Linking NO, N2O and CO2 rewetting pulses to a short-lived soil flush of mineral and organic nitrogen in a Californian grassland
Drought leads to the accumulation of N in soil because (i) diffusion of water-soluble substances is limited as soil moisture decreases, and (ii) microorganisms become physically separated from their substrates when water films are disrupted. This accumulated N can be lost when soils that have been dry for a long period (a couple of weeks to months) are rewetted. Rewetting of dry soil leads to a flush in mineral and organic N, which supports high microbial activity. This leads to a pulse of CO2, NO or N2O gas emissions which usually peaks in the first few hours and can stay elevated for days after a rewetting event.
The present study was done in collaboration with the Josh Schimel lab from UC Santa Barbara, and Pete Homyak from UC Irvine. Our goal was to link the mobilization of N in its anorganic (nitrite, nitrate, ammonium) and organic (amino acids) form to emission peaks of NO, N2O and CO2 after manual irrigation of a semi-arid California grassland soil that had been dry for 6 months. To this end, we used the novel soil microdialysis technique, which allows the determination of soil N in high temporal resolution without disturbing the soil matrix. Simulateously, we measured fluxes of NO, N2O and CO2 every 1-4 hours over the course of 2 days. Rewetting led to a flush of nitrite which was chemo-denitrified rapidly to NO. Nitrate, ammonium and amino acids were immobilized during the first 24 h post-wetting, most likely to support microbial activity and growth. After 24 h, ammonium started to be mineralized, which could indicate a switch from N- to C-limitation.
A manuscript with the results from this study is currently under review. My trip to the US was financed by a short-term scientific scholarship of the Austrian Marshall Plan Foundation, and analysis and field trip costs were covered by the NSF grants DEB-1145875 and DBI-1202894.