Data from: Nutrient enrichment shifts peak water-use efficiency to wetter conditions in an alpine meadow

Ecosystem water-use efficiency, the ratio of gross ecosystem productivity
to plant transpiration, is found to show a hump-shaped response to extreme
water variations (with a peak at moderate water level). This peak
water-use efficiency represents maximum carbon gain per unit of water
consumed, with its associated water threshold indicating the optimal water
level to maximize carbon uptake. However, it remains unclear whether
widespread nutrient enrichment alters this peak water-use efficiency and
its associated water threshold. Here, we conducted a 6-year field
experiment involving precipitation reduction and multiple nutrient
additions in an alpine meadow on the Qinghai-Tibet Plateau. A wide water
gradient is created by combining experimental precipitation manipulation
and interannual precipitation fluctuations. Our results showed a
consistent hump-shaped response of ecosystem water-use efficiency to water
variations in control, N, NP, and NPK treatments. While nutrient addition
had little effect on peak water-use efficiency, it significantly increased
the associated water threshold by 16.9-25.2% (from 540 mm to 631-676 mm).
This increase was primarily caused by a U-shaped transpiration response,
but not by the positive responses of gross ecosystem productivity to
precipitation across all treatments. Specifically, nutrient addition
elevated the transpiration water threshold (the water level for minimum
transpiration) by 10.1-16.0% (from 406 mm to 447-471 mm). This U-shaped
transpiration response to precipitation was mainly regulated by soil
temperature in control plots, but by vertical root distribution in
nutrient treatments. Synthesis. These new findings show that nutrient
enrichment does not alter peak water-use efficiency but elevates its
associated water threshold. This pattern indicates that nutrient-enriched
ecosystems require more water than nutrient-poor ones to reach maximum
carbon uptake, with important implications for predicting the dynamics of
grassland carbon sink under the combined effects of soil water and
nutrients.