Assistant Professor Harvard University, United States
Abstract: Most nitrogen (N) enters tropical ecosystems through symbiotic N fixation (SNF) and SNF is thought to impact future carbon capture by supporting plant growth. Yet, we do not understand what controls where and how much SNF occurs in tropical forests. There are likely critical SNF “hotspots” that remain undocumented because many previous field-based SNF measurements do not sample in some microenvironments that favor SNF upregulation, which could lead to underestimation of N inputs. Our understanding of SNF suggests the altered light, temperature, nutrient dynamics, and increased plant N demand that occur in tropical forest gaps likely upregulate SNF and create these N input hotspots.
Initial evidence suggests significant SNF increases in canopy gaps, but we do not currently know how or the importance of this gap effect on SNF is across the landscape. Previous proxies for landscape-scale SNF estimates, such as legume abundance, may not explain landscape-scale SNF variation well. To robustly estimate the SNF that fuels tropical forest growth, we extensively sampled 18 Neotropical gaps and intact forests to understand the mechanisms driving gap SNF hotspots and the magnitude of this gap effect. Along transects that spanned gaps, transition zones, and intact forest, we measured light, soil N, and soil P availability, soil pH and moisture, and SNF using nodule biomass and isotopically enriched 15N2 gas incubations.
Canopy gap formation changes the microenvironment and SNF. Within our sites, soil moisture is 19% higher, while soil temperature was 1.3% lower in gaps compared to intact forest. Intact forest soil cores were about 2 times more likely to have nodules compared to cores from gaps. However, the average nodule biomass per core was 20 times higher in gaps, resulting in 8.5 times higher nodule biomass m-2 in gaps than understory. If we assume that 1.4%-7.5% of primary Neotropical forests are gaps (Yavitt et al. 1995), we find nodule biomass would be 12.8%-56.4% higher when accounting for this gap effect. This gap effect is likely even higher when considering SNF rates (results to come) as McCulloch et al. 2021 found 42x higher SNF rates, compared to 17x higher nodule biomass, in seedlings growing in gaps compared to intact forest. Thus, this work suggests that canopy gaps serve as significant SNF hotspots in Neotropical forests, and this gap effect needs to be accounted for explicitly in our landscape-scale N cycling estimates.
