Plant-soil feedbacks and nutrient availability mediate microbial decomposition in a nutrient-poor tropical forest

Background/Question/Methods

Soil microbial communities and their associated enzymes mediate the decomposition of organic matter and the release of plant-available nutrients. Resource allocation theory predicts increased microbial enzyme activity when simple nutrients are scarce and complex nutrients are abundant. In the highly weathered tropical oxisols of northeastern Puerto Rico, nutrients cycle rapidly between plants and soil, indicating that the zone directly influenced by plant roots may be an important site of microbial activity. The objective of this study was to investigate linkages between microbial community structure, soil nutrient availability and plant-soil feedbacks. A greenhouse experiment was conducted using surface mineral soil collected in the Bisley Watershed at the Luquillo Experimental Forest. Seedlings of Tabebuia heterophylla, an early-successional Puerto Rican endemic tree, were germinated and grown in individual pots. Planted and unplanted pots were fertilized with different combinations of phosphorus and either ammonia chloride, a simple, plant-available Nsource, or caesin protein, an N-rich organic compound presumed to require microbial degradation to become plant-available. Soils were analyzed over a 12 week period to evaluate changes in microbial community structure and enzyme activities.

Results/Conclusions

Preliminary results of a DGGE community fingerprinting analysis indicate strong decreases in bacterial diversity and shifts towards the dominance of several taxa in the complex-N fertilizer treatment. Soil phosphatase activity declined in response to P additions, but soil peptidase activity increased in response to complex-N additions. Activities of C-, N- and P-acquiring enzymes were greater in planted compared with unplanted soils. These results indicate that for tropical soils, microbial community structure and functional characteristics may be sensitive to both fertilizer addition as well as its form. Furthermore, the presence of plant roots exerts a positive feedback on microbial enzyme activity, underscoring the importance of plant-mediated nutrient cycling in this system.