Saturday, 15 July 2006

Differences in Plant Growth and Phosphorus Uptake Among Three Riparian Grass Species.

John Kovar, USDA-ARS Natl. Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011-4420 and Norbert Claassen, Institute of Agricultural Chemistry, Carl-Sprengel-Weg 1, Goettingen, 37075, Germany.

Phosphorus (P) losses from agricultural landscapes contribute significantly to eutrophication of surface waters. The total amount of P absorbed by plants growing in or near grass filter strips and riparian buffers can be determined, but we have little knowledge of the pattern of accumulation/depletion during the course of a growing season. In this study, we evaluated differences in plant growth and P acquisition of switchgrass (Panicum virgatum L.), smooth bromegrass (Bromus inermis Leyss.), and reed canarygrass (Phalaris arundinacea L.) grown under controlled conditions in a flowing nutrient solution culture system. Pre-germinated seedlings were transplanted into 1.7-L pots that then were placed into 450-L tanks with continuously circulating (>1 L min-1) nutrient solution. Phosphorus concentrations of 1 or 100 然 were maintained by daily analysis of solution, and addition of P as needed via peristaltic pump. Each species/P concentration treatment combination was randomly assigned to one tank, with 16 pots per tank, which allowed four sequential harvests. Plants were grown under a 16-h day length, with 80% relative humidity, and 25C/20C day/night temperature. Plants were harvested at approximately 0, 26, 40, and 53 days after transplanting, depending on the species. At each harvest, shoot and root weight, root length, and shoot and root P content were determined. Assuming that P uptake kinetics of the roots followed a Michaelis-Menten relationship, a method based on the rate of P depletion from solution was used to estimate parameter values. Depletion experiments were conducted before the second, third, and fourth plant harvests. Throughout the course of the experiment, 1 然 P in solution was sufficient for optimum growth of these three species. Shoot dry matter accumulation of switchgrass was greater than that of the other species, the exception being harvest four of canarygrass in 1 然 solution (18.9 g plant-1 vs. 11.4 g plant-1 for switchgrass). Root dry weights tended to follow the same trend. However, by the fourth harvest, root length of canarygrass was significantly greater than that of the other species in both low and high P solutions. This was reflected by shoot P content, which was 51.8 and 56.4 mg plant-1 for canarygrass and switchgrass, respectively in 100 然 P solution, and 58.9 mg plant-1 for canarygrass in 1 然 P solution. Preliminary analysis of the P depletion data suggests that at low solution P concentration (1 然), the maximum rate of P influx (Imax) was generally higher for canarygrass than for the other species. At high solution P concentration (100 然), bromegrass tended to have higher Imax values than the other species. In all cases, Imax values were higher for plants grown in 1 然 P solution. The results of this study suggest that differences in the ability of these riparian grass species to accumulate P were due to both differences in P uptake kinetics and the size of the root systems. Further research is required to determine whether these differences would be observed in the field.

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