The fate of nitrogen-15 ammonium sulfate applied to Kentucky bluegrass and perennial ryegrass turfs

Turfgrasses that possess a spreading growth habit typically produce an organic layer, termed thatch, that has been shown to sequester significant quantities of applied nitrogen. This study was conducted to compare the fate of nitrogen in a thatchy Kentucky bluegrass (Poa pratensis L.) and a non-thatch producing perennial ryegrass (Lolium perenne L.) turf. The effects of sampling time and sampling depths on the distribution of labeled fertilizer nitrogen (LFN) were investigated. Ammonium sulfate was applied to 36 microplots installed in a Marlette sandy loam soil (fine-loamy, mixed, mesic Glossoboric Hapluadalfs) at a rate of 293 kg ha^-1 yr^-1 in six equal applications of 48.8 kg N ha^-1, with the initial application containing ¹⁵N-labeled (NH₄)₂SO₄. Four microplots from each turf species were excavated on 17 June, 6 July, and 10 Aug. 1994 and on 15 June 1995 for nitrogen analysis. Two days after treatment, the Kentucky bluegrass LFN recoveries in shoot tissue, thatch, and soil were 34, 38, and 20% of the total LFN applied. These values changed to 47, 20, and 9% at 365 DAT. For perennial ryegrass, the corresponding values were 25, 28, and 27% at 2 DAT and 43, 10, and 14% at 365 DAT. Thatch was a better N sink than mat, but mat also sequestered significant quantities of LFN. Some downward movement of LFN was observed, particularly in perennial ryegrass. The very small LFN concentrations found in the 20- to 40-cm soil layer confirms that leaching is not a major pathway for N loss from turf. Soil LFN was primarily found in the 0- to 5-cm soil layer and as organic nitrogen. These results indicate that while thatch is important in reducing N leaching in turf, the absence of thatch does not dramatically alter nitrogen dynamics in turf.