The recurring spatiotemporal patterns observed in soil microbial communities largely occur due to legacy effects from long-term responses to annual climate cycles. Temperature (T) effects on soil microbes are well known but often measured using a fixed mean T, yet large diurnal swings (>20° C) occur seasonally from April through September, especially in surface (0-5cm) soils. Here, we looked closely at how soil diurnal T influences microbial N2O and CO2 emissions along with the related gene expression. Triplicate microcosms with soils taken at 0-5 cm and 20-30 cm from an agricultural field (Havana, IL) were amended with urea-ammonium-nitrate and incubated under a 24h T regimen typical of June diurnal cycles; 18.9°C-37°C (∆18.1°C, 0-5 cm depth) and 23°-25°C (∆2°C, 20-30 cm depth). Unamended controls were set up in parallel. N2O and CO2 were measured over 21d along with amplicon sequencing and qPCR of 16S rRNA-, nosZ, amoA, nrfA genes (DNA).
Results/Conclusions
The net cumulative N2O production was highest in the 0-5 cm soil with the 18.1°C diurnal T flux throughout the 21d with the highest net N2O production rate occurring after 24h (76 nmol/d), greater than any other soil treatment. Both 0-5 cm microcosm types were greater than any 20-30 cm soil microcosm at any time. Net CO2 production was also highest in the 0-5 cm soils with the 18.1°C diurnal T change, 1250- vs 1131 nmol CO2 after 21d, respectively. Both N2O and CO2 emissions were similar in the 20-30 cm soils regardless of T. Microbial communities showed distinctions at soil depths, but shifted accordingly with activity measurements upon exposure to treatments. Collectively, the data suggests surface soils harbored populations that were differentially active in response to T and distinct from those at the 20-30 cm soil depth. Gene amplicons affiliated with Clade II NosZ were abundant in all soils from both depths, particularly those associated with Gemmatimonadetes and Anaeromyxobacter subclades. Thus nosZ diversity did not explain the activities observed. We demonstrate, however, the likely presence of a microbial community adapted to diurnal temperature changes which indicates possible issues when measuring N2O and CO2 emissions at constant temperature.