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The fumigation–incubation (FI) and the fumigation–extraction (FE) ninhydrin methods for quantifying the microbial biomass pool were tested and the relationships between microbial biomass characteristics and soil organic matter fractions (separated following particle size) investigated for a range of soils representative for the West-African moist savanna zone (13 soils). Three soils from the humid forest zone were also included. Microbial C values calculated using the CO2-C production of the fumigated soils from d 10 to d 20 as control (Bio-C (II)) were better correlated with the flush of ninhydrin reactive N (NRN) during 10 d of fumigation than those calculated with the CO2-C production in the unfumigated soils from d 10 to d 20 as control (Bio-C (I)). The Bio-C (II) values also showed a more consistent range of values (22–210 mg C kg−1 soil) than the Bio-C (I) values. Using a kC factor of 0.35, which was considered to be more appropriate for our soils than a kC factor of 0.45, kC,NRN could be estimated as 22 and 16, for a 5- and 10-d fumigation, respectively. For all savanna soils, the NRN flush after 5 d of fumigation was closely related to the NRN flush after 10 d, indicating that a 5-d fumigation was sufficient provided that the k-values are adapted. Mineral N flushes during incubation and incubation after fumigation were small. Although microbial N values calculated as [NH4+-N flush of the fumigated soils (0–10 d)]/kN with kN=−0.014×(CO2-C-flush-to-NH4+-N-flush during fumigation)+0.39, which gave values of 14.0–100.7 mg N kg−1 soil, showed the best relationship with the NRN flush after 10 d of fumigation, microbial N values calculated as [(mineral N flush of the fumigated soils after 10 d)−(mineral N flush of the unfumigated soils after 10 d)]/0.68, which gave values of 3.4–46.2 mg N kg−1 soil, including 3 values |
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