Nitrogen and Water Addition Increased Sensitivity of Ecosystem Carbon Fluxes to Manipulated Extreme Drought in a Meadow Steppe of Northern China

Global change factors, such as nitrogen deposition and variation in precipitation regimes, are likely to occur simultaneously and may have profound impacts on ecosystem carbon cycling. In a meadow steppe, which has been treated with nitrogen and water addition for 3 years, we conducted a manipulated extreme drought experiment to assess whether or not nitrogen and water addition has altered sensitivity of ecosystem carbon fluxes to extreme environmental disturbance. Rainout shelters (3.5 m×3.5 m) were deployed in plots received nitrogen addition (N), water addition (W), nitrogen and water addition (NW), or control (C) treatments with each treatment has 6 replications. Drought treatment was carried out from June 17th to August 1st 2014, with a total duration of 45 days. Leaf gas exchange on dominant species (Leymus chinensis), ecosystem carbon fluxes and soil respiration were measured before drought treatment and on Day 4, 9, 16, 21, 27, 36 and 42 after the start of the drought treatment. With the progress of drought treatment, leaf carbon assimilation rate, gross ecosystem productivity, net ecosystem CO2 exchange and soil respiration showed a trend of gradually decreasing. However, the magnitude of reduction and sensitivity to drought differed among treatments with nitrogen addition plots had the greatest reduction and the highest sensitivity to the drought treatment. Ecosystem respiration remained relative constant throughout the drought treatment period for all treatments. Relative to control plots, nitrogen and water addition plots had significant higher root:shoot ratios (C 5.7±0.4; N 2.9±0.4; W 2.7±0.3; NW 3.5±0.4), which may explain the observed greater reduction in carbon assimilation and more sensitive to drought treatment in nitrogen and water addition plots. Our results suggest that the projected increase in nitrogen deposition and the amount of precipitation may alter responses of ecosystem carbon fluxes to extreme disturbance in the meadow steppe of northern China.