**1. Introduction**

Biochar is a stable carbon (C) rich material formed through pyrolysis of organic materials [1,2]. Application of biochar to the soil is suggested to improve soil properties in addition to C sequestration [3–6]. It is reported to be beneficial in improving soil physical, biological, and chemical properties which include, among others, soil organic carbon (SOC), water retention capacity, cation exchange capacity (CEC), total soil nitrogen (TSN), and soil pH, hence contributing to soil fertility [7,8].

Soil organic C, one of the most important biological properties that determines quality of soil, is believed to be improved through application of biochar. Some research reports have documented the contribution of fertilizer-N (FN) in increasing SOC stock. They argue that FN increases quantity of crop residues added to the soil as a result of improved biomass production. Generally, high rates of FN inconsistently affect SOC where increases are observed in some cases while manure application more frequently increases surface soil SOC [9]. Biochar application with >90% of C in recalcitrant forms more consistently increases SOC. However, there are contradictory conclusions on the role of biochar in enhancing SOC storage. Some researchers have reported negative priming effect of biochar to the native SOC as a result of increasing the rate of evolution of carbon dioxide hence less storage [10,11]. This could be due to short term oxidation of the labile biochar compounds [12]. If the soil is inherently poor in SOC, application of biochar will reduce the evolution of CO2 while the opposite would be observed in soils rich in organic C [13].

**Citation:** Omara, P.; Aula, L.; Otim, F.; Obia, A.; Souza, J.L.B.; Arnall, D.B. Biochar Applied with Inorganic Nitrogen Improves Soil Carbon, Nitrate and Ammonium Content of a Sandy Loam Temperate Soil. *Nitrogen* **2022**, *3*, 90–100. https://doi.org/ 10.3390/nitrogen3010007

Academic Editor: Jacynthe Dessureault-Rompré

Received: 27 December 2021 Accepted: 18 February 2022 Published: 23 February 2022

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Besides, C loss is always very small relative to the amount of C stored within the biochar itself [14]. In contrast, Cross and Sohi [12] reported that application of biochar did not, for the most part, indicate negative priming of the native SOC and that application of biochar could stabilize native SOC in grassland soils. Applying a combination of N and biochar could contribute to the increase in the SOC storage.

With evidence of increased SOC following biochar application, soil N is likely to increase. Soil N is present mostly in organic compounds which consist of both particulate organic N and dissolved organic N. The particulate organic N include the N in living organisms and detritus. On the other hand, dissolved organic N consists of a wide range of organic substances, such as free amino acids, and proteins, among others [15]. Biochar soil incorporation is suggested to increase the buildup of organic N. Prommer et al. [16] reported that application of inorganic N in combination with biochar had a synergistic effect by activating the belowground build-up of soil organic N. They explained that biochar reduces the transformation rates of the native soil organic N as plants and microbes draw from the inorganic fertilizer N. Bai et al. [17] added that changes in microbial processes and activities on soil organic N following biochar soil application are mediated primarily by abiotic factors such as rainfall and temperature. Therefore, biochar has a great potential in building soil organic N.

Plants take up N in the inorganic form; NO3 − and NH4 <sup>+</sup> which are susceptible to losses such as volatilization, denitrification, runoff, and leaching [18–20]. Biochar application may improve inorganic N retention through alteration of CEC and anion exchange capacity (AEC) with the greatest benefit on sandy soils and this has been demonstrated by many studies [21–25]. The increased AEC of biochar reduces leaching of NO3 −–N while the CEC increases the adsorption of NH4 +–N. Therefore, the application of inorganic N with biochar may reduce loss and increase uptake of both NO3 −–N and NH4 +–N. The objective of this study was to compare the effect of fertilizer N-biochar-combinations (NBC) and FN on soil NO3 −–N, NH4 +–N, SOC, and TSN. We hypothesized greater soil N content and improved SOC under NBC compared to NF following harvest of maize.

### **2. Materials and Methods**

### *2.1. Experimental Sites and Design*

Field trials were conducted for two years in the summer cropping season of 2018 and 2019 at Efaw Agronomy Research Station (36◦08 12.6 N 97◦06 25.8 W) and Lake Carl Blackwell research farm (36◦08 58.0 N 97◦17 19.3 W), near Stillwater, OK, USA. Efaw Agronomy Research Station had Ashport silty clay loam (fine-silty, mixed, superactive, thermic Fluventic Haplustoll) soil. Lake Carl Blackwell had Pulaski fine-sandy loam (coarse/loamy, mixed nonacid, thermic Udic Ustifluvent) soil [26]. The treatments included; 0, 50, 100, and 150 kg ha−<sup>1</sup> of FN with no biochar; and 5, 10 and 15 t ha−<sup>1</sup> of biochar with no FN. The three NBC treatments were 50 kg N plus 5 t ha−<sup>1</sup> biochar, 100 kg N plus 10 t ha−<sup>1</sup> biochar, and 150 kg N plus 15 t ha−<sup>1</sup> biochar. In the second year, treatments were applied to the same exact plots used in the first year. Biochar was obtained from Wakefield Agricultural Carbon (Columbia, MO, USA), a USDA certified biochar producing company. Physical and chemical properties of Southern Yellow Pine biochar pyrolyzed at 500 ◦C, and the initial soil conditions are included in Table 1. All the N and biochar treatments were applied prior to planting of maize. Total rainfall and average air temperature (April to September) in 2018 and 2019 at Stillwater, OK, USA were obtained from Oklahoma Mesonet (Figure 1).

**Table 1.** Physical and chemical properties of soft wood (Southern Yellow Pine) biochar supplied by Wakefield Biochar, Columbia, Missouri; the initial soil chemical properties at Lake Carl Blackwell (LCB) and Efaw research sites, Stillwater, OK, USA.


TP, total phosphate; TN, total nitrogen; TOC, total organic carbon; BD, bulk density; x, values not determined. Initial soil properties were determined before the first year of biochar application.

**Figure 1.** Total rainfall and average air temperature (April to September) in 2018 and 2019 at Stillwater, OK, USA.

Fertilizer-N was applied as urea ammonium nitrate—UAN (28:0:0). Fertilizer-N, biochar and NBC treatments were surface applied. Biochar was broadcast and incorporated at a 15 cm soil depth using a 2720 John Deere Disk Ripper (John Deere, Moline, IL, USA). This incorporation ensured an in-depth mixing of the biochar-N fertilizer complex with soil materials for the respective treatments.

#### *2.2. Data Collection and Analysis*

Composite soil samples, 15–20 cores per plot at 0–15 cm, were collected five months after biochar application following harvest of maize in 2018 and 2019. Soil samples were oven-dried for 48 h at 65 ◦C, and ground to pass through a 1 mm sieve size to remove larger aggregates and plant roots. The extraction of inorganic N (NO3 −–N and NH4 +–N) was carried out from 5 g of soil with 25 mL 1 M KCl after shaking for 30 min on a rotary shaker at 200 rpm. The extracts were filtered with 0.45 μm Whatman filter paper and then analyzed using automated Lachat QuickChem 8500 Series 2 Flow Injection Analyzer (Hach Co., Loveland, CO, USA). The SOC and TSN contents were determined from 200 mg of

soil using dry combustion [27] at 950 ◦C with LECO Truspec CN dry combustion analyzer LECO CN628 (LECO Inc., St. Joseph, MI, USA).

#### *2.3. Statistical Analysis*

In this study, the independent variables were contrasts, treatments, and replications while the dependent variables were NO3 −–N, NH4 +–N, TSN, and SOC. Data were analyzed separately for each year and separated by location. The GLM procedure of the SAS statistical package was used in the analysis of variance (ANOVA) [28]. The combined ANOVA tested for the effect of the independent variables as well as key interactions on the response variables. For all the response variables, the difference between treatment means from NBC and FN were compared using single-degree-of-freedom orthogonal contrasts [29,30]. In addition to the level of statistical significance from ANOVA, the standard error (S.E) of means for each treatment and the coefficient of variation (CV) were used to indicate the precision of measurement and the extent of variability within and between groups, respectively. For each response variable, treatment means and the corresponding S.E were presented in a table that combined experimental sites and years. Additionally, contrasts that compared specific treatments of interests were presented in the bottom half of the table with corresponding F and *p*-values for each site and year.
