Nitrogen uptake by crops, soil distribution and recovery of urea-N in a sorghum-wheat rotation in different soils under Mediterranean conditions

The N uptake by crops, soil distribution and recovery of N-15 labelled urea -N (100 kg N ha(-1)) were investigated in a sorghum-wheat rotation in two s ilty clay soils (Foggia and Rieti Casabianca) and one silt loam soil (Rieti Piedifiume) under different mediterranean conditions. Non-exchangeable lab elled NH4-N represented an important pool at both Rieti sites with higher v alues (p<0.05) under sorghum (14.0 and 24.6% of the urea N in the 0-20 cm l ayer at the end of the cropping season) than wheat whereas it was much less important in the Foggia soil (10.0% in the surface soil under sorghum). Th is is probably related to the clay minerals composition of the three soils; because vermiculite was present in both Rieti sites but not in the Foggia soil. At harvest from 4.4 to 5.3% of the urea N initially applied was prese nt as microbial biomass N in the surface soil layer with no generally signi ficant differences due to location and type of crops. Both sorghum and whea t N yields were higher in the driest site (Foggia) probably due to better l ight conditions, higher temperatures and irrigation during summer of the so rghum cropping period. The recovery of plant fertilizer N (about 21% for so rghum and 27% for wheat) and the percentage of N in the plant derived from the fertilizer (NDFF) were the lowest at Rieti-Casabianca probably as the r esult of the protection of immobilized fertilizer N against microbial miner alization by the swelling clays. The fertilizer N unaccounted for was nil o r very low (10.8% at Rieti-Casabianca under wheat and 11.8 and 4.9% at Riet i-Piedifiume under sorghum and wheat, respectively). Urea-N losses occurred when Rieti Piedifiume and Rieti Casabianca soils were kept bare. In this c ase the urea N unaccounted for ranged from 12 to 56% of the urea N with hig her losses in Rieti-Piedifiume than in Rieti-Casabianca. The higher recover ies in the latter soil were probably confirmed by the stabilizing effect of clays on the immobilized urea N.