Effects of nitrogen application and intercropping on spatio-temporal distribution of soil inorganic nitrogen in pea/maize intercropping field

The aim of this study was to determine nitrogen use efficiency and reduce environmental pollution caused by excessive use of nitrogen fertilizer in pea/maize intercropping fields in irrigated areas of Hexi Corridor, Gansu Province. To that end, a field experiment was conducted in 2011 to determine the spatial and temporal distributions of soil inorganic N (N_min) in orthic anthrosol soils under pea/maize intercropping at different N application rates (N 0 kg·hm^-2, 75 kg·hm^-2, 150 kg·hm^-2, 300 kg·hm^-2, 450 kg·hm^-2). Soil samples were collected in the 0 20, 20 40, 40 60, 60 80, 80 100 and100 120 cm soil layers and N_min concentrations measured by flow analysis of extracted CaCl₂. The results showed that NO₃^--N was the major form of N_min in orthic anthrosol soils with a concentration 7.55 times that of NH₄⁺-N. During maize growth season, NO₃^--N concentration increased by 29.7%, 67.5%, 88.2% and 134.3% respectively under N rates of 75, 150, 300 and 450 kg·hm^-2 over no N fertilization treatment. Compared with that after pea harvest, NO₃^--N concentration decreased by 44.2% after maize harvest. Soil NO₃^--N concentration in the 0-120 cm soil layer under intercropped pea/maize decreased respectively by 6.1%/5.1% over that of each corresponding monocultured crop. The highest soil N_min accumulation occurred in the 0-20 cm soil layer at different nitrogen application rates and growth stages. After pea harvest, N_min accumulation in the 0-60 cm soil layer under intercropped pea/maize respectively decreased by 4.9%/1.9%, and in the 60-120 cm soil layer by 10.8%/9.2% compared with that of each corresponding monocultured crop. After maize harvest, soil N_min accumulation in the 0-60 and 60-120 cm soil layers under intercropped pea/maize decreased respectively by 28.2%/9.4% and 23.5%/12.5% over each corresponding monocultured crop. Also soil N_min residue in the 0-60 and 60-120 cm layers under intercropped pea decreased respectively by 4.9% and 10.9% over monocultured pea. The study showed that nitrogen fertilizer application significantly increased soil inorganic nitrogen concentration and accumulation with higher effect on soil NO₃^--N. Also excessive N application resulted in environmental pollution in the study area. Pea/maize intercropping significantly reduced soil inorganic nitrogen concentration and accumulation. It was obvious that intercropping reduced soil inorganic nitrogen accumulation especially in the later growth stages of the crops.