Soil Chemical Analysis

Humans are altering the composition of biological communities through a variety of activities that increase rates of species invasions and species extinctions, at all scales, from local to global. These changes in components of the Earth's biodiversity cause concern for ethical and aesthetic reasons, but they also have a strong potential to alter ecosystem properties and the goods and services they provide to humanity. Ecological experiments, observations, and theoretical developments show that ecosystem properties depend greatly on biodiversity in terms of the functional characteristics of organisms present in the ecosystem and the distribution and abundance of those organisms over space and time. Species effects act in concert with the effects of climate, resource availability, and disturbance regimes in influencing ecosystem properties. Human activities can modify all of the above factors; here we focus on modification of these biotic controls. The scientific community has come to a broad consensus on many aspects of the re- lationship between biodiversity and ecosystem functioning, including many points relevant to management of ecosystems. Further progress will require integration of knowledge about biotic and abiotic controls on ecosystem properties, how ecological communities are struc- tured, and the forces driving species extinctions and invasions. To strengthen links to policy and management, we also need to integrate our ecological knowledge with understanding of the social and economic constraints of potential management practices. Understanding this complexity, while taking strong steps to minimize current losses of species, is necessary for responsible management of Earth's ecosystems and the diverse biota they contain.

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Effects of biodiversity on ecosystem functioning: a consensus of current knowledge

A doubling in global food demand projected for the next 50 years poses huge challenges for the sustainability both of food production and of terrestrial and aquatic ecosystems and the services they provide to society. Agriculturalists are the principal managers of global useable lands and will shape, perhaps irreversibly, the surface of the Earth in the coming decades. New incentives and policies for ensuring the sustainability of agriculture and ecosystem services will be crucial if we are to meet the demands of improving yields without compromising environmental integrity or public health.

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Agricultural sustainability and intensive production practices

The most unique feature of Earth is the existence of life, and the most extraordinary feature of life is its diversity. Approximately 9 million types of plants, animals, protists and fungi inhabit the Earth. So, too, do 7 billion people. Two decades ago, at the first Earth Summit, the vast majority of the world's nations declared that human actions were dismantling the Earth's ecosystems, eliminating genes, species and biological traits at an alarming rate. This observation led to the question of how such loss of biological diversity will alter the functioning of ecosystems and their ability to provide society with the goods and services needed to prosper.

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Biodiversity loss and its impact on humanity

When a person looks at an object while exploring it with their hand, vision and touch both provide information for estimating the properties of the object. Vision frequently dominates the integrated visual-haptic percept, for example when judging size, shape or position, but in some circumstances the percept is clearly affected by haptics. Here we propose that a general principle, which minimizes variance in the final estimate, determines the degree to which vision or haptics dominates. This principle is realized by using maximum-likelihood estimation to combine the inputs. To investigate cue combination quantitatively, we first measured the variances associated with visual and haptic estimation of height. We then used these measurements to construct a maximum-likelihood integrator. This model behaved very similarly to humans in a visual-haptic task. Thus, the nervous system seems to combine visual and haptic information in a fashion that is similar to a maximum-likelihood integrator. Visual dominance occurs when the variance associated with visual estimation is lower than that associated with haptic estimation.

Humans integrate visual and haptic information in a statistically optimal fashion.

Summary 1 We established monocultures and 16 unique mixed communities of 12 native grasses, legumes and non-N 2 -fixing forbs. We identified species having a greater or lesser yield in the mixed communities than expected from monoculture data as winners and losers, respectively. To test our hypothesis that performance of the subordinate species (losers) is mainly controlled by light availability, whereas the dominant species (winners) are sensitive to the availability of below-ground resources, we traced the effects of number of species, light transmission in the community and foliar N concentration on plant photosynthesis through leaf C isotope composition ( δ 13 C). 2 Phalaris arundinacea and Phleum pratense , the two tallest grass species, yielded more in mixtures than expected, as, initially, did Dactylis glomerata . Festuca ovina , the smallest grass, and Ranunculus acris , a forb, had smaller yield in mixtures than expected. For most species, observed mixture yields did not deviate significantly from those expected. 3 Decreases in transmitted light decreased δ 13 C in D. glomerata , Lotus corniculatus and Rumex acetosa . The δ 13 C of Trifolium pratense and L. corniculatus was affected by increasing number of species in the plant community even after accounting for the transmitted light. In P. arundinacea , δ 13 C increased with increasing foliar %N, as expected for the tallest, dominant species. 4 Species showing a positive, significant relationship between δ 13 C and transmitted light were relatively low growing and unable to establish dominance in multispecies communities due to shading by larger dominants. 5 We conclude that above-ground competition is crucial in determining C isotope composition among the subordinate species, whereas the dominant species are more strongly affected by below-ground resources. Different factors thus dictate the physiological performance of species according to the size-distribution hierarchy in the community.

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Winners and losers in herbaceous plant communities: insights from foliar carbon isotope composition in monocultures and mixtures

We report here on an 8-year study examining links between plant and nematode communities in a grassland plant diversity experiment, located in the north of Sweden on previous agricultural soil. The examined plots contained 1, 4 and 12 common grassland plant species from three functional groups; grasses, legumes and forbs. The same plant species composition was maintained in the plots through weeding and resowing during the experimental period. The hypotheses were (i) that the nematode community would shift towards a more diverse and mature fauna over time and (ii) that the effects of both plant species identity and plant species richness would increase over time. As hypothesized, the Bongers’ Maturity Index (a measure of nematode responses to disturbance) increased over time, but not nematode diversity. Instead, the nematode community development in the present grassland experiment seemed to be more characterized by shifts in dominance patterns than by colonization of new taxa. Clear temporal trends were found for plant-feeders and Adenophorea bacterial-feeders which increased in abundance over time in almost all plant treatments. Rhabditidae bacterial-feeders decreased in abundance over time, in particular in plots with legumes. Fungal-feeders, omnivores/predators and the two nematode indices PPI (Bongers’ Plant Parasitic Index) and NCR (Nematode Channel Ratio) had significant interactions between plant composition and time giving some support to our second hypothesis. Our results highlight the need for long-term experiments to examine plant species effects on soil fauna, especially on taxa belonging to higher trophic levels. The results also stress the importance of plant composition for belowground soil faunal communities.

Temporal dynamics of soil nematode communities in a grassland plant diversity experiment

Transfer of fixed N from legumes to non-legume reference plants may alter the 15N signature of the reference plant as compared to the soil N available to the legume. This study investigates how N transfer influences the result of 15N-based N2 fixation measurements. We labelled either legumes or non-legumes with 15N and performed detailed analyses of 15N enrichment in mixed plant communities in the field. The results were used in a conceptual model comparing how different N transfer scenarios influenced the 15N signatures of legumes and reference plants, and how the resulting N2 fixation estimate was influenced by using reference plants in pure stand or in mixture with the legume. Based on isotopic signatures, N transfer was detected in all directions: from legume to legume, from legume to non-legume, from non-legume to legume, from non-legume to non-legume. In the scenario of multidirectional N transfer, N2 fixation was overestimated by using a reference plant in pure stand. Fixed N transferred to neighbouring reference plants modifies the 15N signature of the soil N available both to the reference plant and the N2-fixing legume. This provides strong support for using reference plants growing in mixture with the legumes for reliable quantifications of N2 fixation.

Does nitrogen transfer between plants confound 15N-based quantifications of N2 fixation?

Huss-Danell, K. and Sellstedt, A. 1985. Nitrogenase activity in response to darkening and defoliation of Alnus incana.—l. exp. Bot. 36: 1352-1358. In the Alnus-Frankia symbiosis the nitrogen-fixing root nodules are one of the sinks for carbon compounds newly formed in photosynthesis and exported from the leaves (source). The source-sink ratio of cloned plants of Alnus incana was reduced by darkening or by total or partial defoliation and the resulting nitrogenase activity (C2H2-reduction) was measured. Nitrogenase activity had nearly ceased 5 h after total defoliation but not until ca. 5 d after total darkening. Most of the activity was lost during the initial hours and days, respectively. When leaf area was reduced approximately by half nitrogenase activity decreased slightly less than by half. Removal of upper leaves seemed less harmful

Nitrogenase Activity in Response to Darkening and Defoliation of Alnus incana

Methods based on amplification and recognition of Frankia -specific DNA have recently been developed to quantify Frankia in soil. But measuring numbers of infective Frankia in soil still requires bioassays involving nodulation of appropriate host plants. We compared three alder species, Alnus glutinosa (L.) Gaertn., A. incana (L.) Moench and A. rubra Bong, grown in two nutrient solutions under controlled conditions in a climate chamber. Plants were either kept in the same cultivation tube or, to remove most of the inoculum, they were moved into a new tube 2 weeks after inoculation. Soil dilution series prepared from a forest soil served as inocula. Roots were inspected for nodules weekly for 8 weeks and the number of nodulation units (NU) g −1 soil was calculated according to the most probable number (MPN) method and according to a nodulation capacity method. Alnus glutinosa required the longest time to nodulate. The number of NU detected was in the order A. rubra > A. incana > A. glutinosa in each of the nutrient solutions used. Eight weeks after inoculation, number of NU ranged from 5 to 400 as determined by the MPN-method, and from 10 to 380 as determined by the nodulation capacity method in the different species, solution and tube combinations used. The bioassay seems appropriate for comparisons between soils when the same plant genotypes are used under the same conditions, but otherwise comparisons should be made with caution. The MPN method and the nodulation capacity method essentially gave the same results. The MPN method is somewhat less labour intensive.

Kerstin Huss-Danell

Papers

Winners and losers in herbaceous plant communities: insights from foliar carbon isotope composition in monocultures and mixtures

Temporal dynamics of soil nematode communities in a grassland plant diversity experiment

Does nitrogen transfer between plants confound 15N-based quantifications of N2 fixation?

Nitrogenase Activity in Response to Darkening and Defoliation of Alnus incana

Intrageneric variation in nodulation of Alnus: consequences for quantifying Frankia nodulation units in soil