Individual trees are known to influence soil chemical properties and processes, creating a sphere of influence that varies with distance from the tree trunk. The effects of particular species have been well documented in low-diversity systems such as temperate forests or tropical plantations. However these spatial patterns are less well studied in species-rich tropical forests. Legume tree species, which are abundant in tropical dry forest, may be a critical regulator of soil nutrient dynamics because of high foliar nitrogen and potential for symbiotic fixation. We investigated whether legumes have distinct soil chemistry under their crowns compared to nearby non-legume species when grown in both a diverse secondary forest or alone in 18-yr old plantations in northwestern Costa Rica.  In each site, we collected samples from the top 10 cm of mineral soil beneath the canopy of trees of three legume species and three non-legume species that were at least two crowns away from any of legume. We measured soil moisture, pH, microbial carbon, inorganic nitrogen (NH4+and NO3−), total carbon and nitrogen, inorganic phosphorous concentrations and enzyme activity.  We predicted that legumes would have higher nutrient availability under their crowns, but that this effect would be more pronounced for plantations.  

Results/Conclusions

The range of concentrations of labile soil carbon varied considerably in the forest (64.02 to 147.2 µg C/g soil in legumes, 71.7 to 161.4 µg C/g soil in non-legumes) as well as in plantations (89.6 to 247.2 µg C/g soil in legumes and 98.9 to 169.1 µg C/g soil in non legumes), and it was significantly different among species in each site (forest p>0.05). Similarly, soil pH ranged from 4.00 to 5.75 but did not vary significantly among different species in the mature forest or plantation. Other analysis are pending.  Since age of trees, light environment and soil physical properties were standardized, abiotic controls or microbial processes may contribute to the dilution of individual species effects and explain the lack of variation on soil properties and nutrient pools especially in species rich forests.