Lithology controls of soil N availability and plant N acquisition strategies in subtropic forests

Soil nitrogen (N) availability and plant N uptake strategies are central to species adaptation and community assembly, yet how lithology shapes soil N supply and plant N uptake remains poorly understood. Here, we used ¹⁵N labeling to compare soil N supply and plant N uptake preferences in forests on soils developed from limestone and clastic rocks, respectively. Our results revealed significant contrasts in inorganic N pools: nitrate (NO₃^–) dominated in limestone soils, while ammonium (NH₄⁺) was more abundant in red soils developed from clastic rocks. We attributed these differences to different soil N transformation pathways. Increased nitrification rates in limestone soils increased NO₃^– content, whereas red soils exhibited high mineralization but lower nitrification rates, leading to NH₄⁺ dominating inorganic N. Forest plants in both limestone and red soils preferentially utilized NH₄⁺ as their primary N source. However, plant in limestone soils took up significantly higher proportions of NO₃^– and glycine. Moreover, total N uptake rates by plants were significantly larger in limestone than red soils, suggesting a more efficient N acquisition strategy. Structural equation modeling indicated that lithology significantly affected soil N mineralization and nitrification by regulating soil pH and total N, thereby driving differences in soil inorganic N pools and ultimately plant N uptake. Our results provide evidence that lithology-driven variations in soil N supply can strongly affect plant N acquisition strategies.