| Soil resources are heterogeneously distributed in natural systems, both in space and time. Much research has centered on understanding how roots respond to this heterogeneity and on comparing these responses between species. However, the mechanisms regulating these responses, and thus, the potential for root responses to vary within and between species depending on environmental conditions, have been largely unaddressed. Based on optimal foraging models and research in crop systems, three experiments were conducted to test the overall hypothesis that plant growth and function is regulated so uptake of non-limiting resources is minimized causing multiple resources to be limiting. As a consequence, the magnitude of root foraging for a specific heterogeneously distributed resource should vary depending on demand for that resource. In one study, water and macronutrients were applied to the shrub Atriplex parryi in a factorial design. As predicted, plant growth and function (i.e. allocation, gas exchange, and water relations) was regulated so that multiple resources simultaneously limited growth. Although N is limiting in deserts, N capture depended on the availability of other soil resources. Based on these results and work in crop systems showing N capture is linked to growth rate, in another study, I applied 15N-labled pulses through the growing season to two desert shrubs and a grass and manipulated soil water and P availability. Water and P supply affected N capture from pulses, as predicted, but these effects differed among species. All study species, however, showed a large decline in ability to capture N from pulses as growth rates slowed, suggesting root responses to pulsed N supply are tightly linked to growth rate. Path analysis was then used to show that multiple root parameters (i.e. rooting density and N inflow rate per unit length) were affected by growth rate. These root responses, in combination with changes in soil water content and microbial nitrification, affected plant N capture from pulses. Together, these data suggest root foraging ability for a heterogeneously distributed resource is not a fixed ecological characteristic, as is commonly assumed by theory and empirical studies, but instead depends on plant demand for that resource. |
