Most terrestrial plant communities form symbioses with soil microbial communities by exchanging carbon allocated below-ground for nutrients acquired by microbial symbionts. Plant communities often respond to increased demand for nutrients by allocating additional carbon below-ground to increase nutrient acquisition, but mechanisms driving these responses are not well understood. Nutrient acquisition is often thought to be similar in magnitude to below-ground carbon allocation in microbial symbioses; however, there is growing evidence that nutrient acquisition dynamics are dependent on symbiosis type and not tightly coupled with below-ground carbon investments. Here, we investigated relationships between below-ground carbon allocation and nitrogen acquisition in cotton (arbuscular mycorrhizal) and soybean (nitrogen-fixer) grown under four light and four nitrogen fertilization treatments in a greenhouse experiment. We hypothesized that increased light will increase nitrogen acquisition due to increased demand to build photosynthetic machinery. We predicted this would require greater below-ground carbon allocations to satisfy heightened resouce demand, particularly in low soil nitrogen conditions. We also hypothesized that carbon costs to acquire nitrogen would not change with light if acquired nitrogen is proportional to below-ground carbon allocations, but would vary by nutrient acquisition strategy and soil nitrogen fertilization.
Results/Conclusions
Individuals of both species grown at the lowest light level (80% shade) acquired less nitrogen than individuals grown under any higher light level (0%, 30%, 50% shade), supporting our first hypothesis. This was only observed in groups receiving nitrogen fertilization treatments greater than 0 ppm. Increased light also increased below-ground carbon allocation, where cotton individuals allocated more carbon below-ground than soybean at each light level. Increasing nitrogen fertilization did not change soybean below-ground carbon allocation and decreased below-ground carbon allocation in cotton. Increased light availability increased carbon costs to acquire nitrogen in both species, while nitrogen fertilization increased carbon costs to acquire nitrogen, supporting our second hypothesis. Our results indicate that below ground carbon allocation is driven by increased resource demand, while nutrient acquisition is driven by increased resource availability. These results provide preliminary evidence that carbon costs to acquire soil resources likely differ by nutrient acquisition strategy and highlight a need for future investigations of the mechanisms underlying plant-microbe interactions.