Nitrogen addition suppresses microbial respiration but enhances the release of water-extractable organic carbon in a non-allophanic Andosol

Soil carbon (C) stocks are suggested to be declining due to agricultural land use; however, the factors driving the decline remain unclear. Mineralization of soil organic matter (SOM), regulated by inorganic nitrogen (N), is believed to play a key role in this process. This study investigated the effects of inorganic N (ammonium nitrate, NH₄NO₃) on SOM decomposition, with a focus on conditions where labile C was added. The present study used a soil sample from the surface horizon of a non-allophanic Andosol, which was rich in stable C but poor in available N because the soil has not been fertilized. After 60 days of incubation at 25°C and 60% of water holding capacity, it was found that the addition of 38.5, 116, and 347 mg kg⁻¹ of N inhibited the microbial respiration by 7%, 23%, and 31%, respectively. Path analysis revealed that the inhibition was caused by N directly, not through soil pH changes, and the addition of 1600 mg kg⁻¹ of d-glucose did not change the suppressive trend by the N addition. The N addition increased the amounts of water-extracted organic C by 43% to 84% (p < 0.05) and thermodynamically reactive SOC by 5% to 7% (p < 0.05), and the increases were ascribed to the reduced soil pH caused by the inorganic N addition together with the direct effect of N addition. This was coupled with the release of iron, aluminum, and silicon from the soil (p < 0.05). The present study indicated that the N addition destabilizes SOM, but it does not always accelerate the decomposition of the SOM. In other words, the addition of exogenous N does not directly reduce the C content in non-allophanic Andosols, but it can indirectly decrease the stability of organic C, increasing the risk of C loss by leaching.