Intra-specific diversity shapes ecosystem carbon exchange in an old-field ecosystem

Background/Question/Methods

Intra-specific diversity can influence community structure and ecosystem functions such as net primary productivity, but the conditionality of such diversity effects on ecosystem properties, such as ecosystem carbon exchange, has not been addressed. In our study, we manipulated both intra-specific diversity  (one vs. six genotypes in a population) and soil nutrient availability (nitrogen addition- 10 g m² yr^-1, phosphorus addition- 10 g m² yr ^-1, nitrogen and phosphorus addition- 5 g N g m² yr ^-1 and 5 g P g m² yr ^-1, and control- no manipulation) to test both the main and interactive effects of intra-specific diversity and resource availability on whole ecosystem carbon (C) exchange. We measured both net primary productivity and net ecosystem CO₂ exchange across 92  1-m² plots to test for the effects of diversity and nutrients. We predicted that: (1) Intra-specific diversity would promote productivity and whole ecosystem C exchange, (2) Soil nutrients would promote productivity and whole ecosystem C exchange, and (3) Soil nutrients would interact with intra-specific diversity to shape productivity and whole ecosystem C exchange.

Results/Conclusions

Intra-specific variation and intra-specific diversity but not nutrients or their interactive effects, shaped net ecosystem C exchange. Diverse plots had 30% greater whole ecosystem C exchange (i.e., greater ecosystem C sink) relative to monoculture plots at the beginning of the growing season. This result may indicate that complementarity effects play a role in whole ecosystem C uptake such that variance in ecological traits maximizes resource uptake. However, differences in whole ecosystem C exchange early in the growing season in mixture vs. monoculture plots did not result from differences in net primary productivity or leaf area index. To our surprise, soil nutrients did not influence C dynamics in this old-field ecosystem suggesting that biomass production in this system is not limited by macronutrients. Taken together, old-field ecosystem function is determined by intra-specific variation in below ground traits or perhaps in the structure of associated soil microbial communities. These results suggest an important role in variation within species in C dynamics at larger spatial scales.