*3.1. Soil Nitrate N*

The results at Efaw location in 2018 did not show any significant difference (*p* = 0.0534) in soil NO3 −–N content between treatments (Table 2). At each fertilizer rate, soil NO3 −–N was higher under NBC compared to FN by 5 and 7% at 50 and 100 kg N ha<sup>−</sup>1, respectively. At 150 kg N ha−1, orthogonal contrast showed that NBC was significantly (*p* = 0.0259) lower than FN by 31%. The highest soil NO3 <sup>−</sup>–N of 6.4 mg kg−<sup>1</sup> was observed under FN at 150 kg N ha−<sup>1</sup> while the lowest (3.8 mg kg<sup>−</sup>1) was observed at 10 t ha−<sup>1</sup> biochar with no N applied. In 2019, results showed significant differences in soil NO3 −–N content (*p* < 0.0001) between treatments (Table 2). However, contrasts between NBC and FN did not show significant difference in soil NO3 −–N at all N rates. Soil NO3 −–N was lower under NBC than observed FN by 5, 9, and 5% at 50, 100, and 150 kg N ha−1, respectively. The highest soil NO3 <sup>−</sup>–N (5.9 mg kg−1) was observed under FN at 150 kg N ha−<sup>1</sup> while the lowest (4.1 mg kg<sup>−</sup>1) was observed under 5 t ha−<sup>1</sup> of biochar with no N applied.

At LCB, the 2018 ANOVA results showed significant differences in soil NO3 −–N (*p* < 0.0001) between treatments (Table 2). For each fertilizer rate, contrasts between NBC and FN did not show significant difference in soil NO3 <sup>−</sup>–N at 50 kg N ha−<sup>1</sup> (*p* = 0.1702). However, significant differences were seen at 100 kg N ha−<sup>1</sup> (*p* = 0.0003) and 150 kg N ha−<sup>1</sup> (*p* < 0.0001). Nitrate under NBC was higher than that observed under FN by 11, 29, and 40% at 50, 100, and 150 kg N ha−1, respectively. The highest NO3 <sup>−</sup>–N (3.9 mg kg−1) was seen under NBC at 100 kg N ha−<sup>1</sup> while the lowest (2.0 mg kg<sup>−</sup>1) was observed at 150 kg N ha−<sup>1</sup> under FN. In 2019, results were similar to that of 2018 where significant differences in NO3 −–N (*p* = 0.001) were observed between treatments (Table 2). Contrasts between NBC and FN did not show a significant difference in NO3 <sup>−</sup>–N at 50 kg N ha−<sup>1</sup> (*p* = 0.3134) and 100 kg N ha−<sup>1</sup> (*p* = 0.0891), while significant difference was observed at 150 kg N ha−<sup>1</sup> (*p* = 0.02). The observed differences showed higher NO3 −–N under NBC than FN by 16, 23, and 27% at 50, 100, and 150 kg N ha<sup>−</sup>1, respectively. The highest soil NO3 <sup>−</sup>–N (7.0 mg kg<sup>−</sup>1) was observed under NBC at 150 kg N ha−<sup>1</sup> while the lowest (3.7 mg kg<sup>−</sup>1) was observed at the check plot.


**Table 2.** Mean nitrate N for treatments plus the associated contrasts between N fertilizer and biochar-N combinations at Efaw and Lake Carl Blackwell, Stillwater, Oklahoma. 2018 and 2019.

C.V, coefficient of variation between treatments; S.E, standard error for replicated means (±SE, *n* = 3). Nitrogen fertilizer was applied as urea ammonium nitrate—UAN (28:0:0). Biochar was applied immediately following UAN and incorporated to a depth of 15 cm.
