Nitrogen addition decreases soil aggregation but enhances soil organic carbon stability in a temperate forest

Soil aggregates and the stability of their associated soil organic carbon (SOC) are important factors mediating soil carbon (C) sequestration and soil functions. However, the response of SOC stability to nitrogen (N) deposition is highly divergent, and the combined influences of N deposition and soil aggregates on SOC stability are poorly understood. The mechanisms underlying these influences were explored in a six-year field N-addition experiment covering a wide range of soil aggregates, root morphologies, soil properties and several SOC stability indices (represented by heterotrophic respiration and δ13C or δ15N) in a deciduous broad-leaved forest in Northeast China. The results showed that N addition significantly decreased the proportion of large macroaggregates (2–8 mm), reducing the soil aggregate mean weight diameter (MWD) by 10.0–17.2 %, which was negatively correlated with root length density (RLD) and root weight density (RWD). Organic C stability in soil aggregates was enhanced by N addition, as indicated by the decrease in the decomposition rate of organic C and the increase in the δ13C values but not the C content in microaggregates within macroaggregates (mM) or δ15N values. Furthermore, the promotion of mineral-associated organic C formation after N addition was detected by stable isotopic mixed model analysis (SIMM), indicating increased C protection by minerals. With the use of a structural equation model (SEM), the variation in the C stability of large macroaggregates (2–8 mm) was explained by the changes in fine roots, MWD and N availability but not those in small macroaggregates (0.25–2 mm) due to the instability of large macroaggregates. These results demonstrate that N addition may enhance soil C stability in all the soil aggregate sizes by promoting mineral sorption of C, with the C stability in large macroaggregates being more vulnerable to multiple environmental changes than that in small macroaggregates. Therefore, the soil C stability response to N deposition in the temperate forests of Northeast China could be modulated by soil aggregate sizes and may cause negative feedbacks to global warming.