This paper uses a refined soil gradient method to estimate soil CO2 efflux. Six different models are used to determine the relative gas diffusion coefficient (ξ). A weighted harmonic averaging is used to estimate the soil CO2 diffusion coefficient, yielding a better estimate of soil CO2 efflux. The resulting soil CO2 efflux results are then compared to the soil CO2 efflux measured with a soil chamber. Depending on the choice of ξ model used, the estimated soil CO2 efflux using the gradient method reasonably approximates the efflux obtained using the soil chamber method. 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, in general agreement with previous findings. These results suggest that the gradient method is a practical cost‐effective means to measure soil CO2 emissions. Results from the present study suggest that the gradient method can be used successfully to measure soil CO2 efflux provided that proper attention is paid to the judicious use of the proper diffusion coefficient.

Abstract. Continuous measurements of net ecosystem CO 2 exchange (NEE) using the eddy-covariance method were made over an agricultural ecosystem in the southeastern US. During optimum environmental conditions, photosynthetically active radiation (PAR) was the primary driver controlling daytime NEE, accounting for as much as 67 to 89% of the variation in NEE. However, soil water content became the dominant factor limiting the NEE-PAR response during the peak growth stage. NEE was significantly depressed when high PAR values coincided with very low soil water content. The presence of a counter-clockwise hysteresis of daytime NEE with PAR was observed during periods of water stress. This is a result of the stomatal closure control of photosynthesis at high vapor pressure deficit and enhanced respiration at high temperature. This result is significant since this hysteresis effect limits the range of applicability of the Michaelis-Menten equation and other related expressions in the determination of daytime NEE as a function of PAR. The systematic presence of hysteresis in the response of NEE to PAR suggests that the gap-filling technique based on a nonlinear regression approach should take into account the presence of water-limited field conditions. Including this step is therefore likely to improve current evaluation of ecosystem response to increased precipitation variability arising from climatic changes.

Food legumes, an excellent source of protein and soil fertility improvement, offer small farmers a means of intensifying cropping on rice lands in semiarid and tropical regions. Unfortunately food legume productivity is often limited by variation in the amount and distribution of rainfall. The study was conducted to compare differential responses of mungbean (Vigna radiata L.), soybean [Glycine max (L.) Merr.], cowpea [Vigna unguiculata (L.) Walp.], peanut (Arachis hypogaea L.), and pigeonpea (Cajanus cajan L.) to a soil water gradient imposed during the reproductive growth phase. Field studies were conducted on Lipa clay loam, an isohyperthemic Typic Hapludoll silt loam soil, using a line source sprinkler irrigation system at the International Rice Research Institute, Philippines, from January to May during 1985 and 1986. Among the five species, peanut yielded significantly higher across the irrigation regimes in both years. Lack of water in the driest regime (50% water deficit replacement) reduced the seed yield of mungbean, soybean, cowpea, and peanut by an average of 43, 39,34, and 32%, respectively. However, pigeonpea seed yield increased by an average of 31% in the driest regime. Seed yield increases per mm of total irrigation water plus rainfall were 3.46 and 5.61 kg ha−1 in soybean, and 3.55 to 5.67 kg ha−1 in peanut in 1985 and 1986, respectively. However, pigeonpea yield per mm of total irrigation water plus rainfall declined 1.53 and 1.27 kg ha−1 in 1985 and 1986, respectively. Peanut performed best in irrigated as well as in rainfed environments, followed by cowpea, soybean, and mungbean. Pigeonpea was suitable only for the relatively dry, rainfed environment. Results indicate the need to match the suitable food legumes to maximize the rice land use.

Continuing increases in atmospheric carbon dioxide concentration, [CO 2 ], will likely be accompanied by global warming. Thus, it is important to quantify and understand the consequences of elevated [CO 2 ] and temperature on crop growth and yield to develop suitable varieties and agronomic management practices for future climates. The objective of this study was to investigate the growth and development responses of shoots and roots of peanut (Arachis hypogaea L.) grown under different combinations of atmospheric [CO 2 ] and temperature. The study comprised a long-term experiment, in which plants were grown in growth chambers for 112 days, and a short-term experiment, in which growing plants in rhizotrons for 17 days. In the long-term experiment, peanut cultivar Tainan 9 was grown in 20-L containers fitted with minirhizotron observation tubes at 5 cm soil depth and placed in controlled environment chambers under three levels of [CO 2 ] (400, 600, and 800 µmol mol -1 ) and two levels of air temperature (25/15ºC and 35/25ºC day/night temperature). In the short-term experiment, two peanut seedlings were grown in each of 18 acrylic rhizotrons with a 6-mm thick soil layer. Rhizotrons with plants were placed in the same growth chambers as above. At 3-to 4-day intervals, rhizotrons were placed on a flatbed scanner to collect digital images from which root length and number were measured using RMS software. At 25/15ºC, plants grown at 600 and 800 µmol mol -1 CO 2 had main stems that were 24 and 44% longer than those grown at 400 µmol mol -1 , while at 35/25ºC the main stem length was similar in all [CO 2 ] levels. At 25/15ºC, plants showed greater area and dry weight per leaf than at 35/25ºC. At harvest, high temperature significantly reduced total leaf area to 574 cm 2 for 35/ 25ºC compared with 921.2 cm 2 for 25/15ºC. Specific leaf area at low temperature was 22% less than at high temperature. Above ground biomass was increased by elevated CO 2 in both temperature treatments. At high temperature, above ground biomass was 56%, 24%, and 16% higher than at low temperature at [CO 2 ] of 400, 600 and 800 µmol mol -1 , respectively. Pod dry weight increased with increasing [CO 2 ] at 25/15ºC, but was not different among [CO 2 ] levels at 35/25ºC. At 25/15ºC, pod dry weight was 50% higher than at 35/ 25ºC. As the temperature increased from 25/15ºC to 35/25ºC, pod dry weight was reduced by 40% at 400, 53% at 600, and 54% at 800 µmol mol -1 CO 2 . High temperature produced more root length in the containers, whereas low temperature did in the rhizotrons. There were significant interactions between temperature and [CO 2 ] for their effects on main stem length and above ground biomass. High temperature enhanced growth of shoots and roots, but decreased pod dry weight. There was no interaction of elevated [CO 2 ] with higher temperature on the reproductive growth, despite a tendency for beneficial temperature by [CO 2 ] interaction on vegetative growth and total shoot dry weight. The beneficial effects of increased ...

Abstract. Continuous measurements of net ecosystem CO2 exchange (NEE) using the eddy-covariance method were made over an agricultural ecosystem in the southeastern US. During optimum environmental conditions, photosynthetically active radiation (PAR) was the primary climatic factor controlling daytime NEE, accounting for 67 to 89% of variations in NEE. However, soil water content (SWC) was the dominant factor limiting the NEE-PAR response during the peak growth stage, as NEE was 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 was observed between daytime NEE and PAR during periods of water-stress resulting from high vapor pressure deficit (VPD). This is 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. The systematic presence of hysteresis in the response of NEE to PAR 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 evaluations of ecosystem response to climate change.