Long-term effects of nitrogen fertilization on aggregation and localization of carbon, nitrogen and microbial activities in soil

Highlights: • N fertilization improved soil structure by forming large macroaggregates (> 2 mm). • N addition increased TN, fungi and NAG activity in macro- but not in micro-aggregates. • However, SOC in all aggregates remained constant, suggesting accelerated C turnovers. • Macroaggregates are more sensitive to N fertilization than microaggregates. • N fertilization shifted localization of microorganisms to the macroaggregates. Long-term nitrogen (N) fertilization affects soil aggregation and localizations of soil organic carbon (SOC), N and microbial parameters within aggregates. The mechanisms of these N effects are poorly understood. We studied these processes in a loamy soil from a 23-year repeated N addition field experiment under a rice–barley rotation. Nitrogen fertilization increased plant productivity and the portion of large macroaggregates (> 2 mm). However, SOC contents in macro- and micro-aggregates remained constant despite an N-induced increase of 27% in root C input into soil. Therefore, N fertilization accelerated SOC turnover. Nitrogen addition increased total N (TN) content in bulk soil and two macroaggregates (> 2, and 1–2 mm), but not in microaggregates (N-acetyl-β-D-glucosaminidase (NAG) activities in the two larger macroaggregate size classes (> 2, and 1–2 mm), but not in the aggregates (< 1 mm). In both control and N fertilization, the large macroaggregates localized more TN, microbial PLFAs, and NAG activities than the microaggregates. In conclusion, long-term N fertilization not only directly promotes soil N resource but also indirectly improves soil structure by forming large macroaggregates, accelerates SOC turnover, and shiftes localization of microorganisms to the macroaggregates.