Role of bedrock nitrogen in regulating asymbiotic nitrogen fixation and plant tissue chemistry

Background/Question/Methods

Nitrogen (N) is considered to be one of the most frequently limiting elements to terrestrial ecosystem productivity worldwide. Biological nitrogen fixation (BNF) can provide a substantial input of usable N, especially in ecosystems with few other N inputs. In temperate forests lacking legume or actinorhizal species, which fix N symbiotically, BNF is mainly performed by heterotrophic diazotrophs in decomposing plant litter. Rates of asymbiotic BNF are controlled by factors such as N availability and plant litter quality. Generally, BNF decreases as N availability increases, reflecting the extreme energetic cost of BNF. Litter quality may also exert an influence on BNF, as recalcitrant C compounds are not readily metabolized by N fixers. Plant litter quality can change in response to nutrient limitation; in situations of extreme nutrient limitation, plants may produce more recalcitrant compounds such as lignin, limiting microbial decomposition of plant matter. Recent research in our group has demonstrated that bedrock can serve as a pool of plant-available N. Here, we examine the role of bedrock N in regulating both plant litter chemistry and asymbiotic BNF in temperate coniferous forests in northern California underlain by different parent materials. Using a combination of nitrogenase enzyme assays, rock chemical analyses, and plant tissue chemical analyses, we ask: 1) if rates of asymbiotic BNF change across this geologic N gradient; and 2) if rates of asymbiotic BNF are primarily constrained by N availability or by plant litter quality.

Results/Conclusions

We measured rates of BNF across a set of sites that varied substantially in rock N concentrations. The sites are dominated by common plant species, share identical climates, yet display significantly different foliar N concentrations (ranging from 0.62% to 1.82% total foliar N). We used the acetylene reduction assay to examine free-living BNF rates in the litter across the sites. Our results pointed to significantly higher rates of asymbiotic BNF at the highest rock N site than at the lowest rock N site (1.87 kg N ha^-1 yr^-1 ± 2.38 vs 0.28 kg N ha^-1 yr^-1 ± 0.21, p=0.03).  This result runs counter to expectations from N input controls on free-living fixation rates, and was in contrast to the changes in foliar N concentrations observed. An examination of plant litter-chemistry pointed to feedbacks between the plant-soil-rock system in regulating patterns of BNF. These results suggest that plant feedbacks to rock N are a more important control than N inputs in regulating patterns of free-living BNF.