Nutrient acquisition from soils with patchy nutrient distributions as assessed with simulation models

Effectiveness of nitrate (NO3-) and phosphate (P) uptake in a patchy nutrie nt environment by tussocks of Agropyron desertorum (crested wheatgrass) was assessed with simulations using existing nutrient-uptake models parameteri zed with field data. Simulations conducted with Monte Carlo techniques were used to expose roots to different levels of nutrient variability with numb ers of patches set at 10 large or 1000 small patches over the rooting zone of a single tussock. With constant uptake kinetics, simulated whole-plant u ptake of both NO3- and P by established or growing roots was highest in soi ls with the nutrients distributed uniformly but declined with increasing so il nutrient variability for both nutrients. The decline was greater for NO3 - than for P and was greater for both ions in soils with nutrients distribu ted in the 10 large patches. The reduction in predicted nutrient uptake wit h increasing variability was more pronounced with higher mean concentration s of P, while mean concentration had little effect on the pattern of NO3- u ptake. However, allowing increased uptake kinetics with increasing nutrient concentration, as measured earlier for Agropyron desertorum, indicated tha t changes in kinetics could offset the lower whole-plant uptake rates in so ils with patchy nutrient distributions. The effects of soil moisture and root distribution on whole-plant uptake of NO3- and P were also simulated. With constant kinetics, nutrient uptake in drier soil with uniformly distributed soil moisture was less affected by p atchy nutrient distributions than was nutrient uptake in moister soil. Bene fits associated with enhanced uptake kinetics with patchy nutrients distrib utions were reduced in drier soils for NO3-, but were similar for P. Variab ility in soil moisture had little effect on patterns of P and NO3- uptake. Differences in distributions of distances between fine roots had little inf luence on the patterns of NO3- and P uptake.