We used 15 NH 4 tracer additions to determine travel distances of ammonium (NH 4 ) and suspended particulate organic nitrogen (SPON) in six streams ranging from second to fifth order located within a single watershed on the North Slope of Alaska. Based on the distribution of 15 N stored in stream bottom compartments (primary producers or grazers), we estimated NH 4 travel lengths. We used a two-compartment model to estimate the travel length of SPON based on the distribution of source 15 N on the stream bottom and SPO 15 N in the water column. Both NH 4 and SPON travel lengths (S w and S p , respectively) increased with discharge primarily due to changes in depth and velocity. Variation in the vertical mass transfer coefficient ( f ) of both NH 4 and SPON did occur among the streams but was not related to stream size and was relatively small compared to the change in physical characteristics. Thus, in the Kuparuk watershed, physical gradients outweighed biological or chemical changes as controls on NH 4 and SPON travel length. The one exception was the Kuparuk fertilized reach, where phosphorus fertilization greatly increased biological activity and NH 4 processing compared to unaltered streams. Longitudinal gradients in major biological driving variables such as litter inputs, debris dams, and shading are absent in the Arctic, perhaps explaining the relatively uniform NH 4 -f . Watersheds in other biomes may show differing degrees of physical versus biological/chemical controls. A conceptual model is presented for comparing the relative strength of these controls among different watersheds. Strong relationships between discharge and travel length should greatly aid development of watershed models of nutrient dynamics.

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The concentration of nitrate in the lower portion of corn stalks 1 to 3 wk after physiological maturity is a reliable tissue test for detecting optimal or above‐optimal supplies of available N for corn (Zea mays L.) grown for grain. This test, the end‐of‐season corn stalk test, has not been evaluated for silage corn. The test would be especially valuable for silage corn because this crop is frequently grown on manured fields where assessing the N status is difficult due to unquantified rates of N application and variable mineralization of organic N. This study reports nitrate concentrations in lower corn stalks shortly before harvest of silage and evaluates their use as an end‐of‐season corn stalk test for silage corn. Also explored is the possibility of collecting corn stalks up to 24 h after a field is harvested. Lower corn stalk samples were collected from a total of 19 N‐response trials on corn fields with a history of manure applications in Connecticut in 1994 and 1995. In 1996, corn stalk samples were collected at harvest and 24 h after harvest from 20 farmers’ fields. Three commonly used methods for defining a critical nitrate concentration, the quadratic‐plateau model, the linear‐plateau model, and the Cate‐Nelson method were used to describe the relationship between relative yield and stalk nitrate concentrations. Three different critical concentrations were calculated by the three methods, which suggests that an optimal range of 500 to 1000 ppm nitrate N is appropriate for this test. There was no significant change in the nitrate concentration of stalk samples collected at harvest and 24 h after harvest. These results suggest that the end‐of‐season corn stalk test can be used as a convenient method to define excess N availability and to improve N fertilizer recommendations for silage corn fields. Research Question This study evaluates the use of nitrate concentrations in the lower portion of corn stalks 3 to 4 wk before physiological maturity to estimate the N status of corn grown for silage. Also explored is the possibility of collecting corn stalks up to 24 h after a field is harvested. Literature Summary The concentration of nitrate in the lower portion of corn stalks 1 to 3 wk after physiological maturity is a reliable tissue test for detecting optimal or above‐optimal supplies of available N for corn grown for grain. This test, the end‐of‐season corn stalk test, has not been evaluated for use with silage corn. The test would be especially valuable for silage corn because it is more frequently grown on manured fields where assessing the N status is difficult due to unquantified rates of N application and variable mineralization of organic N. Excess N availability in corn fields has been linked with nitrate loss from the soil profile. Most diagnostic tissue tests indicate N deficiency. While these tests can be economically beneficial for corn fields with N deficiency, environmental problems and unneeded economic outlays can exist where excess N availability is detected. Diagnostic tools for identifyi...