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 cropping sequence of winter wheat (Triticum aestivum L.)-sorghum [Sorghum bicolor (L.) Moench]-fallow-winter wheat. The purpose of this N-15 field study was to provide as complete accounting as possible of fertilizer N dynamics (changes in the NO3-N pool, fertilizer N uptake by plants, fertilizer N carryover effects, and mineralization and uptake of labeled N from wheat residues) for this intensive cropping sequence. Main-plot treatments were 0, 56, and 112 kg N ha(-1). Eight microplots were established within all main plots. For the main plots that received 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 with 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. Mineralization, fertilizer application, and plant uptake had dramatic effects on the soil NO3-N pool. Both N rates increased the total N concentration and N uptake of aboveground biomass of the first wheat crop and the sorghum crop over the unamended treatment, whereas only the highest fertilizer N rate increased the total N uptake of the unfertilized second wheat crop. Plant N uptake transferred the majority of fertilizer NO3 to aboveground biomass, and crop residue deposition, immobilization, and mineralization maintained the fertilizer N in the top 60 cm of soil, At the end of the 4-yr cropping sequence, 90 and 87% of the applied fertilizer N was accounted for at the 56 and 112 kg N ha(-1) rates, 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 was probably lost by denitrification or NH3 volatilization.