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We investigated arbuscular mycorrhizal fungi ( AMF) alteration of microbial mediation of litter decomposition. AMF ( Glomus hoi) were either allowed access to or excluded from Plantago lanceolata L. root litter embedded in soil; litter was labeled with either 13 C only or 13 C and 15 N. Plant N uptake was significantly increased if AMF accessed the litter, and 15 N analysis of the plant material indicated that 2-3% of plant N originated from litter. Succession of the soil community mediating decomposition was assessed by phospholipid fatty acids ( PLFA) combined with 13 C- PLFA. During the first 21 days of decomposition, saprotrophic fungi and Gram-negative bacteria were the dominant consumers of litter C. As decomposition progressed however, 13 C content of the fungal biomarkers declined substantially, and Gram-negative and Gram-positive bacteria became the primary reservoirs of labeled litter C. The putative PLFA marker for AMF (16:1ω5 c) originated primarily from non- AMF sources. In AMF-invaded root litter, Gram-negative, Gram-positive, and 16:1ω5 c markers became less 13 C-enriched relative to their counterparts in non- AMF-invaded microcosms during active decomposition. These patterns of 13 C: 12 C enrichment may result from AMF supply of 12 C from the plant to the decomposing soil microbial community; such C inputs could alter the microbial mediation of litter decomposition. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal (AM) fungi can enhance the rate of decomposition of organic material, and can acquire nitrogen (N) from organic sources, although they are not saprotrophs. These fungi may instead indirectly influence decomposition through interactions with other soil microorganisms. We investigated the impact of both AM hyphae and a bacterial filtrate on N capture by a host plant from sterilized organic material ( Lolium perenne shoots dual labelled with N and C), using compartmented microcosms. The addition of a bacterial filtrate considerably suppressed AM hyphal growth in the patch and reduced the root phosphorus content, demonstrating that bacterial populations can reduce symbiotic effectiveness. In contrast, AM hyphae had only a limited impact on bacterial community structure. Uptake of N greatly exceeded that of C, demonstrating that fungi acquired N in an inorganic form. We also examined the ability of AM fungi in gnotobiotic hairy root culture to acquire N directly from organic materials of varying complexities (glutamic acid, urea, bacterial lysate and L. perenne shoots). AM colonization did not enhance root N capture from these materials, although the bacterial lysate reduced both total AM colonization and arbuscule frequency. Collectively, these data demonstrate antagonistic interactions between AM fungi and bacteria that reflect resource competition for decomposition products. [ABSTRACT FROM AUTHOR]

The distribution and partitioning of dry matter and photoassimilate of Lolium perenne was investigated under two light regimes providing photosynthetically active radiation of 350 μmol m-2 s-1 (low light treatment) or 1000 μmol m-2 s-1 (high light treatment). Plants were grown at specific growth conditions in either soil or sand microcosm units to follow the subsequent release of carbon into the rhizosphere and its consequent incorporation into the microbial biomass (soil system) or recovery as exudates (sand system). The distribution of recent assimilate between the plant and root released carbon pools was determined using 14CO2 pulse-chase methodology at both light treatments and for both sand- and soil-grown seedlings. A significant (P≤0.05) increase in partitioning of 14C-label below-ground occurred for both soil- and sand-grown seedlings at the increased light treatment. Incorporation of recent assimilate into the microbial biomass, however, was unaltered by light treatment. Total plant biomass of L. perenne seedlings grown in the sand microcosm unit was unaffected by light treatment, but differences in partitioning of biomass did occur resulting in an increased root-to-shoot ratio under high light. Soil-grown L. perenne seedings showed a large (81%) and significant (P≤0.001) increase in shoot biomass under high light with a consequent decrease in root-to-shoot ratio. Specific leaf area was altered in the soil grown plants only, where it decreased under high light. Total organic carbon (TOC) content of the recovered exudate material was measured throughout the 14 d experimental period and during the 14C-chase period. Comparison of plant C budgets using these two measurements is discussed. [ABSTRACT FROM PUBLISHER]

A preface to the "Journal of Experimental Botany" is presented.