A high soil potential net nitrification was observed at 4 m depth: What are the driving factors?

Studies about nitrogen (N) mineralization and nitrification in deep soil layers are rare because N processes are considered to occur mainly in topsoil that hosts active and diverse microbial communities. This study aimed to measure the soil potential net N mineralization (PNM) and nitrification (PNN) down to 4 m depth and to discuss factors controlling their variability. Twenty-one soil cores were collected at the Restinclieres agroforestry experimental site, where 14-year-old hybrid walnut trees were intercropped with durum wheat. Soil cores were incubated in the dark in the laboratory at both 6 and 25 degrees C. The soil was a deep calcic fluvisol with a fluctuating water table. It featured a black layer that was very rich in organic matter and permanently water saturated at depths between 3.0 and 4.0 m. The mean soil mineral N content was 3 mg N kg(-1) soil in the upper 0.0-0.2 m layer, decreasing until a depth of 2 m and increasing to the maximum value of 25.8 mg N kg(-1) soil in the black layer. While nitrate (NO3-) was the dominant form of mineral N (89%) in the upper 0.0-0.2 m layer, its proportion progressively decreased with depth until ammonium (NH4+) became almost the only form of mineral N (97%) in the saturated black layer. Laboratory soil incubation revealed that PNM and PNN occurred at all depths, although the latter remained low at 6 degrees C. The soil nitrate content in the black layer was multiplied by 48 times after 51 days of incubation at 25 degrees C, whereas it was almost inexistent at the sampling date. While the soil total N, the pH and the incubation temperature explained 84% of the variation in PNM, only 29% of the percent nitrification variance was explained by the incubation temperature (T-inc) and the soil C-to-N ratio. These results point out the necessity to consider soil potential net N mineralization and nitrification of deep soil layers to improve model predictions.

Automated summary

This study aimed to measure the soil potential net nitrogen mineralization and nitrification in deep soil layers and discuss factors controlling their variability. The results showed that both mineralization and nitrification occurred at all depths, with a significant increase in nitrate content in the black layer after 51 days of incubation at 25 degrees C. These findings highlight the importance of considering potential net nitrogen mineralization and nitrification in deep soil layers for improving model predictions.