Modelling the physical states, element stoichiometries and residence times of topsoil organic matter

Abstract

Soil organic matter (SOM) is a major ecosystem component, central to soil fertility, carbon balance and other soil functions. To advance SOM modelling, we devised a steady‐state model of topsoil SOM, with explicit descriptions of physical states and properties, and used it to simulate SOM concentration, carbon:nitrogen:phosphorus (C:N:P) stoichiometry, bulk density and radiocarbon content. The model classifies SOM by element stoichiometry (αSOM is poor in N and P, βSOM is rich), mean residence times (1–2000 years) and physical state (free, occluded, adsorbed, hypoxic). The most stable SOM is either βSOM preferentially adsorbed by mineral matter, or αSOM in strongly hypoxic zones. Soil properties were simulated for random combinations of plant litter input (amount and C:N:P stoichiometry), mineral sorption capacity, propensity for hypoxia, and bulk density of non‐adsorbed αSOM. To optimize model parameters, outputs from 5000 simulations were used to construct bivariate relations among soil variables, which were compared with those found in data for 835 survey sites, covering all common land uses. The bivariate relations, and patterns of data scatter, were reproduced, and also variations in soil radiocarbon with soil type, suggesting that apparent scatter in measured data might reflect SOM diversity. The temporal acquisition by soil of ‘bomb 14C’ could also be simulated. The steady‐state model is the basis for a dynamic version, suitable for simulating changes in SOM through time. It provides insight into the possible manipulation of soil organic carbon (SOC) sequestration; for example, increasing litter inputs might only increase moderately‐stable SOC pools, whereas encouraging the creation of βSOM by adsorption to mineral matter from deeper soil could lead to long‐term stabilization. Highlights Models of SOM should include explicit descriptions of physical states and properties. Our new topsoil SOM model is constrained by C:N:P stoichiometry, SO14C, and physical fractionation data. Simulated soil properties, randomly generated, account for measured trends and patterns of scatter in SOM data. SOM properties depend upon litter input, interactions with mineral matter, hypoxia, and bulk density.

Radiocarbon analysis reveals underestimation of soil organic carbon persistence in new-generation soil models

“…14 C is therefore a powerful tool to study the exchanges and storage of carbon from decadal to millennial time scales. However, new-generation models do not generally implement 14 C simulations, and only a handful have systematically assimilated observed 14 C data (e.g., Tipping & Rowe, 2019;Braakhekke et al, 2014;Ahrens et al, 2020).…”

Reusing Fe water treatment residual as a soil amendment to improve physical function and flood resilience

“…The water-holding capacity and structural stability of a soil are not just dependent on organic matter, but also on particle size distribution and soil structure, which is largely governed by the inorganic mineral component of the soil. In addition, there is grow-H. C. Kerr et al: Reusing Fe water treatment residual as a soil amendment ing evidence that it is inorganic minerals which present the best opportunity to stabilize soil structure (Peng et al, 2007), stabilize carbon, and help us achieve net zero (Tipping and Rowe, 2019).…”

“…Al/Fe salts and organic matter are able to beneficially modify soil characteristics due to their effect on aggregation and cementation between soil particles, effect on bulk density (due to low density of organic matter), and water retention characteristics. In particular, Fe oxides are known to be good at stabilizing carbon in marine systems (Lalonde et al, 2012), and thus their potential role in carbon stabilization and soil structure is also very interesting (Tipping and Rowe, 2019). As such, we present an investigation on the physical properties (water retention properties, hydraulic conductivity, and shear strength) of a sandy loam soil amended with both Fe WTR and a Fe WTR / compost mix.…”

Relationships between riverine and terrestrial dissolved organic carbon: Concentration, radiocarbon signature, specific UV absorbance

“…Available evidence suggests that most terrestrial DOC is produced from litter and topsoil organic matter, as part of the turnover of soil organic carbon (Qualls and Haines, 1992;Neff and Asner, 2001;O'Donnell et al, 2010;Tipping and Rowe, 2019;Pschenyckyj et al, 2020), and may also come from root exudates. Its soil concentrations are controlled largely by mineralisation, adsorption and export in outflow.…”