Seasonal Changes in Smooth Bromegrass Top and Root Growth and Fate of Fertilizer Nitrogen

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Abstract
While information is available at the end of the growing season on the fate of fertilizer N applied to perennial grasslands, little detailed information is available on the transformations and translocation of N during a growing season. The purpose of this research was to measure the effect of N fertilization on time, quantities, and rates of dry‐matter accumulation by smooth bromegrass (Bromus inermis L.) tops and roots, water use, and fertilizer N transformations and uptake. Ammonium nitrate was surface broadcast at 0, 90, and 270 kg N ha−1 on established smooth bromegrass at Mandan, ND. At approximately 3‐wk intervals, top and root (0–900 mm) dry weights were measured, N uptake calculated, soil inorganic N concentrations determined, and apparent fertilizer N in tops, roots, and as soil inorganic N was calculated (fertilized minus check). Growth and N uptake of tops increased until physiological maturity in July, decreased during a midsummer drought, and increased after August rains. Fertilization greatly increased growth rate. For roots, growth stopped or even declined slightly at time of floral emergence in June, resumed until physiological maturity, remained constant or decreased slightly through the summer drought (when soil water reserves were depleted), then increased rapidly in September after fall rains. Dry matter accumulated at rates in excess of 300 kg ha−1 d−1 in May and September, when N uptake rates exceeded 4 kg ha−1 d−1. Over 40% of the fertilizer N at the 270 kg N rate was immobilized in organic forms other than root growth within 10 d after application, but by physiological maturity, 51% was in tops, 27% in roots, 3% as soil inorganic N, and only 19% was immobilized in other organic forms or lost by gaseous means (no leaching occurred). Results suggest that gaseous losses were no more than 10%. Nitrogen fertilization appeared to enhance three labile N pools—readily available (inorganic), highly labile (probably including microbial biomass), and moderately labile (possibly primarily grass roots).
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