The optimal Redfield N: P ratio caused by fairy ring fungi stimulates plant productivity in the temperate steppe of China

The Redfield ratio (or stoichiometry) is widely used as a reference to differentiate between nitrogen (N) and phosphorus (P) limitations in plankton and in the deep oceanic waters. Whether the Redfield ratio is applicable to plant and soil fungal growth is rarely considered. Fairy rings (FRs) caused by Agaricus gennadii can influence plant productivity and soil nutrient composition. However, the internal mechanisms associated with the optimal Redfield N: P ratio remain unclear. Changes in plant and soil stoichiometry in three concentric zones of FRs occurring in the temperate steppe of China, namely outside the ring (Out), on the ring (On), and inside the ring (In), were assessed in order to confirm the effects of FR fungi on plants and soil and explain the mechanisms using nutrient stoichiometry. We observed a significantly higher aboveground biomass only in the On zone. The plant N: P ratio was 14.5, 10.4, and 7.3 in the Out, On, and In zones, respectively. This indicates that the limiting element of the aboveground biomass was P in the Out zone and N in the In zone, while the On zone possessed an optimal plant N: P ratio and relatively high aboveground biomass. As with the plant N: P ratio, the On and In zones displayed a significantly lower soil inorganic N: available P ratio than the Out zone. The plant N: P ratio was highly positively correlated with the soil inorganic N: available P ratio. Our results suggest that FR fungi with low N: P ratio shift the soil ecosystem from P- to N-limited, thereby accelerating the uptake of soil N by plants. As with the plant N: P ratio, the soil inorganic N: available P ratio can also be used to evaluate whether N or P is more limiting for plant production in grassland ecosystems.