Abstract
Biodiesel Co-Product (BCP) is a complex organic material formed during the transesterification of lipids. We investigated the effect of BCP on the extracellular microbial matrix or ‘extracellular polymeric substance’ (EPS) in soil which is suspected to be a highly influential fraction of soil organic matter (SOM). It was hypothesised that more N would be transferred to EPS in soil given BCP compared to soil given glycerol. An arable soil was amended with BCP produced from either 1) waste vegetable oils or 2) pure oilseed rape oil, and compared with soil amended with 99% pure glycerol; all were provided with 15N labelled KNO3. We compared transfer of microbially assimilated 15N into the extracellular amino acid pool, and measured concomitant production of exopolysaccharide. Following incubation, the 15N enrichment of total hydrolysable amino acids (THAAs) indicated that intracellular anabolic products had incorporated the labelled N primarily as glutamine and glutamate. A greater proportion of the amino acids in EPS were found to contain 15N than those in the THAA pool, indicating that the increase in EPS was comprised of bioproducts synthesised de novo. Moreover, BCP had increased the EPS production efficiency of the soil microbial community (μg EPS per unit ATP) up to approximately double that of glycerol, and caused transfer of 21% more 15N from soil solution into EPS-amino acids. Given the suspected value of EPS in agricultural soils, the use of BCP to stimulate exudation is an interesting tool to consider in the theme of delivering sustainable intensification.
Highlights
The greatest challenge for soil science over the few decades is to help alleviate competing demands on soil for security of water, food, fuel, fibre, and ecosystem pressures without expanding the total area of soil under exploitation (Tilman et al, 2011)
Comparing extracellular polymeric substances’ (EPS)-amino acids (AAs) concentrations per unit soil determined by GC–flame ionisation detector (FID) alone, shows no statistically significant differences in mean EPS-AA concentrations between treatments Biodiesel Co-Product (BCP) produced from foodgrade ‘virgin’ oilseed rape (BCPV), BCP made from recycled vegetable oils (BCPR), and glycerol (Supplementary Table 3) — with all giving statistically significant increases in EPS-AA relative to ‘N only’ (l.s.d. = 0.095 μg g−1 soil; data transformed log10)
This work provides a clear demonstration using 15N stable isotope probing with GC/C/IRMS that microbial allocation of nitrate-N to EPSAA is stimulated through provision of BCP, and most notably by BCPR
Summary
The greatest challenge for soil science over the few decades is to help alleviate competing demands on soil for security of water, food, fuel, fibre, and ecosystem pressures without expanding the total area of soil under exploitation (Tilman et al, 2011). The need to engineer soils to deliver sustainable intensification objectives is increasingly urgent. It is increasingly achievable as the shortfalls between soil capability and actual condition become more apparent (McBratney et al, 2014). We propose that some alleviation to the pressure on soil functions may be found through employing native soil microbial populations to better utilise the waste streams from otherwise competing industries. If successful, this approach will challenge the simplistic perception that one agricultural output (e.g., food) necessarily conflicts with another (e.g., fuel). The global demand for biodiesel is driven by the need to
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