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Litter addition effects on root biomass and nutrient concentrations in a wet tropical forest in Costa Rica: an experimental approach.
Hubbard, Jakara*,1, Wood, Tana1, Lawrence, Deborah 1, 1 University Of Virginia, Charlottesville, VA
ABSTRACT- Tropical forests typically occur on weathered, nutrient poor soils. In order to compensate for low nutrient availability, it is likely that fine roots play an important role in maximizing nutrient acquisition in these systems. Litterfall is the major mechanism for transferring nutrients from the above ground vegetation to the soil. Hence, we hypothesized that increased litter fall would lead to an initial increase in root biomass followed by root death once nutrients are mineralized from the litter. Additionally, we expected that an increase in nutrients released from the additional litter would lead to higher nutrient concentration in roots than sites with less litterfall. To simulate this experimentally in the field, we established six sites varying in age and soil type at La Selva Biological Station in Costa Rica. Leaf litter was removed from a 400m2 plot and added it to a 100m2 plot with two replicates per site. We sampled fine roots to 5cm depth in the addition and control plots both 7 months and 1 year after litter addition. Originally, a comparison of live and dead roots showed that live root biomass was significantly lower in the addition plots (t-test; p=0.005) and dead root biomass was significantly higher in the addition plots (p=0.023) than in the control plots. However, after one year of decomposition, the control and addition plots did not differ significantly in their live/dead root ratio. These results support our hypothesis that increased litterfall leads to greater initial exploration by roots followed by root death as nutrients become more available. The live roots were digested using a modified Kjeldahl digestion and the digestate was analyzed colorometrically for phosphorus concentration. Mean root phosphorus was 0.503 mg/g and, despite slightly higher phosphorus concentrations in the addition plot, did not differ significantly between addition and control plots as expected. By repeating this experiment in other locations over a longer period of time with a greater level of litter addition, perhaps we will gain a better understanding of the nutrient cycling process in wet tropical forest.
Key words: tropical rainforest, litterfall, nutrient cycling, root biomass