On the origin of carbon dioxide released from rewetted soils

When dry soils are rewetted a pulse of CO2 is invariably released, and whilst this phenomenon has been studied for decades, the precise origins of this CO2 remain obscure. We postulate that it could be of chemical (i.e. via abiotic pathways), biochemical (via free enzymes) or biological (via intact cells) origin. To elucidate the relative contributions of the pathways, dry soils were either sterilised (double autoclaving) or treated with solutions of inhibitors (15% trichloroacetic acid or 1% silver nitrate) targeting the different modes. The rapidity of CO2 release from the soils after the drying:rewetting (DRW) cycle was remarkable, with maximal rates of evolution within 6 min, and 41% of the total efflux over 96 h released within the first 24 h. The complete cessation of CO2 eflux following sterilisation showed there was no abiotic (dissolution of carbonates) contribution to the CO2 release on rewetting, and clear evidence for an organismal or biochemical basis to the flush. Rehydration in the presence of inhibitors indicated that there were approximately equal contributions from biochemical (outside membranes) and organismal (inside membranes) sources within the first 24 h after rewetting. This suggests that some of the flux was derived from microbial respiration, whilst the remainder was a consequence of enzyme activity, possibly through remnant respiratory pathways in the debris of dead cells

Distinct respiratory responses of soils to complex organic substrate are governed predominantly by soil architecture and its microbial community

Factors governing the turnover of organic matter (OM) added to soils, including substrate quality, climate, environment and biology, are well known, but their relative importance has been difficult to ascertain due to the interconnected nature of the soil system. This has made their inclusion in mechanistic models of OM turnover or nutrient cycling difficult despite the potential power of these models to unravel complex interactions. Using high temporal-resolution respirometery (6 min measurement intervals), we monitored the respiratory response of 67 soils sampled from across England and Wales over a 5 day period following the addition of a complex organic substrate (green barley powder). Four respiratory response archetypes were observed, characterised by different rates of respiration as well as different time-dependent patterns. We also found that it was possible to predict, with 95% accuracy, which type of respiratory behaviour a soil would exhibit based on certain physical and chemical soil properties combined with the size and phenotypic structure of the microbial community. Bulk density, microbial biomass carbon, water holding capacity and microbial community phenotype were identified as the four most important factors in predicting the soils’ respiratory responses using a Bayesian belief network. These results show that the size and constitution of the microbial community are as important as physico-chemical properties of a soil in governing the respiratory response to OM addition. Such a combination suggests that the 'architecture' of the soil, i.e. the integration of the spatial organisation of the environment and the interactions between the communities living and functioning within the pore networks, is fundamentally important in regulating such processes

Spatial variability in properties affecting organic horizon carbon storage in upland soils

Quantifying the amount and distribution of soil organic carbon (SOC) within natural soils is important for sample design, C budgeting/pool estimation, and understanding SOC turnover at a process level. We examined the distribution of SOC across a typical UK upland, moorland catchment to establish the amount and spatial structure of variability in key soil properties affecting SOC stocks, namely O horizon C content, bulk density (DB) and horizon depth. Organic horizons of Histosols and Gleysols had greater SOC contents but smaller bulk densities than Podzols and Leptosols. Consequently, SOC density differences between soils were minimized and horizon depth variation became crucial to the measurement of SOC stocks. However, individual Podzol profiles stored appreciable amounts of SOC in O horizons (up to 50 kg m–2). Geostatistical analyses showed spatially structured variance in many properties relating to SOC storage at both plot (variograms reaching sills at ranges 3–8 m) and catchment scales (ranges 437–529 m). The increase in variance from plot to catchment scales was large for O horizon depth. However, DB showed complex scale and soil type inter-relationships, with similar variance at different scales. We show that detailed soil investigations spanning multiple spatial scales are necessary to quantify soil C storage properties for purposes of hydro-ecological modeling and C budgeting at small catchment scales. This has implications for upscaling to regional or national soil C databases

A novel three-dimensional lattice Boltzmann model for solute transport in variably saturated porous media

This paper presents a lattice Boltzmann method for the advection and dispersion of solute in three-dimensional variably saturated porous media. The proposed method is based on the BGK model and discretizes the particle velocity space with a cuboid lattice in which the particles move in 19 directions and 7 speeds. In the proposed model a directionally dependent relaxation time is introduced to produce the second-order dispersion tensor, and a modification of the equilibrium distribution functions is given to model the solute in porous media where the volumetric water content varies over space and time. The concentration is calculated from a weighted summation of the particle distribution functions to ensure that the BGK collision does not result in mass change. The accuracy and consistency of the proposed model are verified against benchmark problems and the finite difference solution of solute transport in an unsaturated heterogeneous soil. The results show that the proposed model conserves mass perfectly and gives efficient and accurate solutions for both advection-dominated and dispersion-dominated problems