Net ecosystem carbon exchange for Bermuda grass growing in mesocosms as affected by irrigation frequency
Section snippets
INTRODUCTION
Conversion of non-irrigated grasslands to high-intensity grazing systems with irrigation is a major land-use change in dryland areas of New Zealand to increase feed supply for cattle during periods with low rainfall (MacLeod and Moller, 2006). However, despite increased aboveground production (Condron et al., 2014), there is increasing evidence that irrigation in grazed grasslands of New Zealand leads to a decrease (Houlbrooke et al., 2008; Mudge et al., 2017) or no change (Condron et al., 2014
Mesocosm preparation and experimental design
Topsoil (0–150 mm depth) was collected from a grazed grassland site dominated by perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) from the Lincoln University Demonstration Farm (43° 65′ S, 172° 48′ E, 34 m above sea level). The soil was classified as a Typic Immature Pallic Soil (Hewitt, 2010) or a Typic Haplustept (Soil Survey Staff, 2014). The soil was mixed and sieved (≤ 4 mm), with all visible plant material removed, prior to storage at 4 °C. Soil total C and N
Soil water content and cumulative W
As anticipated, soil θv values decreased gradually during the period when no water was applied and then returned to a value between 0.32 and 0.35 m3 m-3 immediately after water was applied (Fig. 1). The minimum values reached for θv decreased with decreasing irrigation frequency to 0.23, 0.20, 0.18, and 0.08 m3 m-3 in the I1, I2, I3, and I6 treatments, respectively. The pronounced effects of the I6 treatment on CO2 exchange components following the lack of response in the I2 treatment after the
Responses of CO2 exchange components to irrigation frequency
The threshold values of W for the broken-stick model below which FN, FG, and RE remained constant at the maximum values were similar regardless of treatment and ranged from 47 to 57 mm, suggesting similar threshold values for photosynthesis and ecosystem respiration. The lack of any treatment differences in cumulative FN, FG, and RE during the first 12 d demonstrated that these processes were insensitive to mild increases in W up to 30 mm. This coincides closely with the breakpoints between no
CONCLUSIONS
The findings from growing Bermuda grass in mesocosms showed that decreases in net ecosystem CO2 exchange (FN) with increasing cumulative soil water deficit (W) were moderated by the offset between a strong decrease in gross C uptake by plants (FG) and a less sensitive response in ecosystem respiration (RE). However, this did not result in changes in aboveground biomass production with increasing W. Although cumulative RS did not increase with increasing W, the use of a 13C natural abundance
ACKNOWLEDGEMENTS
This work was funded by the New Zealand Agricultural Greenhouse Gas Research Centre (NZAGRC) and National Natural Science Foundation of China (No. 32101431) to support Yuan Li. The authors especially thank Roger Cresswell, Kethsiri Alwis, Emily Huang at Lincoln University, New Zealand for laboratory analyses. We thank Alan Stewart at PGG Wrightson Seeds, New Zealand for providing grass seeds. We are grateful to Neil Smith, Graeme Rogers, Trevor Hendry, Brent Richards, Andrea Leptin, David Rex,
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