Effects of Different Tillage Practices and Nitrogen Fertiliser Application Rates on Soil-Available Nitrogen

The dynamics of nitrogen availability in soil result from the net amount of organic and inorganic inputs undergoing decomposition, mineralization, and immobilization. This varies with soil type, climate, and cropping system management practices. Hence, the interaction of these factors in a specific environment is important for nitrogen management and availability in any cropping system. This study evaluated the concentration of nitrate (NO3−) and ammonium (NH4+) in a maize monocropping system in a long-term trial under different tillage practices, i.e., no-till (NT), conventional tillage (CT), and rotational tillage (RT), and with different nitrogen (Urea) fertiliser application rates (0, 100, and 200 kg/ha N). The experimental area was a split plot with randomized tillage strips as the main plots, with the nitrogen fertiliser application rates forming the subplots. The results found the highest concentration of NO3− (7.728 mg/kg) and NH4+ (0.762 mg/kg) under the NT (p < 0.05) compared to the other treatments. CT recorded the lowest amount of NH4+ (0.648 mg/kg) (p < 0.05) compared to RT (0.758 mg/kg), while the concentration of NO3− (6.755 mg/kg) under the CT was relatively higher than under the RT (4.720 mg/kg). The results also showed that the 0–10 cm depth had a significantly (p < 0.05) higher concentration of NO3− (6.787 mg/kg) and NH4+ (0.851 mg/kg), followed at the 20–30 cm depth with 6.229 mg/kg NO3− and 0.832 mg/kg NH4+. Soil samples taken at 10–20 cm depth recorded the lowest NO3− (6.188 mg/kg) and NH4+ (0.485 mg/kg). The study also found the highest NO3− concentration (0.452 mg/kg) with 200 kg/ha N (p < 0.05), while the highest concentration of NH4+ (0.954 mg/kg) was found with 0 kg/ha N. The lowest concentrations of both NO3− (6.333 mg/kg) and NH4+ (0.561 mg/kg) were found with 100 kg/ha N. the no-till also had a significantly (p < 0.05) higher NO3− (8.518 mg/kg) concentration with 200 kg/ha N compared to the CT (6.005 mg/kg) and RT (4.833 mg/kg). On the other hand, more NH4+ (1.478 mg/kg) was found with 0 kg/ha N under the NT compared to the CT (0.717 mg/kg) and the RT (0.669 mg/kg). The concentration of NH4+ was relatively low with 200 kg/ha N in the NT (0.545 mg/kg) and CT (0.598 mg/kg) but higher in the RT (0.813 mg/kg). In conclusion, the NT favoured the availability of NO3− and NH4+ in the studied cropping system; however, the concentration of these was affected by fertiliser application rate and depth. This suggested that each tillage system should be compatible with the type and the rate of inorganic nitrogen fertiliser for better crop production.