Water pulse sampling of irrigated and dryland alfalfa/grass perennial plots for carbon and nitrogen dynamics and greenhouse gas fluxes in the high plains

Background/Question/Methods

Summer rainfall events are a key factor influencing soil carbon and nitrogen dynamics and greenhouse gas (GHG) emissions in semi-arid agro-ecosystems.  The typical agricultural production in High Plains is composed of alfalfa/perennial grass (Medicago spp.) hay under irrigated and dryland management. However, we do not clearly understand the magnitude of belowground biochemical changes and associated GHG emissions following rainfall events in systems that naturally lack high nitrogen inputs that, in this case, are contributed through atmospheric nitrogen fixation by planted legumes. Therefore the main goal of this study was to determine the influence of moisture pulse on the functional responses of GHG emissions from irrigated and dryland systems that have legume inter-planted with perennial grass.  We hypothesized that water-pulsed conditions under irrigated alfalfa would have increased nitrous oxide production and less carbon dioxide emissions than grass species from the same treatment.  The opposite is predicted for pulsed dryland plots.  It is hypothesized that the irrigated water pulsed plots will have a greater methane sink than the equivalent dryland treatment. The magnitude of GHG emissions would depend on belowground dynamics driven by moisture and differential soil C and N availability in irrigated and dryland hay production systems

Results/Conclusions

Trace gas emissions from irrigated and dryland managed systems varied considerably across life forms and water treatment.  Trace gas fluxes of carbon dioxide showed immediate release from pulsed irrigated plots in both grass and dominate alfalfa species but were not significantly different (p-value=0.05).  Dryland management systems showed a 2 fold carbon dioxide flux within 24 hours of water pulse where dominate grass species was higher than alfalfa.  Dryland and irrigated carbon dioxide emissions are not significantly different.  The dryland alfalfa/grass mix hay cropping system was an effective sink for methane.  Nitrous oxide emissions from both systems varied but are highly correlated to belowground nitrogen dynamics of microbial biomass nitrogen and soil available nitrate and ammonium concentration.  Nitrous oxide emissions were significantly higher in pulsed alfalfa from the irrigated system than grass of the system.  The preliminary data that is available for this study indicates that water (irrigation) application is highly correlated to trace gas fluxes in alfalfa/grass hay production in the High Plains.  Further analysis of soil total carbon and nitrogen and biomass will allow for a full picture of the nitrogen budget in dryland and irrigated alfalfa/grass hay production under water pulsed conditions.