Precipitation variability is expected to increase as a result of climate change, resulting in longer, higher severity droughts and changes in seasonality. While the effects of prolonged drought have been widely investigated, less is known about ecosystem responses to changes in rainfall seasonality. Meanwhile, precipitation is an important driver of soil respiration, a key component of the global carbon cycle. In the southwestern US, most annual rainfall occurs during the summer monsoon, which spans July-September. However, regional climate models indicate a delayed start to the summer monsoon, thus prolonging drought conditions during the growing season. To assess the impacts of chronic drought and changes in monsoon seasonality on soil respiration, we established concurrent rainfall manipulation experiments in blue and black grama dominated grasslands in central New Mexico, USA. A drought treatment, consisting of a ~66% reduction in monsoon rainfall, was imposed April-September each year (2013-2017), while a delayed seasonality treatment consisted of a 100% reduction in monsoon rainfall during July-August, during which ambient rainfall was captured and stored, then reapplied during September-October each year. Soil respiration in both grasslands was continuously measured in all treatments using sensor arrays positioned within the canopy of the respective dominant grass.
Results/Conclusions
In blue grama grassland, mean growing season soil respiration across all years ranged from 1.51 μmol CO2 m-2 s-1 under ambient conditions to 1.48 μmol CO2 m-2 s-1 under delayed seasonality to 1.06 μmol CO2 m-2 s-1 under drought. Soil respiration was consistently lower in black grama grassland, with five year means ranging from 0.91 μmol CO2 m-2 s-1 under ambient conditions to 0.94 μmol CO2 m-2 s-1 under delayed seasonality to 0.55 μmol CO2 m-2 s-1 under drought. Soil respiration under both treatments in both grasslands differed significantly (p≤0.05) from ambient, with higher fluxes observed in response to delayed seasonality and lower fluxes in response to drought. We found a positive correlation between soil respiration and soil moisture at both blue (R2=0.58; p≤0.001) and black (R2=0.46; p≤0.001) grama grasslands under ambient conditions, as well as under delayed seasonality (R2=0.63; p≤0.001) and drought (R2=0.15; p≤0.001) in blue grama grassland. However, soil respiration was not significantly correlated with soil moisture under drought in black grama grassland, and beginning in 2015, delayed seasonality resulted in strong negative correlations (p≤0.001) between these variables. These results suggest that these semi-arid grasslands have functionally different responses to drought and changes in monsoon rainfall seasonality.