COS 50-1 - Mechanisms controlling nitrogen dynamics along a climate gradient

Tuesday, August 8, 2017: 1:30 PM
D132, Oregon Convention Center
Peter M. Vitousek1, Kabir G. Peay1, Karen Casciotti2, Christian von Sperber3 and Oliver A. Chadwick4, (1)Department of Biology, Stanford University, Stanford, CA, (2)Earth System Science, Stanford University, (3)Institute for Crop Science and Resource Conservation, University of Bonn, (4)Department of Geography, University of California, Santa Barbara, CA
Background/Question/Methods

We used a well-defined and well-studied climate gradient from 280-3300 mm/yr rainfall on the island of Hawaii to evaluate proximate and ultimate controls of N availability and cycling. Earlier research demonstrated that there is a soil process domain in which net N mineralization in intermediate-rainfall sites is elevated relative to wetter and drier sites. Available P and Ca also are enriched in this domain, but the same processes are unlikely to control the availability of rock-derived nutrients and of N. To evaluate the short- and long-term controls of N availability on the gradient, we determined natural abundance N isotopes in total soil N and in nitrate, gross mineralization by isotope dilution, microbial community composition, and the consequences of wetting up dry soils for gaseous pathways of N loss. 

Results/Conclusions

Net N mineralization was a small fraction of gross N mineralization in wet and dry portions of the gradient – suggesting that microbial demand for N contributes proximately to the overall pattern of N availability. Natural abundance δ15N in bulk soil increased from ~+4 per mil in the wettest sites to ~+14 per mil in sites receiving ~400 mm/yr of rainfall, then decreased to ~+9 per mil in the driest sites. Other researchers observed a similar pattern – with the same climatic break point – in a continental-scale gradient across North China, suggesting the processes underlying N cycling on the Hawaii gradient are general ones. δ15N in nitrate was depleted relative to total soil N  in the wetter sites and matched total soil N closely in drier sites. In laboratory experiments, we observed that N2O-N and NH3-N emissions peaked in the first half-hour following wet-up of dry soils. We suggest that the balance between inputs of N, largely by biological N fixation, and losses of N by biologically uncontrollable pathways ultimately causes the pattern of high N availability in intermediate-rainfall sites and lower availability in wetter and drier sites. We propose the dominant loss pathway is dissolved organic N leaching in the wet sites, consistent with patterns in δ15N, while the dominant loss pathway in dry sites is gaseous fluxes during wet-up of dry soils.