Gross nitrogen mineralization and fungitobacteria ratios are negatively
Gross nitrogen mineralization and fungi-to-bacteria ratios are negatively correlated in boreal forests Mona N. Högberg, Yu Chen, Peter Högberg; Biol Fertil Soils, 2007
Introduction In terrestrial ecosystems gross N mineralization is: • positively correlated to microbial biomass • negatively correlated to soil organic matter C/ N ratios (also to inorganic N loss) C/N ratio • Bacteria: around 5: 1 • Fungi: more variable
Fungi • Mycorrhizal fungi: C from the host plant, N from the soil • Saprotrophic fungi: C/N ratio correlates with the soil substrates mycorrhizal fungi are stronger N sinks under N-limitation
Hypothesis • soil fungi/bacteria- and C/N-ratios are interrelated • fungi have strong influences on soil N cycling in boreal forests
Materials and Methods 1) Betsele (Northern Sweden): 3 natural forest types DS… nutrient poor dwarf shrub forest SH… intermediate short herb type TH… and a rich tall herb forest 2) Norrliden (Northern Sweden): long term N-loading experiment (1971 -2003) a Pinus sylvestris forest with an understorey of ericaceous dwarf shrubs (similar to site DS of Betsele); N 0 - N 3 (DS)
Materials and Methods • Microbial community: PLFA (Phopholipid fatty acids) Fungi: 18: 2ω6 (linoleic acid) Bacteria: sum of 12 specific bacterial PLFAs • Gross N mineralization: 15 NH + 4 pool dilution technique
Results
Discussion • very strong negative correlation between gross N mineralization and fungi/bacteria ratio • key role of fungi in regulating N cycling in boreal forests • weaker and less significant positive correlation between gross N mineralization and soil p. H • (gross N mineralisation is very strong negatively correlated to soil organic matter C: N ratios)
Gross nitrogen mineralization -, immobilization-, and nitrification rates as a function of soil C/N ratio and microbial activity G. Bengtsson, P. Bengtson, K. F. Månsson; Soil Biology & Biochemistry, 2002
Introduction • Low NO 3 - concentrations in forest soils – Rapid turnover of small NO 3 - pool • NO 3 - (NH 4+) immobilization in SOM pool – By biotic (dominant) and abiotic processes • Variation of N immobilization (35 -95 %) • N mineralization and immobilization factors: – C/N ratio, . . – Heterotrophic soil bacteria
Heterotrophic soil bacteria • High biomass and activity – Competition with heterotrophs – Low nitrification rates • Heterotrophic soil bacteria: low C/N ratio (10) – Soil C/N ratio: > 20 N limited < 20 C limited (Tate, 1995)
Hypothesis • A laboratoy experiment to challenge: – C/N ratio and N immobilization – C/N ratio and N mineralization & nitrification POSITIVE relationship NEGATIVE relationship • Main Hypothesis: Gross nitrogen immobilization and mineralization rates are more related to the respiration rate and ATP content than to the C/N ratio. High influence: soil temperture and moisture • Subhypothesis: – variation in N transformation rates – variations in microbial biomass and activity
Materials and Methods • 3 deciduous forest soil samples (soil I, II and III) Southwestern Sweden (1998) pool dilution experiment • Air dried and rewetted – Burst of microbial growth • 15 N
Measurements • Gross mineralization, - nitrification, and NO 3 - and NH 4+ immobilization • Respiration Rate • ATP production (net ATP production) • C/N ratio
Results & Discussion Hypothesis: The study suggested that differences in respiration rate and ATP content are more indicative of the magnitude of gross immobilization and mineralization Respiration rate
Results & Discussion • Gross nitrification rates were negatively related to ATP content and C/N ratio Gross nitrification – Immobilization = Net nitrification
Conclusio • Covariation between mineralization, immobilization, ATP content and respiration • Tight connection between N turnover and retention and the microbial activity • Positive correlation between ATP content and net nitrification rate Property of forest soils may be more variable with its microbial biomass and activity than on its C/N ratio
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