Nutrient Acquisition from Soils with Patchy Nutrient Distributions: Importance of Patch Size, Degree of Variability and Root Uptake Kinetics

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Effectiveness of nitrate (NO3-) and phosphate (P) uptake in a patchy nutrient environment by tussocks of Agropyron desertorum (crested wheatgrass) was assessed with simulations using existing nutrient-uptake models parameterized with field data. Simulations conducted with Monte Carlo techniques were used to expose roots to different levels of nutrient variability with numbers 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 uptake of both NO3- and P by established or growing roots was highest in soils with the nutrients distributed uniformly but declined with increasing soil nutrient variability for both nutrients. The decline was greater for NO3- than for P and was greater for both ions in soils with nutrients distributed in the 10 large patches. The reduction in predicted nutrient uptake with increasing variability was more pronounced with higher mean concentrations of P, while mean concentration had little effect on the pattern of NO3- uptake. However, allowing increased uptake kinetics with increasing nutrient concentration, as measured earlier for Agropyron desertorum, indicated that changes in kinetics could offset the lower whole-plant uptake rates in soils 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 patchy nutrient distributions than was nutrient uptake in moister soil. Benefits associated with enhanced uptake kinetics with patchy nu- trients distributions were reduced in drier soils for NO3-, but were similar for P. Variability in soil moisture had little effect on patterns of P and N03- uptake. Differences in distri- butions of distances between fine roots had little influence on the patterns of NO3- and P uptake.

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