Complexity in water and carbon dioxide fluxes following rain pulses in an African savanna

The idea that many processes in arid and semi-arid ecosystems are dormant until activated by a pulse of rainfall, and then decay from a maximum rate as the soil dries, is widely used as a conceptual and mathematical model, but has rarely been evaluated with data. This paper examines soil water, evapotranspiration (ET), and net ecosystem CO2 exchange measured for 5 years at an eddy covariance tower sited in an Acacia–Combretum savanna near Skukuza in the Kruger National Park, South Africa. The analysis characterizes ecosystem flux responses to discrete rain events and evaluates the skill of increasingly complex “pulse models”. Rainfall pulses exert strong control over ecosystem-scale water and CO2 fluxes at this site, but the simplest pulse models do a poor job of characterizing the dynamics of the response. Successful models need to include the time lag between the wetting event and the process peak, which differ for evaporation, photosynthesis and respiration. Adding further complexity, the time lag depends on the prior duration and degree of water stress. ET response is well characterized by a linear function of potential ET and a logistic function of profile-total soil water content, with remaining seasonal variation correlating with vegetation phenological dynamics (leaf area). A 1- to 3-day lag to maximal ET following wetting is a source of hysteresis in the ET response to soil water. Respiration responds to wetting within days, while photosynthesis takes a week or longer to reach its peak if the rainfall was preceded by a long dry spell. Both processes exhibit nonlinear functional responses that vary seasonally. We conclude that a more mechanistic approach than simple pulse modeling is needed to represent daily ecosystem C processes in semiarid savannas

Nitrate deposition in northern hardwood forests and the nitrogen metabolism of Acer saccharum marsh

It is generally assumed that plant assimilation constitutes the major sink for anthropogenic Nitrate NO 3 − deposited in temperate forests because plant growth is usually limited by nitrogen (N) availability. Nevertheless, plants are known to vary widely in their capacity for NO 3 − uptake and assimilation, and few studies have directly measured these parameters for overstory trees. Using a combination of field and greenhouse experiments, we studied the N nutrition of Acer saccharum Marsh. in four northern hardwood forests receiving experimental NO 3 − additions equivalent to 30 kg N ha −1 year −1 . We measured leaf and fine-root nitrate reductase activity (NRA) of overstory trees using an in vivo assay and used 15 N to determine the kinetic parameters of NO 3 − uptake by excised fine roots. In two greenhouse experiments, we measured leaf and root NRA in A. saccharum seedlings fertilized with 0–3.5 g NO 3 − −N m −2 and determined the kinetic parameters of NO 3 − and NH 4 + uptake in excised roots of seedlings. In both overstory trees and seedlings, rates of leaf and fine root NRA were substantially lower than previously reported rates for most woody plants and showed no response to NO 3 − fertilization (range = non-detectable to 33 nmol NO 2 − g −1 h −1 ). Maximal rates of NO 3 − uptake in overstory trees also were low, ranging from 0.2 to 1.0 μmol g −1 h −1 . In seedlings, the mean V max for NO 3 − uptake in fine roots (1 μmol g −1 h −1 ) was approximately 30 times lower than the V max for NH 4 + uptake (33 μmol g −1 h −1 ). Our results suggest that A. saccharum satisfies its N demand through rapid NH 4 + uptake and may have a limited capacity to serve as a direct sink for atmospheric additions of NO 3 − .Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47695/1/442_2004_Article_BF00334659.pd

Yield and physiological aspects of 17 varieties of corn grown in runoff farming

A micro-catchment water harvesting agrisystem in Avra Valley, west of Tucson, Arizona, was utilized to grow 17 varieties of drought tolerant corn (Zea mays). The primary objective of this study was to isolate and evaluate the grain yield performance of these cultivars. Additional measurements were also taken on transpiration rate (TR), leaf water potential (0), stress degree day (SDD), and crop water stress index (CWSI), during the period 24 October to 2 November, between two irrigations, in search of possible physiological mechanisms of drought adaptability and their impact on production. The system's performance in terms of economical crop growth is subject to further research. The analysis of grain yield indicates a significant varietal difference. Physiological parameters monitored also show trends of differences among cultivars. It was found that cultivars capable of maintaining a higher plant water content, by preserving their TR, Ψ1, CWSI, and SDD are not necessarily the better yielding cultivars. Possible justifications of this phenomena are discussed. It is suggested that a distinction has to be made between crop adaptability to drought and preservation of a high grain yield since under limited moisture conditions, one might be attained through the suppression of the other.hydrology collectio

Plant nitrogen budget under elevated carbon dioxide level: Regulation by nitrogen absorption and assimilation. Progress report, October 1, 1995--July 31, 1996

The overall objective is to assess root physiological and morphological characteristics that may alter plant N acquisition capacity in response to rising atmospheric CO{sub 2} concentration. There is increasing evidence that plant and ecosystem responses to elevated levels of CO{sub 2} will ultimately depend on availability and acquisition rate of other resources such as N. Therefore, knowledge of any changes in root capacity to acquire N is crucial in predicting plant and ecosystem responses to high CO{sub 2}. Here the authors are testing two major hypotheses: (1) elevated CO{sub 2} will enhance root N uptake kinetics and (2) CO{sub 2} enrichment will increase root preference for NO{sub 3}{sup {minus}} as opposed to NH{sub 4}{sup +}. High CO{sub 2} enhances root energy status which should in turn favor energy-intensive processes such as NO{sub 3}{sup {minus}} uptake and assimilation. The above hypotheses are being tested on a range of species from native and agricultural ecosystems using a combination of field, lab and growth chamber studies. The authors have demonstrated a considerable interspecies variation in root N uptake responses to CO{sub 2} enrichment and attempts are now underway to evaluate if such variations are correlated with different functional groups. A comprehensive growth model, using physiological and allocation parameters, has been largely completed and will be used to analyze the completed experimental data