*3.2. Soil Ammonium N*

The ANOVA at the Efaw location in 2018 did not show significant difference in soil NH4 +–N content (*p* = 0.9268) between treatments (Table 3). At each fertilizer rate, soil NH4 +–N content was higher under NBC compared to FN by 8 and 9% at 50 and 100 kg N ha<sup>−</sup>1, respectively, while a decrease under NBC by 3% was observed at 150 kg N ha−<sup>1</sup> compared to FN. The highest NH4 +–N of 21.2 mg kg−<sup>1</sup> was seen under NBC at 100 kg N ha−<sup>1</sup> while the lowest (17.6 mg kg−1) was observed at the check plot. In 2019, results showed significant differences in NH4 +–N content (*p* = 0.0009) between treatments (Table 3). However, contrasts between NBC and FN did not show significant difference in NH4 +–N at all contrasted N rates. Soil NH4 +–N content decreased under NBC by 13% at 150 kg N ha−<sup>1</sup> compared to FN. The highest NH4 +–N (5.5 mg kg−1) was observed under FN at 150 kg N ha−<sup>1</sup> while the lowest (3.9 mg kg<sup>−</sup>1) was observed under 5 t ha−<sup>1</sup> of biochar with no N applied.

At LCB, the 2018 results showed significant differences in soil NH4 +–N (*p* = 0.016) between treatments (Table 3). For each fertilizer rate, contrasts between NBC and FN did not show significant difference in soil NH4+–N at 50 kg N ha−<sup>1</sup> (*p* = 0.3546) while significant differences were seen at 100 kg N ha−<sup>1</sup> (*p* = 0.026) and 150 kg N ha−<sup>1</sup> (*p* = 0.0182). Soil NH4 +–N was higher under NBC than FN by 6, 14, and 14% at 50, 100, and 150 kg N ha<sup>−</sup>1, respectively. The highest NH4 +–N (31 mg kg−1) was observed at 150 kg N ha−<sup>1</sup> under NBC while the lowest (23 mg kg−1) was observed at 10 t ha−<sup>1</sup> of biochar with no N fertilizer applied. In 2019, results were similar to that of 2018 where significant difference in NH4 +–N (*p* < 0.0001) was observed between treatments (Table 3). Contrasts between NBC and FN did not show any significant difference in soil NH4 +–N at 50 kg N ha−<sup>1</sup> (*p* = 0.8881) and 100 kg N ha−<sup>1</sup> (*p* = 0.1078) while significant differences were seen at 150 kg N ha−<sup>1</sup> (*p* = 0.0026). Soil NH4 +–N was higher under NBC than FN by 1, 8, and 15% at 50, 100, and 150 kg N ha−1, respectively. The highest soil NH4 +–N (5.2 mg kg−1) was observed under NBC at 150 kg N ha−<sup>1</sup> while the lowest (4.1 mg kg<sup>−</sup>1) was observed at the check plot.


**Table 3.** Mean ammonium 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.
