INVESTIGADORES
ZABALOY Maria Celina
congresos y reuniones científicas
Título:
Community-level physiological profiling of soil communities based on oxygen consumption
Autor/es:
GARLAND, JAY L.; ZABALOY, MARÍA CELINA
Lugar:
Buenos Aires
Reunión:
Taller; Taller Internacional de Rizosfera, Biodiversidad y Agricultura Sustentable; 2010
Institución organizadora:
AAM
Resumen:
The relative importance of carbon and nitrogen limitation on microbial activity in soils has significant consequences for effective agricultural nutrient management and potential strategies to enhance soil carbon (C) sequestration. While C is often considered the primary factor limiting microbial activity nitrogen (N) availability can have important influences on the fate of processed carbon. The traditional view emphasized the linkage between respiration and growth, leading to definition of C/N thresholds which control the relative amounts of microbial N immobilization or mineralization. Recent work has examined the effects of the uncoupling between respiration and growth on C and N processing in soil. Studies in chemostat culture and in the field have demonstrated that the efficiency of microbial utilization of carbon decreases under nutrient limited conditions. Decreased efficiency can involve several mechanisms, including incomplete oxidation of substrates (i.e, overflow metabolism), uncoupled oxidation of NADH from ATP production (e.g, branches in the electron transport chain), and decreased membrane resistance resulting in reduced efficiency of ATP generation via chemiosmosis (i.e., energy spilling). The potential selective advantage of ?wasting? energy is the maintenance of ATP levels and other precursor molecules to enable rapid initiation of growth once N (or other nutrients) becomes available. A similar rationale of ?metabolic alertness? has been invoked to explain the rapid increase in respiration by soil microorganisms in response to low levels of carbon substrates. The most commonly employed approach to assess nitrogen limitation involves measuring the difference in substrate induced respiration (SIR) of soil samples with and without N supplementation. CO2 is monitored during 5-10 day incubations following addition of ~30-40 mg C g-1 soil to ensure that growth occurs. This approach is limited because it indirectly assesses growth, and due to both the selective enrichment and induction of N limitation resulting from the high levels of C amendment. Direct measurement of growth via leucine or thymidine incorporation using much lower levels of C amendment (< 3 mg C g-1 soil) provides a more effective means of assessing C and nutrient limitation on growth. The observation that cells regulate metabolism to maximize survival, rather than optimize efficiency, suggests that methods are needed to assess potential impacts of C and N on microbial communities adapted to both uncoupled and coupled respiration-growth modes. Cell production and/or respiration assays based on radiolabeled substrates are powerful tools to address these research questions, largely because they allow for physiological assessment of minimally enriched communities using ecological relevant substrate levels. A simple yet versatile platform for assessing microbial respiration which shares some of these potential strengths has been recently introduced. This technique builds on the community-level physiological profiling (CLPP) approach by replacing the Biolog redox dye chemistry with an oxygen-sensitive fluorophore. The new method reduces selective enrichment bias by detecting substrate-induced respiration in soils at amendment levels two to three orders of magnitude lower than those required in Biolog plates (~100 ug C g-1 soil), and even allows for rapid assessment of basal soil respiration. Since the new assay does not employ any proprietary, ?growth supporting? nutrients, soil composition, such as available N, can be readily manipulated. Therefore, the assay allow for rapid assessment of C (in a variety of forms) and N amendments on microbial respiration. Preliminary studies have generally reported stimulatory effects of N supplementation on soil respiration with the assay. Recent work found that the difference in oxygen consumption by soils with and without N supplementation (Ndiff) during short term (≤ 8 h) incubations transiently decreased following fertilization of lab microcosms. A predictive relationship between Ndiff and bioavailable N would provide a rapid tool for assessing soil N status. While work to date has reported a general inverse relationship between Ndiff and levels of extractable inorganic soil N, significant variability in the relationship exists, particularly when comparing different soil types. The variability may partially result from an incomplete assessment of bioavailable N in work to date. Further studies are needed to correlate Ndiff with alternative, but more time-consuming, approaches for measuring bioavailable N (i.e., potentially mineralizable N. Results to date have also indicated a complex relationship between the soil microbial community and N availability. The rapid nature of the assay has allowed for testing of various substrates, and data has indicated substrate-specific variation in N-limitation, suggesting differential sensitivity of various functional pathways or groups of microorganisms to N-limitation. In addition, decreased respiration with N amendment has been observed in a few soil microbial communities, and appears to be related to either acute or chronic exposure to organic forms of nitrogen. The decreased response would be predicted if the communities were dominated by individuals which were 1) wasting energy to maintain active cell machinery under N limitation chronic, and/or 2) using energy to take up organic forms of N. Despite the increased number of methods and studies related to biological soil quality, a recent review concluded that it is unclear if effective indicators of soil biological quality exist, or are actually needed. This conclusion appears largely based on the concern that the current suite of biological indicators do not predictively respond to management approaches in an easily interpretable manner which can lead to cost-effective monitoring/control programs. Perhaps this conclusion shouldn?t be surprising given the multifaceted nature of soil quality and the complexity of microbial communities. The rapid microbial respiration assay described here is a promising new approach to address these concerns by directly linking microbial activity and the bioavailability of nutrients critical to sustaining agricultural production.