Impacts of an anomalously warm year on soil nitrogen availability in experimentally manipulated intact tallgrass prairie ecosystems

Global climate change can potentially increase the frequency of climate anomalies. Anomalously warm years may cause an increase in soil nitrogen (N) availability by stimulating N mineralization. To date, most studies addressing the effects of ecosystem warming have been conducted in relatively cold ecosystems and few studies have addressed impacts of interannual as opposed to continuous, multiyear warming. In this study, 12 intact soil monoliths were excavated from a tallgrass prairie site near Purcell, Oklahoma, USA and divided among four large flux chambers (EcoCELLs). During the first year, all four EcoCELLs were subjected to Oklahoma air temperatures and precipitation. During the second year, air temperature in two EcoCELLs was increased by 4 degrees C throughout the year resulting in an increase in soil temperature of 2.3 degrees C at 7.5 cm depth. During the third and fourth years, temperatures in the warmed EcoCELL returned back to 'normal' conditions. During the warming year, vegetation N content was not significantly affected by the warming treatment suggesting no change in N availability. Other N availability indicators (soil solution chemistry, leaching, and N adsorption by ion exchange resins) did not show any effect of warming. Soil solution, leaching, and ion exchange resins showed a large pulse of NH4+ at the start of the study most likely due to disturbance caused by monolith excavation and transport but these effects were short-lived and had disappeared before the treatment started. The lack of a clear warming effect may be explained by a reduction in soil moisture in the warming treatments compared with the controls offsetting a potential stimulation of N mineralization in response to increased temperatures. As a result, effects of an anomalously warm year on N availability in warmer ecosystems may be small compared with colder ecosystems but are likely to depend on soil moisture status.