Combined Application of Organic Fertilizer and Quicklime Enhances Soil Nitrogen Retention in Acidified Apple Orchards

Soil acidification in apple orchards disrupts nitrogen (N) transformation and raises the risk of N loss. Using in situ 15N-urea microcosms in an acidified orchard (pH 4.46) in North China, we tested four treatments: N, phosphorus (P), and potassium (K) (CK, control); N, P, K plus organic fertilizer (NPKM); N, P, K plus quicklime (NPKL); and N, P, K plus organic fertilizer and quicklime (NPKML). We measured soil ammonium (AN), nitrate (NN), fixed AN (FAN), amino sugars and their 15N abundance to quantify fertilizer N distribution and the mechanisms of N retention. The results indicated that co-amendment with organic fertilizer and quicklime increased microbial necromass N (MNN) accumulation, with bacterial necromass N accounting for 83–87% of total MNN based on amino sugar biomarkers, suggesting a predominance of bacterial-derived residues in the retained fertilizer N pool. The NPKML treatment achieved the highest fertilizer N accumulation in the biological pool, maintained a stable pH of 6.20 and a total N content of 1.34 g kg−1, and showed the highest FAN and MNN retention. Based on the observed variations in soil physicochemical indicators and N pool distribution, we infer that quicklime may enhance abiotic ammonium fixation through pH elevation, while organic fertilizer may promote microbial residue accumulation by supplying carbon substrates. Their combined application was associated with the highest N retention across both biotic and abiotic pools, suggesting a complementary effect that warrants further mechanistic investigation. This study provides a theoretical basis for and practical strategies for efficient N management in acidified orchards.