Net ecosystem carbon exchange for Bermuda grass growing in mesocosms as affected by irrigation frequency

Intensification of grazed grasslands following conversion from dryland to irrigated farming has the potential to alter ecosystem carbon (C) cycling and affect components of carbon dioxide (CO2) exchange that could lead to either net accumulation or loss of soil C. While there are many studies on the effect of water availability on biomass production and soil C stocks, much less is known about the effect of the frequency of water inputs on the components of CO2 exchange. We grew Bermuda grass (Cynodon dactylon L.) in mesocosms under irrigation frequencies of every day (I1 treatment, 30 d), every two days (I2 treatment, 12 d), every three days (I3 treatment, 30 d), and every six days (I6 treatment, 18 d, after I2 treatment). Rates of CO2 exchange for estimating net ecosystem CO2 exchange (FN), ecosystem respiration (RE), and soil respiration (RS) were measured, and gross C uptake by plants (FG) and respiration from leaves (RL) were calculated during two periods, 1–12 and 13–30 d, of the 30-d experiment. During the first 12 d, there were no significant differences in cumulative FN (mean ± standard deviation, 61 ± 30 g C m-2, n = 4). During the subsequent 18 d, cumulative FN decreased with decreasing irrigation frequency and increasing cumulative soil water deficit (W), with values of 70 ± 22, 60 ± 16, and 18 ± 12 g C m-2 for the I1, I3, and I6 treatments, respectively. There were similar decreases in FG, RE and RL with increasing W, but differences in RS were not significant. Use of the C4 grass growing in a C3-derived soil enabled partitioning of RS into its autotrophic (RA) and heterotrophic (RH) components using a 13C natural abundance isotopic technique at the end of the experiment when differences in cumulative W between the treatments were the greatest. The values of RH and its percentage contributions to RS (43% ± 8%, 42% ± 8%, and 8% ± 5% for the I1, I3, and I6 treatments, respectively) suggested that RH remained unaffected across a wide range of W and then decreased under extreme W. There were no significant differences in aboveground biomass between the treatments. Nitrous oxide (N2O) emission was measured to determine if there was a trade-off effect between irrigation frequency and increasing W on net greenhouse gas emission, but no significant differences were found between the treatments. These findings suggest that over short periods in well-drained soil, irrigation frequency could be managed to manipulate soil water deficit in order to reduce net belowground respiratory C losses, particularly those from the microbial decomposition of soil organic matter, with no significant effect on biomass production and N2O emission.