Nitrate Immobilization in Anaerobic Forest Soils along a North–South Transect in East China

Abstract

It has been reported that 15N‐NO3− added to soils cannot be completely recovered in the remaining NO3− and its measurable products under anaerobic conditions. Abiotic immobilization of NO3− into dissolved organic N (DON) has been hypothesized to explain this phenomenon. Six forest soil samples were collected from temperate, subtropical, and tropical regions in East China, amended with K15NO3, and incubated at 25°C under anaerobic conditions for 264 h. The results showed that only 36 to 56% of the added N was recovered in the temperate soils, while up to 90% of the added N was recovered in the subtropical and tropical soils 1.5 h after incubation commenced. Compared with the initial concentration, the DON concentration dramatically increased when measured in temperate soils at 1.5 h, but increased only slightly in the subtropical and tropical soils. In all the soils, there was a strong positive correlation between recovery of added N at 1.5 h and soil redox potential (Eh) (R2 = 0.87, P < 0.01), and a strong negative correlation between added N recovery at 1.5 h and dissolved organic C (measured at the beginning of incubation) (R2 = 0.72, P < 0.05). There was a significant decrease in recovery of added N along with an increase in DON as soil Eh fell in the subtropical and tropical soils during long‐term incubation, suggesting that a majority of unrecovered N is incorporated into dissolved organic matter. All these results indicate added N was rapidly converted to DON via abiotic immobilization and was an important component of the N cycle in temperate forest soils but not in subtropical and tropical soils with high soil Eh, which was the controlling factor affecting this process.