*3.3. Soil Organic C*

At Efaw, the 2018 results indicated an overall significant difference (*p* = 0.0016) in SOC between treatments (Table 4). Contrast comparing NBC and FN at 50 kg N ha−<sup>1</sup> did not indicate significance difference (*p* = 0.6542) while significance differences in SOC were seen at 100 kg N ha−<sup>1</sup> (*p* = 0.0064) and 150 kg N ha−<sup>1</sup> (*p* = 0.0018). Higher SOC observed under NBC than FN correspond to 5, 27, and 31% at 50, 100, and 150 kg N ha<sup>−</sup>1, respectively. The highest SOC of 9.6 g kg−<sup>1</sup> was observed under NBC at 150 kg N ha−<sup>1</sup> while the lowest of 6.6 g kg−<sup>1</sup> was obtained at 150 kg N ha−<sup>1</sup> under FN. Similar observations were made in 2019 where significant differences (*p* = 0.0007) in SOC were seen between treatments (Table 4). Contrasts revealed significant differences between NBC and FN at 100 kg N ha−<sup>1</sup> (*p* = 0.018) and 150 kg N ha−<sup>1</sup> (*p* = 0.0007). The NBC registered higher SOC than FN by 22 and 35% at 100 and 150 kg N ha−1, respectively. The highest SOC of 11 g kg−<sup>1</sup> was observed under NBC at 150 kg N ha−<sup>1</sup> while the lowest of 6.86 g kg−<sup>1</sup> was obtained at the control plot with no biochar and N applied.

Results at LCB for 2018 did not show an overall significant difference (*p* = 0.0758) in SOC between treatments (Table 4). For each fertilizer rate, contrast comparing NBC with FN did not show significant difference at 50 kg N ha−<sup>1</sup> (*p* = 0.0858) but differences were seen at 100 kg N ha−<sup>1</sup> (*p* = 0.0058) and 150 kg N ha−<sup>1</sup> (*p* = 0.0006). The SOC under NBC was higher than that observed under FN by 17, 21, and 28% at 50, 100, and 150 kg N ha<sup>−</sup>1, respectively. The highest SOC of 12 g kg−<sup>1</sup> was observed at 150 kg N ha−<sup>1</sup> under NBC while the lowest of 8.2 g kg−<sup>1</sup> was observed at 50 kg N ha−<sup>1</sup> under FN. In 2019, overall ANOVA showed significant differences (*p* = 0.0015) in SOC between treatments (Table 4). Contrasts between NBC and FN showed significant differences at 50 kg N ha−<sup>1</sup> (*p* = 0.0415), 100 kg N ha−<sup>1</sup> (*p* = 0.0241) and 150 kg N ha−<sup>1</sup> (*p* = 0.0335). The observed differences showed higher SOC under NBC more than under FN by 14, 15, and 12% at 50, 100, and 150 kg N ha<sup>−</sup>1, respectively. The highest SOC of 13 g kg−<sup>1</sup> was observed under NBC at 150 kg N ha−<sup>1</sup> while the lowest of 8.2 g kg−<sup>1</sup> was seen at 50 kg N ha−<sup>1</sup> under FN.


**Table 4.** Mean soil organic C 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.
