1 and 917 mm seasonal rainfall. In the gliricidia/maize intercrop, the stationary point had no unique maximum. Ridge analysis revealed that the estimated ridge of maximum grain yield (5.7 t ha-1) in the intercrop is when 69 kg N ha-1, 37 kg P ha-1 is applied and a seasonal rainfall of 977 mm is received. The total P uptake in the intercrop (14.3 kg ha-1) was significantly higher than that in maize monoculture (6.6 kg ha-1). P uptake was significantly (P = 0.008) influenced by P fertilizer rate. Therefore, we conclude that combining inorganic N and P fertilizers with organic inputs from gliricidia has positive and synergistic effects on maize productivity in southern Malawi.