Number Of Species In Mixture Research Articles

Negative to positive shifts in diversity effects on soil nitrogen over time

Soil nitrogen (N) availability is of critical importance to the productivity of terrestrial ecosystems worldwide. Plant diversity continues to decline globally due to habitat conversion and degradation, but its influence on soil N remains uncertain. By conducting a global meta-analysis of 1,650 paired observations of soil N in plant species mixtures and monocultures from 149 studies, we show that, on average across observations, soil total N is 6.1% higher in species mixtures. This mixture effect on total N becomes more positive with the number of species in mixtures and with stand age. The mixture effects on net N mineralization rate and inorganic N concentrations shift from negative in young stands to positive in older stands with greater positive effects in more-diverse mixtures. These effects of mixtures were consistent among cropland, forest and grassland ecosystems and held across climate zones. Our results suggest that plant diversity conservation not only enhances the productivity of current vegetation but also increases soil N retention that will sustain the productivity of future vegetation. Soil nitrogen is vital for terrestrial ecosystems. This study finds that soil nitrogen increases with the number and age of plant species, suggesting another benefit from conserving plant diversity.

Effects of two measures of riparian plant biodiversity on litter decomposition and associated processes in stream microcosms

Plant litter decomposition is a key ecosystem process that can be altered by global changes such as biodiversity loss. These effects can be particularly important in detritus-based ecosystems, such as headwater streams, which are mainly fuelled by allochthonous plant litter inputs. However, experiments examining effects of plant diversity on litter decomposition in streams have not reached consensus about which measures of biodiversity are more relevant. We explored the influence of two of these measures, plant species richness (SR; monocultures vs. 3-species mixtures) and phylogenetic distance (PD; species belonging to the same family vs. different families), on leaf litter decomposition and associated processes and variables (nutrient dynamics, fungal biomass and detritivore growth), in a stream microcosm experiment using litter from 9 tree species belonging to 3 families. We found a negative effect of SR on decomposition (which contradicted the results of previous experiments) but a positive effect on fungal biomass. While PD did not affect decomposition, both SR and PD altered nutrient dynamics: there was greater litter and detritivore N loss in low-PD mixtures, and greater litter P loss and detritivore P gain in monocultures. This suggested that the number of species in mixtures and the similarity of their traits both modulated nutrient availability and utilization by detritivores. Moreover, the greater fungal biomass with higher SR could imply positive effects on detritivores in the longer term. Our results provide new insights of the functional repercussions of biodiversity loss by going beyond the often-explored relationship between SR and decomposition, and reveal an influence of plant species phylogenetic relatedness on nutrient cycling that merits further investigation.

Productivity, Botanical Composition, and Nutritive Value of Commercial Pasture Mixtures

Pastures in the northeastern United States are often planted to mixtures of grasses and legumes. We evaluated several commercial seed mixtures to determine if the number of species in mixture affected yield and botanical composition in central Pennsylvania. Three replicate plots of 25 mixtures, five each of two, three, four, five, and six species of grasses and legumes were planted in August 2007 near State College, PA. Plots were grazed by beef cows for 3 yr. Dry matter (DM) yield was determined at each of the six harvests in each year. Botanical composition was measured at the first, third, and last harvest each year. Annual DM yield was positively related to the number of species planted (r2 = 0.15, 0.18, and 0.26 for 2008, 2009, and 2010, respectively). Mixtures within groups of similar species richness, however, varied in yield. Kentucky bluegrass (Poa pratensis L.), birdsfoot trefoil (Lotus corniculatus L.), and timothy (Phleum pratense L.) did not establish or persist well in mixture with taller grasses [e.g., orchardgrass (Dactylis glomerata L.)]. The short‐lived species festulolium (X Festulolium Asch. & Graebn.) and red clover (Trifolium pratense L.) contributed significantly to production during the first year and were replaced by longer‐lived species such as orchardgrass and white clover (T. repens L.). Forage nutritive value was related to the botanical composition but not the number of species in mixture. Results suggest that a strategy of planting mixtures with a combination of fast‐ and slow‐establishing species can be effective for multispecies pastures in central Pennsylvania.