Low-intensity fire stimulates soil inorganic N supply and adjusts plant N utilization strategy to alleviate plant N limitation in rocky desertification area

Although wildfires bring serious negative environmental and ecological effects, low-intensity fires can promote vegetation recovery to a certain extent, especially in degraded ecosystems. A deeper understanding of the mechanism underlying accelerated vegetation recovery following fire will help provide a reference for the government to formulate ecological restoration strategies and enhance ecological service functions. Low soil nitrogen (N) availability is considered to be a key nutrient factor limiting vegetation recovery. Wildfire may change the coupling relationship between soil N supply and plant N demand to affect vegetation restoration, but little is known about this. We selected the succession sequences of different vegetation recovery stages in low-intensity burned and unburned areas in the karst desertification region of southwest China. We found that low-intensity fire indeed accelerated vegetation recovery, supported by higher plant biomass and diversity in burned than unburned areas. The data of plant leaf N/phosphorus ratio, total N content and δ¹⁵N value collectively indicated that plant growth in degraded ecosystems was severely limited by N, while plant N limitation degree decreased significantly following fire. This difference can be explained by the changes in the composition and content of soil N forms and N transformation processes that control their production. Compared to natural vegetation restoration, low-intensity fire significantly increased external N inputs and soil inorganic N supply capacity, primarily by stimulating free-living N₂ fixation, organic N mineralization, and autotrophic nitrification rates, more pronounced at the early stage of vegetation restoration. These changes were attributed to improved soil conditions, including increased pH, organic matter content, microbial abundances and macroaggregate following low-intensity fire, all of which facilitated inorganic N production. In addition, plant increased the preferential utilization of nitrate following fire. These results suggest that increased soil inorganic N supply and the adjust in plant N utilization strategy after fire reduce plant N limitation, thereby accelerating plant growth and vegetation recovery in degraded ecological areas.

 

Keywords: Degraded ecosystem; Low-intensity fire; Plant N limitation; Plant N utilization strategy; Soil inorganic N supply