Effects and Mechanisms of Granulated Compost on Soil Nitrogen Supply and Crop Uptake: Preliminary Evidence from a 15N Tracing Field Experiment in Tobacco

Organic fertilizer granulation represents a promising strategy for modifying nitrogen (N) release from compost in soil. Nevertheless, there is a lack of large-scale field trials exploring its impact on tobacco production and soil N supply. This research conducted a preliminary study by employing 15N tracing technology to investigate the effects of granular compost on soil N transformation and supply; on the yield and quality of tobacco leaves; and on the distribution of granular compost-derived N among the different soil N pools and tobacco plant organs. The results revealed that the 2 cm diameter granule organic fertilizer treatment (G2) significantly increased tobacco leaf yield by 15% compared to conventional fertilization (CK). However, the 4 cm diameter granule organic fertilizer (G4) treatment resulted in a reduction in leaf yield. Notably, the quality of tobacco leaves remained unaffected compared to conventional fertilization treatment; the N content ranged from 15 to 25 g kg−1, which was within the high-quality range. The results also indicated that direct N supply to the tobacco from granular compost was limited. The G2 and G4 treatments provided 2.8% and 2.2% of the N in the fertilizer to the tobacco plants, respectively, with more than 93% of the N in the tobacco plants derived from the soil. Therefore, both of these particle sizes of granular compost facilitated the absorption of soil N by tobacco plants. At the end of the growth period, the N content derived from the G4 granular fertilizer in the soil was significantly higher than that from the G2 fertilizer. This may be due to the slower nutrient release mechanism and longer release period of the G4 fertilizer compared to G2. Our results suggested that granulated compost fertilizer (both G2 and G4) has the potential to enhance soil N supply. Despite the elevated nitrogen levels observed in leaves treated with 4 cm diameter granular fertilizer, an integrated assessment of yield performance demonstrates that the 2 cm diameter granular organic fertilizer delivers superior economic benefits. However, G2 may also have a higher potential for N loss. Further investigations under field conditions are necessary to validate the applicability of granular fertilizer of different particle sizes and its specific mechanisms of impact.