Fertilizer ¹⁵N balance in a soybean-maize-maize rotation system based on a 41-year long-term experiment in Northeast China

Global awareness of the need to enhance crop production and reduce environmental issues associated with nitrogen (N) fertilizer has increased. However, studies on how the N fate changed with manure addition are still limited. To explore efficient fertilization management for an improved grain yield, N recovery efficiency, and reduced N residual in the soil or that unaccounted for, a field 15N micro-plot trial in a soybean-maize-maize rotation was conducted to evaluate the effect of fertilization regimes on soybean and maize yields and the fertilizer N fate in the plant-soil system during 2017-2019 within a 41-year experiment in Northeast China. Treatments included chemical N alone (N), N and phosphorus (NP), N, P, and potassium (NPK), and those combined with manure (MN, MNP, and MNPK). Application of manure increased grain yield, on average, by 153% for soybean (2017) and 105% and 222% for maize (2018 and 2019) compared to no manure, with the highest at MNPK. Crop N uptake and that from labeled N-15-urea also benefited from manure addition, mainly partitioned to grain, and the average N-15-urea recovery was 28.8% in the soybean season with a reduction in the subsequent maize seasons (12.6%, and 4.1%). Across the three years, the fertilizer N-15 recovery ranged from 31.2-63.1% (crop) and 21.9-40.5% (0-40 cm soil), with 14.6-29.9% unaccounted for, including N losses. In the two maize seasons, manure addition significantly increased the residual N-15 recovery in crop attributed to the enhancing N-15 remineralization, and reduced that in soil and unaccounted for compared to single chemical fertilizer, with MNPK performing the best. Therefore, applying N, P, and K fertilizers in the soybean season and NPK combined with manure (13.5 t ha(-1)) in the maize seasons is a promising fertilization management strategy in Northeast China and similar regions.