Integrating resource heterogeneity and plant plasticity: modeling nitrate and phosphate uptake in a patchy soil environment
Authors: Jackson, RB, MM Caldwell
1 We used the Barber-Cushman model of nutrient uptake to simulate the importance of soil heterogeneity and root plasticity for nitrate (NO3-) and phosphate (P) uptake. Model inputs included root physiological parameters and soil characteristics obtained from five years of field studies in the sagebrush steppe. At an intensively sampled field site the average variation in soil P and NO3- around individual plants was 3-fold and 12-fold (3X and 12X, respectively), the range of soil variability used in our simulations.
2 In soil patches three-fold enriched in P (3X), simulated P uptake was 3 to 4 times greater than in soil of background P concentrations (1X). The importance of soil heterogeneity and root plasticity was even more pronounced for NO3-. In 12X soil patches, NO3- uptake was 7 to 20 times greater than at 1X, depending on simulation conditions. Plasticity (root proliferation and increased uptake kinetics) accounted for up to 75% of NO3- and over 50% of P acquired from enriched soil patches. Even without plasticity, nutrient uptake increased substantially in enriched patches because of higher soil-solution concentrations.
3 Using the same model we simulated P and NO3- uptake for an actual 0.25-m2 soil area in the field. Plant acquisition of P in this area was 28% higher with root plasticity than without, equally attributable to root proliferation and increased uptake kinetics. Plant NO3- uptake was 61% greater with plasticity, due almost exclusively to increased uptake capacity of roots.
4 We also simulated P and NO3- uptake in hypothetical soil arrays containing an equivalent quantity of nutrient distributed homogeneously or heterogeneously. A plant without plasticity always acquired less P or NO3- in the heterogeneous arrays than in the homogeneous arrays. With plasticity, it acquired more nutrients in 3 of 4 cases compared to the homogeneous "control".
5 We present these simulations as a way to integrate field experiments, generate and test hypotheses, and stimulate discussion. Given that heterogeneity is the norm rather than the extreme, our simulations highlight the importance of soil heterogeneity and root plasticity for both nutrient acquisition and plant competition in the field.