Nitrogen acquisition strategies during the winter-spring transitional period are divergent at the species level yet convergent at the ecosystem level in temperate grasslands

Nitrogen (N) is a major limiting element for productivity in temperate grasslands, particularly during early spring when soil N availability is low and the vegetative demand for it is high. Therefore, knowing whether and how plant species adopt different N acquisition strategies during the winter-spring transitional period is essential for understanding ecosystem functioning in temperate grasslands. In this study, parallel experiments with 15N tracer were conducted to examine plant N acquisition strategies during winter-spring transition in a meadow and a typical steppe in northern China. We found that soil microbes immobilized ∼20% of the 15N tracer during the spring thawing period at both sites, and then released half of it back to the soil before late spring, confirming that soil microbes competed effectively with the plant roots for mineral N in early spring. Perennial bunch grasses adopted an active N acquisition strategy at the beginning of the spring thawing period. In contrast, perennial forbs and rhizome grasses began to take up N in the middle of the spring thawing period, and they acquired more N than the bunch grasses. However, sagebrushes and legumes accounted for little 15N recovery, indicating their dependence on internal N accumulation or N fixation. At the ecosystem level, no significant difference in the magnitude of plant 15N uptake was observed between the meadow steppe and typical steppe, although the plant biomass N in the meadow steppe was twice that of the typical steppe during the thawing period. This was attributed to the higher soil inorganic N and faster net N mineralization rate in the meadow steppe than in the typical steppe. Our results suggest that temporal niche differentiation in N acquisition during early spring may facilitate species coexistence in temperate grasslands despite strong plant-microbe or plant-plant competition for N. The divergent N acquisition strategies at the species level and convergent N acquisition strategies at the ecosystem level should be considered for model development to better simulate vegetation growth particularly under spring N stress.