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 ( ) a Pinus sylvestris forest with an understorey of ericaceous dwarf shrubs (similar to site DS of Betsele); N0 - N3 (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 pH (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 POSITIVE relationship – C/N ratio and N mineralization NEGATIVE relationship & nitrification 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) 15 N pool dilution experiment Air dried and rewetted – Burst of microbial growth

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