Threshold dynamics in soil carbon storage for bioenergy crops

A Novel and Non-Invasive Approach to Evaluating Soil Moisture without Soil Disturbances: Contactless Ultrasonic System

Soil moisture has been considered a key variable in governing the terrestrial ecosystem. However, it is challenging to preserve indigenous soil characteristics using conventional soil moisture monitoring methods that require maximum soil contacts. To overcome this issue, we developed a non-destructive method of evaluating soil moisture using a contactless ultrasonic system. This system was designed to measure leaky Rayleigh waves at the air-soil joint-half space. The influences of soil moisture on leaky Rayleigh waves were explored under sand, silt, and clay in a controlled experimental design. Our results showed that there were strong relationships between the energy and amplitude of leaky Rayleigh waves and soil moisture for all three soil cases. These results can be explained by reduced soil strengths during evaporation processes for coarse soil particles as opposed to fine soil particles. To evaluate soil moisture based on the dynamic parameters and wave properties obtained from the observed leaky Rayleigh waves, we used the random forest model. The accuracy of predicted soil moisture was exceptional for test data sets under all soil types (R2 ≥ 0.98, RMSE ≤ 0.0089 m3 m-3). That is, our study demonstrated that the leaky Rayleigh waves had great potential to continuously assess soil moisture variations without soil disturbances.

Molecular properties of the Humeome of two calcareous grassland soils as revealed by GC/qTOF-MS and NMR spectroscopy

A Humeomic fractionation revealed the humus molecular composition of two uncropped calcareous soils of Northern France and differentiated the soils Humeome by extracting humic components first unbound to the organo-mineral matrix and then liberated from their progressively stronger intermolecular and intramolecular ester and ether linkages. We separated organo- (ORG1-3) and water-soluble (AQU2 and AQU4) fractions, a final extractable fraction (RESOM) and soil residues. Organo-soluble fractions were studied by GC coupled with high-resolution mass spectrometry (GC/qTOF-MS), all fractions underwent mono- and two-dimensional liquid-state NMR (except for the iron-rich AQU4 fraction), while solid-state ¹³C-CPMAS-NMR spectroscopy analyzed soil residues. The Calcaric Leptosol (A) showed a larger mass extraction than the Calcaric Cambisol (B), and a greater cumulative C and N content in its Humeome. Both soils showed the greatest weight yield for AQU4 fraction, followed by ORG2, RESOM, ORG1, AQU2, and ORG3. ORG2 was the most differentiating fraction between the two soils for both compound concentration and diversity, showing a larger C content for soil A than for soil B and a different distribution in aromatic compounds, fatty acids, and dicarboxylic acids. No significant differences between soils were found for ORG 3, suggesting similar processes of OM stabilization for its recalcitrant components, mostly hydrophobic esters of alkanoic, hydroxy, and aromatic acids with linear alkanols. We confirmed that Humeomic fractionation coupled to advanced analytical instrumentations enabled a detailed molecular characterization of the soil Humeome and differentiated between the two calcareous grassland soils and the other soils previously subjected to Humeomics.

Effects of nitrogen fertilization and bioenergy crop type on topsoil organic carbon and total Nitrogen contents in middle Tennessee USA

Nitrogen (N) fertilization affects bioenergy crop growth and productivity and consequently carbon (C) and N contents in soil, it however remains unclear whether N fertilization and crop type individually or interactively influence soil organic carbon (SOC) and total N (TN). In a three-year long fertilization experiment in switchgrass (SG: Panicum virgatum L.) and gamagrass (GG: Tripsacum dactyloides L.) croplands in Middle Tennessee USA, soil samples (0–15cm) were collected in plots with no N input (NN), low N input (LN: 84 kg N ha^-1 yr^-1 in urea) and high N input (HN: 168 kg N ha^-1 yr^-1 in urea). Besides SOC and TN, the aboveground plant biomass was also quantified. In addition to a summary of published root morphology data based on a separated mesocosm experiment, the root leachable dissolved organic matter (DOM) of both crops was also measured using archived samples. Results showed no significant interaction of N fertilization and crop type on SOC, TN or plant aboveground biomass (ABG). Relative to NN, HN (not LN) significantly increased SOC and TN in both crops. Though SG showed a 15–68% significantly higher ABG than GG, GG showed a 9.3–12% significantly higher SOC and TN than SG. The positive linear relationships of SOC or TN with ABG were identified for SG. However, GG showed structurally more complex and less readily decomposed root DOM, a larger root volume, total root length and surface area than SG. Collectively, these suggested that intensive N fertilization could increase C and N stocks in bioenergy cropland soils but these effects may be more likely mediated by the aboveground biomass in SG and root chemistry and morphology in GG. Future studies are expected to examine the root characteristics in different bioenergy croplands under the field fertilization experiment.

A molecular zoom into soil Humeome by a direct sequential chemical fractionation of soil

A Humeomics sequential chemical fractionation coupled to advanced analytical identification was applied directly to soil for the first time. Humeomics extracted ~235% more soil organic carbon (SOC) than by the total alkaline extraction traditionally employed to solubilise soil humic molecules (soil Humeome). Seven fractions of either hydro- or organo-soluble components and a final unextractable humic residue were separated from soil. These materials enabled an unprecedented structural identification of solubilised heterogeneous humic molecules by combining NMR, GC-MS, and ESI-Orbitrap-MS. Identified molecules and their relative abundance were used to build up structure-based van Krevelen plots to show the specific contribution of each fraction to SOC. The stepwise isolation of mostly hydrophobic and unsaturated molecules of progressive structural complexity suggests that humic suprastructures in soil are arranged in multi-molecular layers. These comprised molecules either hydrophobically adsorbed on soil aluminosilicate surfaces in less stable fractions, or covalently bound in amorphous organo-iron complexes in more recalcitrant fractions. Moreover, most lipid molecules of the soil Humeome appeared to derive from plant polyesters rather than bacterial metabolism. An advanced understanding of soil humic molecular composition by Humeomics may enable control of the bio-organic dynamics and reactivity in soil.

The Interplay Between Bioenergy Grass Production and Water Resources in the United States of America

We apply a land surface model to evaluate the interplay between potential bioenergy grass (Miscanthus, Cave-in-Rock, and Alamo) production, water quantity, and nitrogen leaching (NL) in the Central and Eastern U.S. Water use intensity tends to be lower where grass yields are modeled to be high, for example in the Midwest for Miscanthus and Cave-in-Rock and the upper southeastern U.S. for Alamo. However, most of these regions are already occupied by crops and forests and substitution of these biome types for ethanol production implies trade-offs. In general, growing Miscanthus consumes more water, Alamo consumes less water, and Cave-in-Rock consumes approximately the same amount of water as existing vegetation. Bioenergy grasses can maintain high productivity over time, even in water limited regions, because their roots can grow deeper and extract the water from the deep, moist soil layers. However, this may not hold where there are frequent and intense drought events, particularly in regions with shallow soil depths. One advantage of bioenergy grasses is that they mitigate nitrogen leaching relative to row crops and herbaceous plants when grown without applying N fertilizer; and bioenergy grasses, especially Miscanthus, generally require less N fertilizer application than row crops and herbaceous plants.