Isotopic studies on the uptake of nitrogen by pasture grasses. II. Uptake of fertilizer nitrogen by Rhodes grass in posts

Abstract

Nitrogen uptake by Rhodes grass was a linear function of the quantity of ¹⁵NH₄N0₃ applied for rates up to the equivalent of 400 lb N/ac, but the proportion of fertilizer nitrogen recovered in the plants fell significantly when the rate was increased to 800 lb N/ac. A nitrogen pretreatment equivalent to 200 lb N/ac had relatively little effect on the uptake of ¹⁵NH₄N0₃ by the grass, despite the fact that it almost doubled the weight of roots in the pots when the ¹⁵NH₄N0₃ was first applied. Over the range 0–400 lb N/ac, 84.1%% of added total nitrogen and 75.5% of added ¹⁵N was taken up by plants that received no nitrogen fertilizer during the pretreatment phase, and 80.3% of added total nitrogen and 71.9% of added ¹⁵N was taken up by plants that received a pretreatment of 200 lb N/ac. Fertilizer nitrogen was distributed between tops and roots in the ratio (averaged for the two pretreatments) of 5.2 : 1 for total nitrogen and 4.5 : 1 for ¹⁵N; these ratios were constant over the range 0–400 lb N/ac and were not significantly different.