Implications of carbon saturation model structures for simulated nitrogen mineralization dynamics

Abstract

Abstract. Carbon (C) saturation theory suggests that soils have a limited capacity to stabilize organic C and that this capacity may be regulated by intrinsic soil properties such as clay concentration and mineralogy. While C saturation theory has advanced our ability to predict soil C stabilization, few biogeochemical ecosystem models have incorporated C saturation mechanisms. In biogeochemical models, C and nitrogen (N) cycling are tightly coupled, with C decomposition and respiration driving N mineralization. Thus, changing model structures from non-saturation to C saturation dynamics can change simulated N dynamics. In this study, we used C saturation models from the literature and of our own design to compare how different methods of modeling C saturation affected simulated N mineralization dynamics. Specifically, we tested (i) how modeling C saturation by regulating either the transfer efficiency (ε, g C retained g −1 C respired) or transfer rate (k) of C to stabilized pools affected N mineralization dynamics, (ii) how inclusion of an explicit microbial pool through which C and N must pass affected N mineralization dynamics, and (iii) whether using ε to implement C saturation in a model results in soil texture controls on N mineralization that are similar to those currently included in widely used non-saturating C and N models. Models were parameterized so that they rendered the same C balance. We found that when C saturation is modeled using ε, the critical C : N ratio for N mineralization from decomposing plant residues (r cr ) increases as C saturation of a soil increases. When C saturation is modeled using k, however, r cr is not affected by the C saturation of a soil. Inclusion of an explicit microbial pool in the model structure was necessary to capture short-term N immobilization-mineralization turnover dynamics during decomposition of low N residues. Finally, modeling C saturation by regulating ε led to similar soil texture controls on N mineralization as a widely used non-saturating model, suggesting that C saturation may be a fundamental mechanism that can explain N mineralization patterns across soil texture gradients. These findings indicate that a coupled C and N model that includes saturation can (1) represent short-term N mineralization by including a microbial pool and (2) express the effects of texture on N turnover as an emergent property.

Legacy effects of long-term nitrogen fertilizer application on the fate of nitrogen fertilizer inputs in continuous maize

“…At physiological maturity, we also observed a decrease in NR μSilt+Clay and NR soil (0-15 cm) with increasing historical N rate. These patterns agree with previous research showing that a decreasing proportion of added N is stored in stable pools as the concentration of mineral-associated or total SOC increases (Castellano et al, 2011;Poirier et al, 2014;White et al, 2014). In plots with historically low N inputs, mineral-associated SOC (particularly μSilt + Clay-C) was lower and thus there was a greater capacity to stabilize new N in organic compounds chemically bound to silt and clay surfaces.…”

A Model–Data Fusion Approach for Predicting Cover Crop Nitrogen Supply to Corn

“…Second, the parameter e does not strictly represent microbial C use efficiency (Sinsabaugh et al, 2013), which is the fraction of C transferred to the microbial pool that is assimilated into the microbial biomass as opposed to respired as CO 2 in one cycle of microbes feeding on an organic substrate. Instead, it is a C humification coefficient (Kemanian et al, 2011;White et al, 2014), which is the proportion of cover crop C transferred to the microbial pool that remains assimilated in the microbial biomass (as opposed to respired) within a given time frame of interest, potentially spanning multiple cycles of microbial feeding and predation. In this study, the time frame of interest is from cover crop termination (either frost killing in winter or managed termination in spring) to the cessation of N uptake by the following corn crop (physiological maturity).…”

“…A longer decomposition period with environmental conditions conducive to microbial respiration, which occurs for winterkilled cover crops relative to winter-hardy cover crops in the mid-Atlantic region, may result in greater C respiration and a lower e. Therefore, the critical C/N ratio may be higher for winterkilled than winter-hardy cover crops. Many other ecosystem properties may play roles in regulating e, including soil texture, microbial community composition and its physiological response to environmental changes, and N availability in the soil and plant litter (Schimel and Weintraub, 2003;Manzoni et al, 2008;Sinsabaugh et al, 2013;White et al, 2014). We did not add additional controls on e to the model proposed here because of relatively homogenous site conditions across the calibration data set and to preserve a model structure that is simple enough to be calibrated with easily collected field data.…”

Reviews and Syntheses: Ironing Out Wrinkles in the Soil Phosphorus Cycling Paradigm

“…Knowledge of C:N ratios has enabled accurate modeling of N limitation and mineralization-immobilization dynamics (White et al, 2014). This modeling success can be attributed to the tight coupling of C and N in soils across ecosystems, which is largely because organic N makes up 95% of soil N (Duxbury et al, 1989;Kirkby et al, 2011;Yang and Post, 2011).…”

“…1) (Hassink and Whitmore, 1997). The saturation of OC has implications for the storage of C, decomposition, and the mineralization of nutrients (Castellano et al, 2012;Kemanian et al, 2011;Kemanian and Stöckle, 2010;White et al, 2014).…”