FERTILIZER NITROGEN RECOVERY IN A NO-TILL WHEAT-SORGHUM-FALLOW-WHEAT SEQUENCE

No-till cropping in the semiarid Central Great Plains increases water storage during fallowing, which allows farmers to use an intensive cro pping sequence of winter wheat (Triticum aestivum L.)-sorghum [Sorghum bicolor (L.) Moench]-fallow-winter wheat. The purpose of this N-15 fi eld study was to provide as complete accounting as possible of fertili zer N dynamics (changes in the NO3-N pool, fertilizer N uptake by plan ts, fertilizer N carryover effects, and mineralization and uptake of l abeled N from wheat residues) for this intensive cropping sequence. Ma in-plot treatments were 0, 56, and 112 kg N ha(-1). Eight microplots w ere established within all main plots. For the main plots that receive d 56 and 112 kg N ha(-1), two microplots received no (KNO3)-N-15, and one of these microplots had its nonlabeled wheat residues exchanged wi th labeled residues. Six microplots were treated with (KNO3)-N-15, one -third received (KNO3)-N-15 the first year, one-third the second year, and one-third both years. There was no detectable NO3 leaching. Miner alization, fertilizer application, and plant uptake had dramatic effec ts on the soil NO3-N pool. Both N rates increased the total N concentr ation and N uptake of aboveground biomass of the first wheat crop and the sorghum crop over the unamended treatment, whereas only the highes t fertilizer N rate increased the total N uptake of the unfertilized s econd wheat crop. Plant N uptake transferred the majority of fertilize r NO3 to aboveground biomass, and crop residue deposition, immobilizat ion, and mineralization maintained the fertilizer N in the top 60 cm o f soil, At the end of the 4-yr cropping sequence, 90 and 87% of the ap plied fertilizer N was accounted for at the 56 and 112 kg N ha(-1) rat es, respectively. Of this N, generally 24 to 28% remained in the soil. The 10 to 13% of the applied fertilizer N that was unaccounted for wa s probably lost by denitrification or NH3 volatilization.