Multiple soil nutrient competition between plants, microbes, and mineral surfaces: model development, parameterization, and example applications in several tropical forests

Abstract

Abstract. Soil is a complex system where biotic (e.g., plant roots, micro-organisms) and abiotic (e.g., mineral surfaces) consumers compete for resources necessary for life (e.g., nitrogen, phosphorus). This competition is ecologically significant, since it regulates the dynamics of soil nutrients and controls aboveground plant productivity. Here we develop, calibrate and test a nutrient competition model that accounts for multiple soil nutrients interacting with multiple biotic and abiotic consumers. As applied here for tropical forests, the Nutrient COMpetition model (N-COM) includes three primary soil nutrients (NH ) and five potential competitors (plant roots, decomposing microbes, nitrifiers, denitrifiers and mineral surfaces). The competition is formulated with a quasi-steady-state chemical equilibrium approximation to account for substrate (multiple substrates share one consumer) and consumer (multiple consumers compete for one substrate) effects. N-COM successfully reproduced observed soil heterotrophic respiration, N 2 O emissions, free phosphorus, sorbed phosphorus and NH + 4 pools at a tropical forest site (Tapajos). The overall model uncertainty was moderately well constrained. Our sensitivity analysis revealed that soil nutrient competition was primarily regulated by consumer-substrate affinity rather than environmental factors such as soil temperature or soil moisture. Our results also imply that under strong nutrient limitation, relative competitiveness depends strongly on the competitor functional traits (affinity and nutrient carrier enzyme abundance). We then applied the N-COM model to analyze field nitrogen and phosphorus perturbation experiments in two tropical forest sites (in Hawaii and Puerto Rico) not used in model development or calibration. Under soil inorganic nitrogen and phosphorus elevated conditions, the model accurately replicated the experimentally observed competition among nutrient consumers. Although we used as many observations as we could obtain, more nutrient addition experiments in tropical systems would greatly benefit model testing and calibration. In summary, the N-COM model provides an ecologically consistent representation of nutrient competition appropriate for land BGC models integrated in Earth System Models.

Predicted Land Carbon Dynamics Are Strongly Dependent on the Numerical Coupling of Nitrogen Mobilizing and Immobilizing Processes: A Demonstration with the E3SM Land Model

“…For many models, even when nitrogen cycling is considered, their representations of coupled nitrogen and carbon cycles have been criticized for several deficiencies, including 1) poor mechanistic representation of plant versus microbe nitrogen competition (Zhu et al 2016a(Zhu et al ,b, 2017 and 2) the lack of representation of other essential processes that affect nitrogen dynamics (e.g., microbial population processes; Grant 2013). Most existing land biogeochemical models, from site level to ESM scale, perform poorly when confronted with observations from ecosystem perturbations that affect the nitrogen cycle (Bouskill et al 2014;De Kauwe et al 2017;Houlton et al 2015;Zaehle et al 2014).…”

“…One example process relates to how plants and microbes compete for nutrients in soil. Our recent survey (Zhu et al 2017) suggests that there are at least four different ways land biogeochemical models represent this process. In this study, however, we focus on uncertainty associated with issues in step 2, which relates to the fact that the same governing equations are often solved differently by different modelers.…”

SUPECA kinetics for scaling redox reactions in networks of mixed substrates and consumers and an example application to aerobic soil respiration

“…1 in Tang, 2015). This problem is particularly acute when consumer abundances are high with respect to their substrates, a situation that may occur in in vivo conditions (Sols and Marco, 1970;Schnell and Maini, 2000) or when consumers interact with mineral surfaces, such as microbial decomposition of soil organic matter or plant-microbial competition for soil nutrients (Schimel and Bennett, 2004;Vitousek et al, 2010;Resat et al, 2012;Tang and Riley, 2015;Zhu et al, 2016a). In the above, we also note that the substrates' mass balance constraints (Eqs.…”

“…ECA kinetics significantly improved the modeling of plant-microbial nutrient competition in the ACME land biogeochemical model (Zhu and Riley, 2015;Zhu et al, 2016aZhu et al, , b, 2017 and was recently cited as one of the most promising methods to improve representation of nutrient competition in ESMs (Achat et al, 2016;Niu et al, 2016). The ECA method also successfully explained why organomineral interactions can slow soil organic matter decomposition rates and how lignincellulose ratios (Melillo et al, 1989) can be stabilized during litter decomposition (Tang andRiley, 2013a, 2015).…”

“…A common feature that underlies these two proposed model structural improvements is substrate-consumer interactions, which affect microbial substrate decomposition (Grant et al, 1993;Tang and Riley, 2013a;Riley et al, 2014;Le Roux et al, 2016), mineral soil interactions with adsorptive substrates (Smith, 1979;Grant et al, 1993;Resat et al, 2012;Tang and Riley, 2015;Dwivedi et al, 2017), and plantmicrobe competition for nutrients (Grant, 2013;Zhu et al, 2016aZhu et al, , b, 2017. In soil, because there are many consumers competing for many substrates in different places at different times, soil biogeochemical models must be able to scale consistently across space, time, and processes.…”