Fungal denitrification dominates soil N2O emissions after vegetation restoration in the Karst region

Fungal denitrification play a key role in the regulation of nitrogen cycle. Knowledge of the fungal denitrification of soil microbes and the driving mechanisms will be essential to understand the impact of soil N₂O emissions in the karst region. This study demonstrates that soil fungal nirK-derived N₂O in the karst region are significantly distinct between cropland and forest, soil NO₃^‒-N and sand are important drivers of the soil N₂O emissions variation, especially after vegetation restoration. The researchers’ finding appeared September 9, 2025 in Soil Ecology Letters.

A series of studies on the carbon and nitrogen cycling process in subtropical forest have been conducted by Huifang Xuʼs and Dejun Liʼs team at the Institute of Subtropical Agriculture, Chinese Academy of Sciences, Changsha, and many interesting findings have been obtained. For example, for the first time they found that the important role of comammox in regulating ammonia oxidation in karst forest soils. However, the relative contribution of fungal denitrification to soil N₂O emissions in the karst region is not well understood.

Professor Xu said, “We chose to study the Wuming County and Dushan County located in Guangxi Province and Guizhou Province, which was characterized by continuous karst formation. Unlike the previous studies in the other ecosystem (e.g., upland and paddy soils), the karst region has resulted in unique soil properties such as high Ca content. Therefore, the karst region provides a unique and meaningful platform for understanding the mechanisms of N2O emissions from fungal denitrification.”

In this study, they found that after vegetation restoration, the abundance of nirK-containing denitrifying fungi (7.72 ± 1.82 × 10⁹ copies g^‒1) was nearly threefold higher than in cropland (2.61 ± 0.29 × 10⁹ copies g^‒1). This finding is closely related to soil pH, as the higher pH of the forest soil compared to the acidic environment of the cropland likely reduced the toxicity of labile aluminum (Al³⁺). In addition, they found that the abundance of fungal nirK was far greater than that of p450nor in forest. The reason for this result is that nirK is a copper-dependent enzyme, and the local micro-acidic environment caused by the decomposition of litter is conducive to the release of Cu²⁺.

“We also used the substrate-induced inhibition method to examine the relative contributions of fungi and bacteria to N₂O emissions in the karst region,” said Professor Xu. Compared to cropland, the relative contribution of fungi to soil N₂O emissions after vegetation restoration was significantly higher. In this study, the higher sand content in the forest soil creates more macropores, these hyphae can navigate through air-filled pores while their branches penetrate anoxic soil microaggregates to perform denitrification. This unique ‘cross-scale’ survival strategy may give fungi a competitive advantage over bacteria in coarse-textured soils, allowing them to dominate N₂O production.

“The study is an extension of our understanding of soil microbes during vegetation restoration, increasing our understanding of the characteristics of fungal denitrification influencing soil N₂O emissions in the context of karst region.”

N₂O is a potent greenhouse gas; it not only depletes stratospheric ozone levels but also significantly contributes to global warming. Monitering and controlling soil N₂O emissions is more than ever our current missions.