conceptual papers (3)

papers to read on N fert and plant diversity

Plant diversity affects culturable soil bacteria in experimental grassland communities — by Jenny Talbot — last modified 2008-03-06 15:58

1 Utilization of carbon sources by culturable soil bacteria can be assessed with BIOLOG microtiter plates (contain 31 C sources). We used this technique to investigate bacterial community structure at various levels of plant diversity. Plant diversity levels were replicated and we investigated the influence of three plant functional groups, grasses, legumes and non-leguminous herbs, as well as the influence of individual plant species. 2 Catabolic activity and catabolic diversity of culturable soil bacteria were used to estimate their density (abundance) and functional diversity, respectively. Both increased linearly with the logarithm of plant species number and with the number of plant functional groups in experimental grassland ecosystems. These e€ffects may have been caused by an increased diversity and quantity of material and energy flows to the soil. They may also have been mediated by increased diversity of soil microhabitats via a stimulation of the soil fauna. 3 The presence of particular plant species or functional groups in the di€fferent experimental communities stimulated the activity and functional diversity of the culturable soil bacteria in addition to their contribution via plant diversity. The legume Trifolium repens had the strongest eff€ect and may be regarded as a keystone species with regard to plant±microbial interactions in the systems studied.

Functional- and abundance-based mechanisms explain diversity loss due to N fertilization — by Jenny Talbot — last modified 2008-03-06 16:17

Human activities have increased N availability dramatically in terrestrial and aquatic ecosystems. Extensive research demonstrates that local plant species diversity generally declines in response to nutrient enrichment, yet the mechanisms for this decline remain unclear. Based on an analysis of >900 species responses from 34 N-fertilization experiments across nine terrestrial ecosystems in North America, we show that both trait-neutral and trait-based mechanisms operate simultaneously to influence diversity loss as production increases. Rare species were often lost because of soil fertilization, randomly with respect to traits. The risk of species loss due to fertilization ranged from >60% for the rarest species to 10% for the most abundant species. Perennials, species with N-fixing symbionts, and those of native origin also experienced increased risk of local extinction after fertilization, regardless of their initial abundance. Whereas abundance was consistently important across all systems, functional mechanisms were often system-dependent. As N availability continues to increase globally, management that focuses on locally susceptible functional groups and generally susceptible rare species will be essential to maintain biodiversity.

A THEORETICAL MODEL OF LITTER DECAY AND MICROBIAL INTERACTION — by Jenny Talbot — last modified 2008-03-06 16:46

Despite the central role of microorganisms in the decomposition of dead organic matter, few models have integrated the dynamics of litter chemistry with microbial interactions. Here we propose a functional resolution of the microbial community that parallels the commonly used chemical characterization of plant litter, i.e., a guild of opportunist microorganisms that grows quickly and has high affinity for soluble substrates, a guild of decomposer specialists that grows more slowly and has high affinity for holocellulose substrates, and a guild of miners that grows very slowly and is specialized for degrading lignin. This guild-based decomposition model (GDM) includes the interactions of holocellulose and lignin, manifest as mutual feedback controls on microbial-based activities. It also includes N limitations on early stages of litter decay resulting from nutritional demands of microorganisms and N inhibition on late stages of litter decay resulting from reduced lignin degradation. Competitive interactions between microbial guilds result from different growth rates and substrate affinities, given limits on microbial colonization of litter. Simulations are consistent with commonly reported and proposed patterns of microbial community succession during litter decay, changes in and controls imposed by litter chemistry, and system responses to N availability. Modest impacts of litter chemistry and N effects on patterns of decay can yield substantial impacts on the relative amount of litter remaining through time, the time required to stabilize litter carbon (i.e., as the lignin content approaches ;70% of the total litter carbon), the relative contributions of different guilds to decay, and the net amount of microbial production. Moreover, seemingly inconsistent patterns in system responses to N regimes can be explained by interactions between litter chemistry and microbial guilds. A validation exercise demonstrated general correspondence of model behavior to field observations. However, relationships among mass loss, litter chemistry, and N availability were more variable in field studies than in simulations. Also, observed changes in litter quality indicated the progressive accumulation of microbial products. Hence, field studies suggest expanding GDM to include dynamics of microbial products and also suggest the utility of GDM in exploring site effects on decomposition as a result of differences in microbial community composition.