Soil aggregate-associated organic nitrogen pools, enzyme activities, and microbial community fertilized with food waste-derived organic fertilizer

Soil aggregates are pivotal sites for nutrient storage and microbial activity within the soil ecosystem. However, how food waste-derived organic fertilizer application changes organic nitrogen (N) pools, enzyme activities, and microbial community in different soil aggregate fractions remain poorly understood. Here, a two-year (2020-2021) field experiment with rice crops in eastern China compared no fertilizer (CK), chemical fertilizer (CF), and 50 % substitution of chemical N with food waste-derived organic fertilizer (CO). Aggregates were categorized into three sizes (> 2 mm, 2-0.25 mm, and < 0.25 mm) using a modified dry-sieving method. Our results indicated that organic substitution (1) increased total N, dissolved organic N, microbial biomass N, and acid hydrolyzable identified N like ammonium N, amino sugar N, and amino acid N across all aggregate sizes, (2) boosted N-acquiring enzyme activities including urease, protease, leucine aminopeptidase, and N-acetyl-beta-Dglucosidase regardless of aggregate fractions, and (3) altered the distribution of bacterial and fungal communities in the aggregates. Additionally, positive correlations with bacterial and fungal communities were found for total N, some organic N fractions and enzymes. The structural equation models revealed that aggregates larger than 2 mm were the primary source of bulk soil organic N pools and microbial community, while aggregates sized 2-0.25 mm contributed mainly to bulk soil enzyme activities. Overall, our study demonstrated that food wastederived organic fertilizer substitution could be an effective practice in improving soil N nutrition and microbial activity, and highlighted the importance of macro-aggregates in bulk soil health and function.