Decreased net N nitrification rate constrains soil N availability during long-term Camellia oleifera cultivation

Red soils in southern China have low nitrogen (N) content, and long-term Camellia oleifera cultivation has resulted in nutrient depletion, soil acidification, reduced microbial activity, and disruption of soil aggregate structure. These factors reduce N utilization in the soil. However, the specific N transformation process and mechanism are unclear. This research aimed to elucidate the potential mechanisms of N transformation with different Camellia oleifera cultivation periods in various soil ecological niches (soil water stable aggregates). We analyzed soil samples in Camellia oleifera sampling period (2 years, Chi-Per), maturity period (10 years, You-Per), and degeneration period (40 years, Old-Per). Samples were sieved into macroaggregates (> 2 mm, LM), small aggregates (2–0.25 mm, SM), and microaggregates (< 0.25 mm, Mi). Changes in inorganic N and net N transformation rates were assessed. Results indicated that soil available nitrogen (AN) and nitrate nitrogen (NO₃⁻-N) contents were 19 % and 32 % lower in the Old-Per treatment compared to the Chi-Per treatment, respectively. In the You-Per treatment, AN and NO₃⁻-N significantly decreased with decreasing aggregate particle size. Additionally, soil net N mineralization rate, net nitrification rate, and denitrification rate decreased by 14 %, 65 %, and 14 % from the Chi-Per to the Old-Per treatment. During the You-Per and Old-Per cultivation periods, both net N mineralization rate and net nitrification rate decreased with decreasing aggregate particle size. The carbon to nitrogen (C/N) ratio increased as the aggregate particle size decreased, leading to reduced net N mineralization and lower AN and NO₃⁻-N content in microaggregates. 16S rRNA gene sequencing results showed that the relative abundance of oligotrophic bacterial communities (Acidobacteria, Verrucomicrobia, and Elusimicrobia) increased in the Old-Per treatment. PLS-PM analysis showed that the net nitrification rate significantly inhibits soil N availability and is closely associated with sucrase, nitrate reductase, and bacterial communities. Overall, this study confirmed that long-term Camellia oleifera cultivation decreases soil N availability by decreasing the net nitrification rate, influenced by changes in soil sucrase, nitrate reductase activities and oligotrophic bacteria communities. These results indicate that reduced net nitrification rate constrains soil N availability during long-term Camellia oleifera cultivation.