Fate of Fertilizer Nitrogen in the Field 2 Years After Biochar Application

This study aimed to clarify the scientific quantification of fertilizer nitrogen (N) uptake and utilization, its destination, and its residual distribution in the soil at a depth of 0-30 cm after biochar application using ¹⁵N tracer technology. The purpose was to provide a theoretical basis for developing a scientific application strategy for N fertilizer and biochar in irrigated farmland areas. Two levels of N fertilizer application were set up using the ¹⁵N labeling method in microareas of large fields: the regular amount of N fertilizer (N1: 300 kg·ha^-1) and a reduction of N fertilizer by 15% (N2: 255 kg·ha^-1). Further, three levels of biochar application were set up: no biochar (B0: 0 kg·ha^-1), a low amount of biochar (B1: 10 × 10³ kg·ha^-1), and a medium amount of biochar (B2: 20 × 10³ kg·ha^-1). The tested biochar was derived from corn stover (maize straw). The natural abundance of ¹⁵N-labeled fertilizer N, the total N content of each aboveground organ, and the total N content of soil at a depth of 0-30 cm in a spring wheat field at maturity were determined, and the yield was measured in the corresponding plots. The proportion of ¹⁵N-labeled fertilizer N uptake by each organ of spring wheat and the soil N uptake was 20.60-35.32% and more than 64.68%, respectively. Moreover, the proportion of soil N uptake showed a decreasing trend with an increase in biochar application. The spring wheat N uptake and utilization rate, the residue rate in the soil at a depth of 0-30 cm, the total utilization rate, and the rate of loss of ¹⁵N-labeled fertilizer N ranged from 15.21% to 29.61%, 23.33% to 28.93%, 38.54% to 58.54%, and 41.46% to 61.46%, respectively. The spring wheat N fertilizer utilization rate, fertilizer N residue rate in soil, and total fertilizer N utilization rate all increased gradually with an increase in biochar application, except for the N loss rate, which decreased gradually. When N fertilizer reduction was combined with medium biochar (B2N2), the yield of spring wheat significantly improved, mainly due to an increase in the number of grains in spikes. Under this treatment, the number of grains in spikes of spring wheat was 41.9, and the yield reached 7075.54 kg·ha^-1, which was an increase of 9.69-28.25% and 10.91-25.35%, respectively, compared with other treatments. Yield increased by up to 25.35%, and nitrogen loss decreased by 48.24% under the B2N2 treatment. Biochar application could promote the amount and proportion of fertilizer N uptake in various organs of spring wheat as well as in the soil at a depth of 0-30 cm. In this study, a 15% reduction in N fertilizer (255 kg·ha^-1) combined with 20 × 10³ kg·ha^-1 biochar application initially helped achieve the goal of increasing spring wheat yield and N fertilizer uptake, as well as improving fertilizer N utilization, providing an optimal scientific application strategy for N fertilizer and biochar in the farmland of the irrigation area. These results substantiate the hypothesis that biochar application enhances spring wheat (Triticum aestivum L.) assimilation of fertilizer-derived nitrogen (¹⁵N) while concomitantly improving fertilizer nitrogen retention in the soil matrix, which could provide a sustainable framework for nitrogen management in irrigated farmlands.