Nitrogen Mineralization Responses to Cropping, Tillage, and Nitrogen Rate in the Northern Great Plains

Nitrogen-mineralization rates are needed to accurately determine N fertilization requirements to meet plant needs while minimizing environmental contamination. A spring wheat (Triticum aestivum L.)-fallow (SW-F) system was compared with a spring wheat-winter wheat-sunflower (Helianthus annuus L.) (SW-WW-SF) system on a Temvik-Wilton silt loam (fine-silty, mixed Typic and Pachic Haploborolls) at three N rates (0, 22, and 45 kg ha‒1 for SW-F and 34, 67, and 101 kg ha‒1 for SW-WW-SF) under conventional, minimum, and no-tillage. After 10 yr, soil samples were incubated to determine N mineralization rates. Cropping intensity, N rate, and tillage intensity interacted to affect N-mineralization rates. Within the SW-F system N-mineralization rates in 0- to 0.05-m depth were 8.2 ± 0.8 kg ha‒1 wk‒1 in the fallow phase vs. 5.0 ± 0.7 kg ha‒1 wk‒1 in the crop phase under conventional tillage and were 6.2 ± 0.3 kg ha‒1 wk‒1 under minimum and no-tillage in both phases. The N-mineralization rates were 2.3 ± 0.4 kg ha‒1 wk‒1 in 0.05- to 0.15-m depth soils of the SW-F system. In spring wheat, N-mineralization rates in 0- to 0.05-m depth soil were 9.9 ± 0.8 kg ha‒1 wk‒1 in the SW-WW-SF system vs. 5.6 ± 0.4 kg ha‒1 wk‒1 in the SW-F system and in the 0.05- to 0.15-m depth were 3.6 ± 0.1 kg ha‒1 wk‒1 in the SW-WW-SF system vs. 2.4 ± 0.2 kg ha‒1 wk‒1 in the SW-F system Within the SW-WW-SF system, N-mineralization rates in the 0- to 0.05-m soil layer were 6.8 ± 0.5 kg ha‒1 wk‒1 under winter wheat vs. 9.9 ± 0.8 kg ha‒1 wk‒1 under spring wheat and 9.2 ± 0.6 kg ha‒1 wk‒1 under sunflower. In the 0.05- to 0.15-m soil layer, N-mineralization rates were 3.3 ± 1.0 kg ha‒1 wk‒1. More intensive cropping and conservation tillage increased N-mineralization rates in this soil and may ameliorate the decline in N fertility associated with crop-fallow systems