Relationships between soil microbial indices in secondary tropical forest soils

In Bacnotan, Philippines, we investigated soils from six secondary tropical forest sites developed from the same parent material, but differing in the present vegetation, in stand age, and in land use history. The aim was to assess the effects of these differences on the activity, biomass and community structure of soil micro-organisms in the A-horizon. The six sites investigated revealed strong differences in soil organic matter and soil microbiological properties. Although these were all within the range described in the literature, our six tropical forest sites reveal several marked differences to forest soils from a temperate climate. The average ratio of microbial biomass C-to-soil organic C was 2.8%, exceeding that of microbial biomass N-to-total N of 2.0%. This means that the average microbial biomass C-to-N ratio of 14.1 exceeded the average soil organic C-to-total N ratio of 10.1. The mean ratios of ergosterol-to-microbial biomass C and ATP-to-microbial biomass C were 0.19% and 4.3 mumol g(-1), respectively, both very low. Adenylate energy charge (AEC): (ATP + 0.5ADP)/(ATP + ADP + AMP) and metabolic quotient qCO(2) reached average levels of 0.71 and 36 (mg CO2-C g(-1) biomass per day), respectively. The different ratios of soil biological and soil chemical properties could be assigned to three different factors by principal component analysis. The ratios soil organic C-to-total N, ergosterol-to-microbial biomass C, and microbial biomass N-to-total N formed the first factor, reflecting a decomposition pathway characterised by fungi. The ratios ATP-to-microbial biomass C, AEC and qCO(2) were combined as the second factor, characterising the specific metabolic activity of soil micro-organisms. The third factor was loaded by the ratios microbial biomass C-to-soil organic C and microbial biomass C-to-N, characterising soil organic matter availability in relation to nutrient availability. (C) 2002 Elsevier Science B.V. All rights reserved.