The role of nutrient, light, and soil-moisture limitation on root allocation across global grasslands

Background/Question/Methods

Optimal allocation theory predicts that plant allocation to roots should vary with resource availability; proportional root allocation (PRA) should decline when plant growth is limited by above-ground resources (e.g. light), and PRA should increase when plant growth is limited by below-ground resources (e.g. water and nutrients). To test this hypothesis we calculated PRA across 25 grassland sites within the Nutrient Network, each with experimental manipulations of soil nutrients (factorial N, P & K fertilization).  The sites span a range of mean annual precipitation (MAP, 330 mm/yr to 1898 mm/yr). We extracted roots from soil cores taken to a depth of 10 cm, estimated root production on a per m² basis, and compared these values to above-ground destructive biomass harvest values taken in the same location.  Measures of photosynthetically active radiation (PAR) at the soil surface during the time of peak biomass in each plot were used as a proxy for light limitation.  Data were analyzed with linear mixed models where PRA was predicted by N, P & K and their interactions, site was a random factor, and PAR or MAP were included as covariates.

Results/Conclusions

Regardless of soil nutrient availability, PRA declined with decreasing PAR (p<0.001, r²=0.40), consistent with our expectation of decreased root allocation under conditions of light limitation.  Further, we found that PRA declined with N (p<0.0001, X²=26.9) or P (p=0.006, X²=7.5) fertilization, suggesting reduced allocation to roots as nutrient limitation was alleviated. While PRA did not vary with MAP under ambient conditions, there was a highest order NxPxKxMAP interaction (p=0.003, X²=5.6) whereby root allocation was relatively unresponsive to nutrient fertilization at low MAP, but PRA declined at high MAP in multiple nutrient fertilization treatments. Taken ogether, our preliminary results suggest that in arid or semi-arid grasslands water limitation may constrain allocation responses to altered nutrients, particularly where light is not a limiting factor for plant growth.

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