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We examined the correlation between fungal and bacterial biomass, abiotic factors such as soil moisture, carbon in the light soil fraction and soil nitrogen to a depth of 0–25 cm and heterotrophic soil respiration using a trenching technique – in a secondary forest ( Myrcia splendens, Miconia prasina and Casearia arborea) and a pine ( Pinus caribeae) plantation in the Luquillo Experimental Forest in Puerto Rico. Soil respiration was significantly reduced where roots were excluded for 7 years in both the secondary forest and the pine plantation. Microbial biomass was also significantly reduced in the root exclusion plots. In root exclusion treatment, total fungal biomass was on average 31 and 65% lower than the control plots in the pine plantation and the secondary forest, respectively, but the total bacterial biomass was 24 and 8.3% lower than the control plots in the pine plantation and the secondary forest, respectively. Heterotrophic soil respiration was positively correlated with fungal biomass ( R 2=0.63, R 2=0.39), bacterial biomass (R2=0.16, R 2=0.45), soil moisture ( R 2=0.41, R 2=0.56), carbon in light fraction ( R 2=0.45, R 2=0.39) and total nitrogen ( R 2=0.69, R 2=0.67) in the pine plantation and the secondary forest, respectively. The regression analysis suggested that fungal biomass might have a greater influence on heterotrophic soil respiration in the pine plantation, while the bacterial biomass might have a greater influence in the secondary forest. Heterotrophic soil respiration was more sensitive to total N than to carbon in the light fraction, and soil moisture was a major factor influencing heterotrophic soil respiration in these forests where temperature is high and relatively invariable. [ABSTRACT FROM AUTHOR]

Wet tropical forests play a critical role in global ecosystem carbon (C) cycle, but C allocation and the response of different C pools to nutrient addition in these forests remain poorly understood. We measured soil organic carbon (SOC), litterfall, root biomass, microbial biomass and soil physical and chemical properties in a wet tropical forest from May 1996 to July 1997 following a 7-year continuous fertilization. We found that although there was no significant difference in total SOC in the top 0–10 cm of the soils between the fertilization plots (5.42±0.18 kg m−2) and the control plots (5.27±0.22 kg m−2), the proportion of the heavy-fraction organic C in the total SOC was significantly higher in the fertilized plots (59%) than in the control plots (46%) ( P<0.05). The annual decomposition rate of fertilized leaf litter was 13% higher than that of the control leaf litter. We also found that fertilization significantly increased microbial biomass (fungi+bacteria) with 952±48 mg kg−1soil in the fertilized plots and 755±37 mg kg−1soil in the control plots. Our results suggest that fertilization in tropical forests may enhance long-term C sequestration in the soils of tropical wet forests. [ABSTRACT FROM AUTHOR]

We examined the effects of root and litter exclusion on the rate of soil CO2 efflux and microbial biomass using trenching and tent separation techniques in a secondary forest (SF) and a pine (Pinus caribaeaMorelet) plantation in the Luquillo Experimental Forest in Puerto Rico. Soil surface CO2 efflux was measured using the alkali trap method at 12 randomly-distributed locations in each treatment (control, root exclusion, litter exclusion, and both root and litter exclusion) in the plantation and the SF, respectively. We measured soil CO2 efflux every two months and collected soil samples at each sampling location in different seasons to determine microbial biomass from August 1996 to July 1997. We found that soil CO2 efflux was significantly reduced in the litter and root exclusion plots (7-year litter and/or root exclusion) in both the secondary forest and the pine plantation compared with the control. The reduction of soil CO2 efflux was 35.6% greater in the root exclusion plots than in the litter exclusion plots in the plantation, whereas a reversed pattern was found in the secondary forest. Microbial biomass was also reduced during the litter and root exclusion period. In the root exclusion plots, total fungal biomass averaged 31.4% and 65.2% lower than the control plots in the plantation and the secondary forest, respectively, while the total bacterial biomass was 24% and 8.3% lower than the control plots in the plantation and the secondary forest, respectively. In the litter exclusion treatment, total fungal biomass averaged 69.2% and 69.7% lower than the control plots in the plantation and the secondary forest, respectively, while the total bacterial biomass was 48% and 50.1% lower than the control plots in the plantation and the secondary forest, respectively. Soil CO2 efflux was positively correlated with both fungal and bacterial biomass in both the plantation the secondary forest. The correlation between soil CO2 efflux and active fungal biomass was significantly higher in the plantation than in the secondary forest. However, the correlation between the soil CO2 efflux and both the active and total bacterial biomass was significantly higher in the secondary forest than in the plantation in the day season. In addition, we found soil CO2 efflux was highly related to the strong interactions among root, fungal and bacterial biomass by multiple regression analysis (R2>0.61,P<0.05). Our results suggest that carbon input from aboveground litterfall and roots (root litter and exudates) is critical to the soil microbial community and ecosystem carbon cycling in the wet tropical forests. [ABSTRACT FROM AUTHOR]