Nitrogen index

Nitrogen (N) is the major limiting nutrient determining yield and quality of crops. There are many factors related to the N cycle that jointly determine its overall functioning and plant uptake, which farmers aim to manipulate with different fertilization types (e.g., organic versus mineral) and fertilizer application rates. However, the soil N cycle, encompassing various elements, remains elusive within different soil aggregate fractions for each fertilization type, where the most significant portions of N are stored. This study established a soil N index combining N substrates (physiochemical properties and microbial residual N), N drivers (functional gene abundance and diversity) and N processes (N transformation rates and extracellular enzyme activities) to evaluate the effects of fertilizer types (organic vs. mineral) and application rates on N cycle in three fractions of soil aggregates. Compared to mineral fertilizer application, organic fertilizer application increased the soil N index in microaggregates rather than in large or small macroaggregates. Higher rates of fertilizer application also increased the soil N index in microaggregates instead of in large and small macroaggregates. We conclude that organic fertilizer application boosts the soil N index, which is dependent on aggregate size. Our study indicates that examining soil within microaggregates may be crucial for tracking functional changes in the soil N cycle.