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The efficiency of nitrate use by turfgrasses is likely related to its efficiency of absorption by roots and its rate of metabolism in roots and shoots. This study was conducted to quantify the relationship between nitrate uptake rate and nitrate reductase activity with N use efficiency in a cool‐season turfgrass. Six cultivars of Kentucky bluegrass (Poa pratensis L.), which differ markedly in field performance, were used to measure intraspecific variation in nitrate uptake, in vivo nitrate reductase activity of roots and leaves, and N use efficiency expressed as clipping mass per unit N in clippings. Companion field studies compared N use efficiency and metabolism among 14 Kentucky bluegrass cultivars established on an Enfield silt loam (Coarse loamy over sandy skeletal, mixed, mesic, Typic Dystrochrepts). Nitrate uptake rate was determined by an in situ nitrate depletion method. Nitrate reductase activity was assayed by an optimized in vivo method. Significant differences among cultivars were observed for nitrate absorption, nitrate reductase activity in roots and leaves, and N use efficiency. Ambient nitrate concentrations influenced these parameters and their intraspecific differences. Nitrate uptake and reduction were saturable at external nitrate concentrations in excess of 1 mM. Regression analyses demonstrated that nitrate reductase activity in roots and leaves was strongly influenced by nitrate uptake rate. Nitrogen use efficiency was negatively related to ambient nitrate levels, nitrate uptake rate and nitrate reductase activity, with nitrate reductase activity in leaves having the strongest negative effect on use efficiency. These results suggest that the efficiency of N use by Kentucky bluegrass may be increased by genetically altering nitrate reductase activity and its partitioning between roots and shoots.

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The stability of nitrogen within a turf‐soil ecosystem is important both for efficient turf management and preventing the contamination of ground water by nitrate. The objective of this study was to quantify responses of the microbial community and the mobility of soil nitrogen following the sudden death of established turf. Twelve‐year‐old turf plots comprising four cool‐season turfgrass species fertilized with five N sources were maintained on an Enfield silt loam (coarse‐silty over sandy or sandy‐skeletal, mixed, active, mesic Typic Dystrudept) at Kingston, RI. Half of the plots were killed with glyphosate in early September and any regrowth was removed mechanically. Measurements of soil physical, chemical, and microbiological properties and nitrate leaching in killed and healthy plots were compared for 12 mo. Turf death did not alter soil moisture, temperature, pH, or extractable ammonium. Nitrate levels were higher in both the root zone and at 60 cm following turf death and this difference persisted for the sampling year. Carbon mineralization and microbial biomass C were not different between soils from healthy and killed plots. Killed plots leached three times more nitrate than healthy plots but this amounted to less than 10% of total soil N present. Retention of nitrate in a turf‐soil system depends on absorption by living grass roots, although reasonable N stability is also provided by N cycling within the soil microbiota. Protecting ground water from nitrate contamination is optimized by maintaining a vigorous turfgrass cover.

Efficient utilization of fertilizer-nitrogen (N) by turfgrasses is probably related to N uptake efficiency of roots and metabolic efficiency of absorbed N in roots and shoots. This study evaluated Kentucky bluegrass (Poa pratensis L.) cultivars for potential differences in nitrate uptake rate (NUR), temporal variation in NUR, and the relationship between NUR and N use efficiency (NUE), defined as grams dry matter per gram N. Six cultivars were propagated from tillers of seeded plants, grown in silica sand, mowed weekly, and watered daily with a complete nutrient solution containing 1.0 mm nitrate. A nutrient depletion method from an initial nitrate concentration of 0.5 mm was used to determine NUR of 5-month-old plants. NUR (μmol·h-1 per plant) of the six cultivars ranked as follows: `Blacksburg' > `Conni' > `Dawn' > `Eclipse' = `Barzan' > `Gnome'. When NUR was based on root weight, `Conni' ranked highest; when NUR was based on root length, surface, or volume, `Eclipse' ranked highest. Averaged across cultivars, NUR on the second day was greater than NUR for the first day of nitrate exposure. Temporal variation was greatest in `Blacksburg', while none was noted in `Conni' or `Eclipse'. Cultivar differences in NUE were significant in fibrous roots, rhizomes, and leaf sheaths, but not in leaf blades and thatch. Total nitrate uptake was positively related to total N recovered and total plant dry matter, but NUR based on root weight was negatively correlated with NUE of the whole plant.