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.