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Basic Plant and Soil Process Measurements for Range Ecosystem Modeling and Management—Updates for 2005


Ecosystem carbon exchange: A diurnal budget

 


 

Total ecosystem carbon exchange rate is an important measure to check the status of an ecosystem. However, any model, whether theoretically (Thornley, 1998) or physically based (Campbell and Norman, 1998), has to use some simplified representations for the canopy light distribution, leaf angle distribution, etc. We need to have field measurements that are less theory-dependent to check the general correctness of the model.


This year, we took canopy pictures and a number of detailed measurements following the method of Campbell and Norman (1998), so that we could scale leaf-level measurements of gas exchange to the canopy level. First, we needed to take some direct measurements of net ecosystem carbon gain by considering canopy photosynthesis and soil respiration. In the 2004 Annual Report, we introduced a newly designed plastic chamber attached to the LI-6400 Portable Photosynthesis System for measuring canopy photosynthesis. In this section, we will report on what we have done in 2004 and 2005 using this chamber.


First, a non-rectangular hyperbola (Thornley, 1998. p 28) was fitted to the 83 canopy photosynthesis-light response curves (See Figure 3 for an example). We then used the fitted equation for each of the curves, as well as the hourly radiation data for the particular day, to calculate the daily accumulation of photosynthesis (this ignores the influence of changes in daytime temperature on canopy photosynthesis). Then we used our measurements on soil respiration (measured both near midday and midnight, assuming representation of the diurnal maximum and minimum of soil respiration, respectively) to obtain an empirical equation for the description of diurnal soil respiration:


ole1.gif


in which R is respiration rate per hour; Rx is diurnal maximum respiration, which was the result measured using our plastic chamber when totally darkened at midday; Rn is the diurnal minimum respiration, which was estimated using independent soil respiration measurements (using LI-COR’s special soil respiration chamber) at midday and midnight; and t is time in a day (hours). This equation seems to describe well the measured daytime respiration data on August 12, 2005 with the maximum of the sine wave (which is at 1:00 pm) shifted to 3:00 pm, while other parameters were unchanged (Figure 4). In our calculation, however, we still used 1:00 pm as the diurnal respiration maximum and 1:00 am as the diurnal minimum. The hourly respiration curve was summed to yield daily total respiration estimation. The difference between daily canopy photosynthesis and daily respiration is the net CO2 exchange rate.

 

The averaged daily net CO2 exchange rates for 2004 and 2005 are 0.23 mol CO2 m-2 day-1 and 0.07 mol CO2 m-2 day-1, respectively. These values are similar to the daily rate for a low leaf area index canopy, as shown in Thornley (1998). For the effects of grazing, simulated drought (using a set of “rain-out” shelters), and month, we observed only significant effects from simulated drought in 2004, and the interaction of month and grazing in 2004 (Table 5). We are especially interested in the effect of simulated drought, because the drought was imposed from 2003 until 2004. Beginning in 2005, no drought treatment was imposed and the field plots were experiencing the post-drought recovery. In 2004, the drought plots had a daily CO2 exchange rate of 0.14 mol CO2 m-2 day-1, which was 58% lower than the data for the average rainfall treatment (Table 6). In 2005, the effect of drought that was found in 2004 disappeared.


Table 5. The p values from the ANOVA table for the effects of grazing intensity (non-grazing exclosure, moderate grazing, and heavy grazing), simulated drought (natural rainfall, long-term averaged rainfall and 75% of long-term averaged rainfall) and month (June, July, September) on the daily net ecosystem CO2 exchange rates for the grazing intensity study pastures at the CGREC. The daily net ecosystem CO2 exchange rates for the selected days in 2004 (second year of simulated drought) and 2005 (post-drought recovery) were calculated based on canopy photosynthesis as well as soil respiration measurements conducted on the field site. The effect of “Month” was not evaluated in 2005, because the data was not balanced (due to changing weather conditions, measurements were occasionally interrupted).

 

Source

M

GI

SD

M*GI

M*SD

GI*SD

M*GI*SD

2004

0.062

0.492

0.043

0.012

0.642

0.426

0.771

2005

-

0.669

0.852

-

-

0.32

-

M: Month; GI: Grazing intensity; SD: Simulated drought.

 


Table 6. Influence of simulated drought to the daily ecosystem net CO2 exchange rates on grasslands with long-term cattle grazing at the CGREC: the effects during the 2nd year of drought treatment (2004) and the 1st year of post-drought recovery (2005). The calculation of daily ecosystem CO2 exchange rates were based on canopy photosynthesis measurements during the midday hours (11:00 am to 2:30 pm) on selected clear days (12 days in 2004 and 14 days in 2005), as well as soil respiration measurements in the midday hours and the midnight hours (10:30 pm to 12:30 am).

 

Treatment

Net CO2 exchange rate (mol CO2 m-2 day-1) (2004)

Net CO2 exchange rate (mol CO2 m-2 day-1) (2005)

Long-term average rainfall

0.33a

0.08a

Natural rainfall

0.22ab

0.08a

75% of long-term average rainfall (drought)

0.14b

0.05a


In both 2004 and 2005, canopy photosynthesis as well as net ecosystem CO2 exchange rate peaked in mid to late June. This is understandable because in June, the pasture plants may be most physiologically active. Following the June peak, the net CO2 rate became lower in both years. We analyzed the data from June in detail (Figure 5 and Figure 6), and noticed an interesting trend: the non-grazing exclosure tended to have the lowest net daily CO2 gain and moderately grazed plots had the highest daily rate in both 2004 (p=0.023) and 2005 (p=0.09). The heavily grazed plots are in between. Although protected pastures had quite high canopy photosynthesis in June, presumably owing to the high green leaf area present in the exclosure, ecosystem respiration tended to be equally high, resulting in a low net gain of CO2. The high respiration rate in the exclosure may partly result from the decomposition of the accumulated surface litter layer, which in June would be active. However, soil microbial biomass and nitrogen content in the exclosure tended to be lower than in the grazing pastures (though not significant). The reason for the lower respiration rate relative to photosynthesis as seen in grazed pastures may be related to the disturbance the pastures received from cattle grazing (especially those pastures that were moderately grazed). The mechanical disturbance from cattle grazing may make the grazed pastures more dynamic in terms of eco-physiological activities. More work is needed to determine why this dynamic effect, if it really existed, did not equally increase both photosynthesis and respiration, especially for the moderately grazed pastures.


Diurnal ecosystem CO2 change is important for the growing season as well as the annual changes. From our calculation based on field measurements, we can make the following preliminary observations:

 

1.     The daily net CO2 gain observed in the Missouri Coteau grasslands is affected by drought stress, with drought treatment (75% of the average rainfall) causing a 58% reduction in net daily CO2 gain compared with the average rainfall treatment.

 

2.     The effect of drought on the daily net CO2 exchange disappeared in 2005 when the drought treatment was removed and the pasture plots were in post-drought recovery.

 

3.     Although the effect of grazing intensity on ecosystem net CO2 exchange is not significant for the whole dataset, in June, when the grasslands are probably most physiologically active, the moderately grazed grassland showed a more positive net CO2 gain than did the idled land, for which ecosystem respiration increases can be comparable to the increase in photosynthesis. ●


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