Effects of nutrient and light availability on nitrogen fixation in tropical dry forest legume seedlings

Background/Question/Methods

Recent theoretical models suggest that nitrogen (N) fixation is subject to environmental variables beyond soil N availability, yet there are few empirical data that test this. Evidence suggests that tropical dry forests are not N deficient. Given the ubiquity of legume trees in this ecosystem, symbiotic fixation by individual trees should be down-regulated. We tested the hypothesis that symbiotic fixation in tropical legume trees responds to changes in light and nutrient availability. Seedlings of four dry forest N-fixing legume species and one non-N-fixing legume were grown in a shadehouse for 6 months in a mixture of forest soil and sand.  After the third month, two treatments of light availability and fertilization (+NP, +N, +P, no nutrients) were applied (N=10 replicates per treatment and species). The response variables that we measured were: photosynthetic rates, growth rates, final biomass partitioning, nodule biomass, nodule activity via the acetylene reduction method, leaf C and N, and foliar ¹⁵N. We expected to find nodulation repressed in N-fertilized seedlings and enhanced in P-fertilized seedlings. Because local light availability can pay off the costs of fixation, we expected that legumes would fix more in the high light treatment.

Results/Conclusions

Total seedling biomass ranged from 0.20 to 612.93 g. Nodule biomass varied between 0 and 948 mg. The average height growth rate was 0.45 cm day^-1, with a maximum of 3.3 cm day^-1. Photosynthetic rates ranged from 5.58 to 29.66 µmol m^-2 s^-1. Nodule biomass was affected by both nutrient and light treatments (P < 0.0001) and the interaction between these variables and species was significant. As expected, the lowest nodulation was found in seedlings fertilized with N only and highest in seedlings that received P. The seedlings that received N and P had similar nodule biomass to the seedlings that did not receive any nutrients. Nutrients also had an effect on total seedling biomass (P < 0.0001) with a significant interaction with species. The light treatments had an effect on the variation in the photosynthetic rates (< 0.01), with a significant interaction with species (< 0.05). In conclusion, our data support the hypothesis that multiple factors including light and soil nutrient availability interact to control N fixation, but that the strength of these drivers varies with species.