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Abstract

Combining nutrient dynamics (plant nutrient uptake and soil fertility) can help uncover
mechanisms of shrub–grass interactions and assess the validity of the stressgradient
hypothesis, which predicts that facilitation between plants increases in
stressful environments. However, how facilitation via shrub-mediated nutrient increases
varies with precipitation is poorly resolved.
2. We first synthesized a global dataset from 66 studies and evaluated how shrubs affected
soil organic carbon (C) and nitrogen (N) in grasslands along a precipitation gradient.
We then made new measurements in a single-grassland type encroached by
leguminous shrubs from the same genus (Caragana) to constrain the variations caused
by combining different grassland types and shrubs traits in the meta-analysis. Specifically,
we investigated how shrubs mediated N dynamics and how shrub–grass interactions
varied along a precipitation gradient (147–342 mm) in a temperate steppe of China.
3. At the global scale, leguminous shrub-mediated effects on soil nutrients increased
with precipitation, while no relationship was found for non-leguminous shrub. For
the field experiment, greater N and lower δ15N in Caragana compared to non-leguminous
shrub (reference shrub, Salsola collina) suggested active N-fixation in
Caragana. We found that Caragana enhanced N concentration and leaf quality (low
C:N ratio) in neighbouring plants more on mesic sites than on xeric sites. Thus, facilitation
increased via higher soil N and with decreasing environmental stress, at
least along this relatively arid precipitation gradient.
4. Our results highlight the importance of precipitation in determining the nutritional
facilitation to neighbouring grasses from encroaching leguminous shrubs.
Conceptual frameworks for plant facilitation may therefore need to include shrub
characteristics (N-fixers vs. non-fixers) and positive effects of higher precipitation
on this type of facilitation to characterize plant interactions along stress gradients.