Differental responses of the components of ecosystem carbon exchange to irrigation frequency in mesocosms with a C4 grassland

Ms Yuan Li1,2, Prof. Tim J Clough1, Dr. Gabriel Y K Moinet2, Dr. John E Hunt2, Dr. David Whitehead2

1Lincoln University, Chirstchurch, New Zealand, 2Manaaki Whenua – Landcare Research, Lincoln, New Zealand

Conversion of non-irrigated grasslands to high-intensity farm systems with irrigation is a major land-use change in dryland areas of New Zealand. The conversion has the potential to cause major changes in ecosystem carbon cycling but differences in the sensitivities of the components of net ecosystem carbon dioxide (CO2) exchange (FN) to water availability are not clear. Here, we partitioned the components of FN for the C4 plant Bermuda grass (Cynodon dactylon L.) growing in mesocosms irrigated every 1, 3, and 6 days (I1, I3, and I6, respectively) with a constant intensity (14.8 mm). FN decreased linearly with increasing soil water deficit (W), indicating increasing ecosystem CO2 losses with decreasing irrigation frequency. Over the 30 days of the experiment, FN reached 657 ± 44 (mean ± standard deviation, n = 4), 604 ± 29, and 552 ± 47 g C m−2 for I1, I3, and I6, respectively. This was due to a linear decrease in ecosystem gross primary productivity with increasing W that exceeded the decrease in ecosystem respiration (RE). RE declined non-linearly with increasing W with the aboveground component increasing and the below-ground component (soil respiration rate, RS) decreasing non-linearly with increasing W. Using a 13C natural abundance isotopic technique, RS was partitioned into autotrophic (RA) and heterotrophic respiration (RH) at the end of the experiment, when differences in W bewteen the treatments were greatest. RH contributed 41 ± 17, 23 ± 21, and 30 ± 14% of RS for I1, I3, and I6, respectively. Increasing irrigation frequency resulted in increased ecosystem CO2 uptake as a result of diverging responses of gross primary production and respiration components. However, care in interpreting the implications for changes in soil carbon stocks is warranted as RH was also highest for the I1 treatment with highest CO2¬ uptake, suggesting possible positive priming.

Biography: Yuan Li is currently studying for his PhD in the field of Soil Science at Lincoln University. Professor Timothy Clough and Dr David Whitehead are supervising this research. Currently I am doing a PhD project about “linking carbon and nitrogen dynamics to mitigate carbon dioxide and nitrous oxide emissions in grazed grasslands”. This research aims to determine how irrigation drives the interactions between carbon and nitrogen cycles and the regulation of soil organic matter decomposition and nitrous oxide emissions, and identify the potential for farm management practices to mitigate soil organic matter losses and reduce nitrous oxide emissions.

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