*Article* **Biochar Applied with Inorganic Nitrogen Improves Soil Carbon, Nitrate and Ammonium Content of a Sandy Loam Temperate Soil**

**Peter Omara 1,2,\*, Lawrence Aula 1, Fred Otim 2, Alfred Obia 2, Joao Luis Bigatao Souza <sup>1</sup> and Daryl Brain Arnall <sup>1</sup>**

<sup>1</sup> Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, OK 74078, USA; aula@okstate.edu (L.A.); bigatao@okstate.edu (J.L.B.S.); b.arnall@okstate.edu (D.B.A.)

<sup>2</sup> Department of Agronomy, Gulu University, Gulu P.O. Box 166, Uganda; otimakio@gmail.com (F.O.); a.obia@gu.ac.ug (A.O.)

**\*** Correspondence: peter.omara@okstate.edu

**Abstract:** Biochar is suggested to improve soil properties. However, its combination with inorganic nitrogen (N) fertilizer in temperate soils is not well understood. This study compared the effect of fertilizer N-biochar-combinations (NBC) and fertilizer-N (FN) on total soil N (TSN), soil organic carbon (SOC), soil nitrate (NO3 −–N), and ammonium (NH4 +–N). Soil samples were taken from experiments at Efaw and Lake Carl Blackwell (LCB), Oklahoma, USA with ten treatments consisting of three N rates (50, 100, and 150 kg N ha−1) and three biochar rates (5, 10, and 15 t ha−1). Results at Efaw showed greater TSN and SOC under NBC compared to FN by 3 and 21%, respectively. No percentage difference was observed for NH4 +–N while NO3 −–N was lower by 7%. At LCB, TSN, SOC, NO3 −–N, and NH4 +–N were higher under NBC by 5, 18, 24, and 10%, respectively, compared to FN. Whereas application of biochar improved SOC at both sites, NO3 −–N and NH4 +–N were only significant at LCB site with a sandy loam soil but not at Efaw with silty clay loam. Therefore, biochar applied in combination with inorganic N can improve N availability with potential to increase crop N uptake on coarse textured soils.

**Keywords:** biochar; total nitrogen; nitrate; ammonium; soil organic carbon
