Precipitation is the major determinant of plant productivity in many systems worldwide. Recent research suggests that the effect of precipitation on plant productivity can become non-linear under extremely dry or extremely wet condition. One explanation for this is that plants become nutrient-limited at high precipitation. To evaluate this, we crossed a manipulation of nutrient supply at two levels – NPK + micronutrients, control – with precipitation at three levels – extremely dry, average, extremely wet – in eight genotypes of the perennial grass Panicum virgatum. We predict that nutrient addition would increase productivity under wet conditions, when productivity may be nutrient-limited, but not under drier conditions. To further evaluate whether non-linear responses to precipitation are driven by shifts from water- to –nutrient-limited growth, we measured transcriptomic and compositional responses (LMA, leaf nutrient content). If productivity shifts from water to nutrient limitation as precipitation increases, we predict that transcriptomic and compositional responses will become discordant from whole-plant responses and that nutrient addition would alleviate nutrient limitation, leading to concordance between transcriptomic, compositional, and whole-plant responses.
Results/Conclusions
As predicted, nutrient addition increased productivity under extremely wet conditions, but failed to increase productivity under drier (extremely dry and average) conditions, indicating that productivity shifts from water to nutrient limitation. Supporting this interpretation, a previous precipitation-only manipulation using the same plants found that transcriptomic responses to precipitation differed from the response of plant productivity. Specifically, the transcriptome changed less between extremely dry and average conditions than between average and extremely wet conditions, while productivity increased more between extremely dry and average conditions than between average and extremely wet conditions. These prior transcriptomic and whole-plant results are consistent with a shift from water- to nutrient-limited growth as precipitation increases. The current study will further explore this by examining transcriptomic, compositional, and whole-plant responses to differences in nutrient and water availability.