Measurements of CO2 nocturnal respiration as an indicator of stress response in peanuts

Summary

Measurements of net ecosystem carbon exchange (NEE) and respiration from plants and soil in peanut field were conducted. Results show that NEE and respiration from plants and soil are affected by solar radiation, soil temperature, soil water content, and nocturnal low level jets.

Situation

The trend of rainfall decrease has already been evidenced, and droughts have occurred frequently in Georgia in recent years. Water availability for native vegetation, agriculture, industry, and domestic use continues to reduce. Scientifically and reasonably managing scarce water resources is an increasingly important environmental, social and economic issue. An understanding of the pattern and behavior of water use in an agricultural system can enhance and augment the effectiveness of water resource management. Georgia is a major contributor in the national production of peanuts, but most peanut production systems are rainfed. Water stress can cause abnormal peanut plant respiration. Nocturnal CO2 fluxes measured with the eddy-covariance technique in peanut field gives the nocturnal Net Ecosystem CO2 Exchange (NEE), composed of the respirations from both plants and soil. Further more, soil respiration consists of autotrophic respiration (combined root respiration and the respiration of soil microorganisms residing in the rhizosphere) and heterotrophic respiration (respiration of soil microorganisms and microorganisms not directly under the influence of the live root system. These different respirations are affected by various factors such as plant amount, plant growth stage, air and canopy temperature, soil water content, soil temperature, and rainfall variability. Therefore, it is important to measure and study NEE and nocturnal respiration and their relationships with the plant and environmental factors, which may give an early indicator of water stress and help policy makers and farmers in deciding the type of irrigation management and appropriate irrigation scheduling.

Response

In 2009, field experiments were conducted in a rainfed peanut field at Cordele, GA. Peanut field CO2/H2O fluxes were monitored using the eddy-covariance technique. The system for measuring eddy-covariance consists of a 3D sonic anemometer (CSAT-3, Campbell Scientific, Logan, UT) and an open path infrared gas analyzer (LI7500, Li-COR Inc., Lincoln, NE) at the height of 1.5 m above the ground surface. The 10 Hz and 30-min averaged data are collected with CR1000 data logger. In order to better understand how the water and carbon fluxes are influenced by key forcing parameters, additional meteorological measurements were continuously monitored at the site, which included net radiation, average soil heat flux from two soil heat flux plates, and soil temperature from thermocouples above each heat flux plate. All of these variables are recorded in 30 min average. A weather station was also placed at the measurement site, monitoring important environmental parameters such as air temperature, humidity, wind speed and direction, solar radiation and rainfall. In addition, the leaf area index was measured with a plant analyzer (LAI-2000, Li-COR Inc., Lincoln, NE) on weekly intervals. Soil respiration was continuously measured at two fixed locations using a long-term low-frequency automated soil chamber (Li-8100-101, Li-Cor, Inc., Lincoln, NE) and high-frequency soil CO2 gradient method (GMP343 soil CO2 probes, Vaisala Inc., Vantaa, Finland). Soil temperature and moisture were measured with thermocouples, and CS616 soil moisture sensors, respectively. The root-exclusion method was also used to separate soil respiration into autotrophic respiration (both root respiration and the respiration of soil microorganisms residing in the rhizosphere) and heterotrophic respiration (respiration of soil microorganisms not directly under the influence of the live root system). The measurements were conducted weekly using a survey soil chamber (Li-8100-101, Li-Cor, Inc., Lincoln, NE). To eliminate the problem of soil water and soil temperature differences between control plot and root-excluded plot, holes were drilled in the PVC pipes and filled with 35 micron nylon mesh bag. The 35 micron nylon mesh prevents prevented encroachment of roots while still allows allowing water to pass through.

Impact

We assessed the soil CO2 gradient method for measuring soil CO2 efflux. A weighted harmonic averaging method, used to estimate the soil CO2 diffusion coefficient with six models, is verified to yield a better estimate of soil CO2 efflux, which reasonably approximates the soil CO2 efflux measured with a soil chamber. In addition, the estimated soil CO2 efflux obtained by this improved method is well described by an exponential function of soil temperature at a depth of 0.05 m with the temperature sensitivity (Q10) of 1.81 and a linear function of soil moisture at a depth of 0.12 m, which is in general agreement with previous findings. These results suggest that the gradient method is a practical cost-effective means to continuously measure soil CO2 emissions. Response of soil CO2 efflux to rainfall events in agricultural fields is investigated using long-term low-frequency soil CO2 automated chamber and high-frequency soil CO2 gradient efflux method measurements. Results show that rainfall events decrease soil CO2 efflux during and immediately following rainfall and cause a delay in generating soil CO2 efflux, suggesting a restriction of soil porosity by rainfall infiltration. Growth stage of peanut also plays a critical role in determining the magnitude of the efflux following rainfall. It is found that the growth stage of the plant is more important than rainfall amount and initial soil moisture in generating soil CO2 efflux after rainfall. The present study suggests that high-resolution soil CO2 efflux measurements should be made to capture large pulses in CO2 emissions to adequately model carbon-water cycling from agricultural soils to the atmosphere. During optimum environmental conditions, photosynthetically active radiation (PAR) is the primary climatic factor controlling daytime NEE, accounting for 67 to 89% of variations in NEE. However, hysteresis response of daytime NEE during droughts is systematically observed. Soil water content (SWC) is the dominant factor limiting the NEE-PAR response during the peak growth stage, as NEE is significantly depressed when PAR exceeding 1300 µmol photons m-2 s-1 coincided with a very low soil water content (SWC < 0.04 m3 m-3). Hysteresis is observed between daytime NEE and PAR during periods of water-stress resulting from high vapor pressure deficit (VPD). This might be used as an indicator of water stress response in peanuts. This is also significant since it limits the range of applicability of the Michaelis-Menten equation and the likes to determine daytime NEE as a function of PAR, which suggests that the gap-filling technique based on a non-linear regression approach should take into account the presence of water-limiting field conditions. Including this step is therefore likely to improve current evaluation of ecosystem response to climate change. It was also found that nocturnal low level jets (LLJs) are a common feature at the study location and have a great potential to enhance surface turbulence and CO2 fluxes during the night. Therefore, it should be considered that their influence on the measurements of nighttime NEE and plant respiration as indicator of water stress. We are currently evaluating the impact of LLJ height and speed on those properties during nighttime and morning transition in a quantitative perspective.

State Issue

Conservation & Management of Natural Resources

Details

• Year: 2009
• Geographic Scope: Multi-State/Regional
• County: Crisp
• Program Areas:
  • Agriculture & Natural Resources