Black Soldier Fly Frass Combined with Ammonium Sulfate Enhances Maize Productivity and Nitrogen Use Efficiency Under Calcareous Soil Conditions

Efficient nitrogen (N) management in calcareous soils remains challenging due to limited nutrient availability, poor synchronization between N supply and crop demand, and low fertilizer use efficiency. While black soldier fly frass (BSFF) represents an emerging organic amendment, its comparative effects relative to vermicompost (VC) and mineral N fertilizers under calcareous soil conditions remain insufficiently understood. In this study, we evaluated the effects of BSFF- and VC-based integrated organic–mineral fertilization strategies involving urea (U) and ammonium sulfate (AS) on maize productivity, nitrogen use efficiency (NUE), and selected soil chemical properties. A two-year field experiment was conducted at Quzhou Experimental Station, China Agricultural University, using a randomized complete block design (RCBD) with nine treatments: unfertilized control (CK), sole mineral fertilizers (U and AS), sole organic amendments (BSFF and VC), and integrated combinations (UBSFF, UVC, ASBSFF, and ASVC). All fertilizer treatments received 180 kg N ha−1 and were replicated three times. Our results showed that ASBSFF increased maize grain yield by 101.9–114.6% compared with CK across both growing seasons, while ASVC increased yield by 92.5–103.9%. ASBSFF also resulted in the highest N uptake, increasing it by 121.3% compared with CK, and achieved the highest agronomic efficiency (26.01–27.51 kg grain kg−1 N applied), apparent recovery efficiency (63.2%), and physiological efficiency (57.13–59.18 kg grain kg−1 N uptake). BSFF-based integrated treatments increased soil NH4+-N, NO3−-N, available phosphorus, and available potassium compared with VC-based treatments, likely due to differences in nutrient composition between BSFF and VC. Overall, combining BSFF with mineral N fertilizers, particularly AS, improved maize productivity and NUE in calcareous soils, demonstrating its potential as a sustainable nutrient management strategy.