Explorations of soil microbial processes driven by dissolved organic carbon

Explorations of soil microbial processes driven by dissolved organic carbon Angela L. Straathof June 17, 2015, Wageningen UR ISBN 978-94-6257-327-7 Abstract Dissolved organic carbon (DOC) is a complex, heterogeneous mixture of C compounds which, as a substrate, may influence various processes of the soil microbial community. Microbial respiration and volatile production are two such processes. These have both been linked to general disease suppression (GDS), a phenomenon in agricultural soils which inhibits pathogenic infestation in crops. The underlying hypothesis of this thesis is that the quality of DOC, via regulation of microbial processes, may be an important indicator of soil functions, including GDS. Properties of DOC quality include proportions of hydrophobic and hydrophilic fractions, and aromaticity. This thesis describes a high range in DOC fractions from various types of compost, which is often added to soil as an amendment to promote GDS. Differences in soil microbial respiration rates were attributed to differences in the composition of compost DOC added to soil in a laboratory incubation experiment. Compost DOC high in proportion of the hydrophilic (Hi) fraction promoted respiration rates. Depletion of the hydrophobic humic acid (HA) fraction was also observed. The relationship between DOC and microbial respiration was further explored in a survey of 50 arable soils. Both HA and Hi fractions of DOC that were found to be statistically, significantly related to respiration rates in these soils. Furthermore, in an assay measuring in vitro pathogen suppression by microbial volatile production, DOC concentration and microbial respiration were linked to growth suppression of Rhizoctonia solani, Fusarium oxysporum, and Pythium intermedium via multivariate regression modelling. This thesis provides evidence for the importance of DOC and DOC quality’s influence on microbial respiration and volatile production, thus supporting the hypothesis that DOC is a microbially-relevant soil chemical parameter, and potential indicator of general disease suppression in agricultural soils. </p

Input materials and processing conditions control compost dissolved organic carbon quality

Dissolved organic carbon (DOC) has been proposed as an indicator of compost maturity and stability. Further fractionation of compost DOC may be useful for determining how particular composting conditions will influence DOC quality. Eleven composts ranging in input materials and processing techniques were analyzed; concentrations of DOC ranged from 428 mg kg-1 to 7300 mg kg-1. Compost DOC was qualified by fractionation into pools of humic acids (HA), fulvic acids (FA), hydrophobic neutrals (HoN), and hydrophilic (Hi) compounds. The range in proportion of DOC pools was highly variable, even for composts with similar total DOC concentrations. Longer composting time and higher temperatures consistently corresponded with a depletion of hydrophilics, suggesting a preferential turnover of these compounds during the thermophilic composting phase. Qualification of DOC pools through fractionation may be an informative tool in predicting the effects of a processing technique on compost quality and, ultimately, soil functional processes

Dynamics of soil dissolved organic carbon pools reveal both hydrophobic and hydrophilic compounds sustain microbial respiration

The quality of dissolved organic carbon (DOC) released from soil organic amendments may influence soil microbial activity and the quality of the soil's DOC pools. Measurements of total DOC are often considered in relation to microbial activity levels but here we propose that quantification of DOC fractions is a more informative alternative. In a laboratory incubation, soil received DOC that was extracted from three organic matter sources: fresh compost, mature compost, and a mixture of the two. Soil microbial respiration (CO2 emission), and concentrations of hydrophobic (humic acids (HA), fulvic acids (FA) and neutrals (HoN)) and hydrophilic (Hi) DOC fractions were measured throughout the 35 d incubation. The A254 specific UV absorption of total and HA DOC were measured at the start and end of the incubation as an indicator of aromaticity. Microbial respiration rates were highest in soils amended with fresh compost DOC, which had a higher proportion of Hi compounds. Concentration of Hi was significantly and positively correlated with soil respiration, explaining 24% more variation than total DOC. Humic acid concentrations significantly decreased over 35 d, including a 33% reduction in HA from an unamended control soil. Compost treated soils' HA pools increased in aromaticity, suggesting preferential mineralization of the least aromatic HA molecules. A decrease in SUVA254 values in other HA pools may be the result of HA degradation in the absence of low-aromatic HA. Our observation of depletion of hydrophobic compounds from the HA fraction provides evidence that humic substances can be a relatively reactive pool, which can provide, together with hydrophilic compounds, a readily available C source to the microbial community

Changes in root-exudate-induced respiration reveal a novel mechanism through which drought affects ecosystem carbon cycling

- Root exudates play an important role in ecosystem response to climate change, but the functional consequences of drought‐induced changes in the quality of root exudates are unknown. Here, we addressed this knowledge gap in a unique experimental approach. - We subjected two common grassland species that differ widely in their growth strategies and root systems, the grass Holcus lanatus and the forb Rumex acetosa, to 2 wk of drought. We collected root exudates and soils at the end of the drought and after 2 wk of recovery and readded all root exudates to all soils in a fully reciprocal set‐up to measure root‐exudate‐induced respiration. - We found that soil treatment was unimportant for determining root‐exudate‐induced respiration. By contrast, root exudates collected from plants that had experienced drought clearly triggered more soil respiration than exudates from undroughted plants. Importantly, this increased respiration compensated for the lower rates of root exudation in droughted plants. - Our findings reveal a novel mechanism through which drought can continue to affect ecosystem carbon cycling, and a potential plant strategy to facilitate regrowth through stimulating microbial activity. These findings have important implications for understanding plant and ecosystem response to drought

Are researchers following best storage practices for measuring soil biochemical properties?

It is widely accepted that the measurement of organic and inorganic forms of carbon (C) and nitrogen (N) in soils should be performed on fresh extracts taken from fresh soil samples. However, this is often not possible, and it is common practice to store samples (soils and/or extracts), despite a lack of guidance on best practice. We utilised a case study on a temperate grassland soil taken from different depths to demonstrate how differences in soil and/or soil extract storage temperature (4 or -20 °C) and duration can influence sample integrity for the quantification of soil-dissolved organic C and N (DOC and DON), extractable inorganic nitrogen (NHC+4and NO-3) and microbial biomass C and N (MBC and MBN). The appropriateness of different storage treatments varied between topsoils and subsoils, highlighting the need to consider appropriate storage methods based on soil depth and soil properties. In general, we found that storing soils and extracts by freezing at -20 °C was least effective at maintaining measured values of fresh material, whilst refrigerating (4 °C) soils for less than a week for DOC and DON and up to a year for MBC and MBN and refrigerating soil extracts for less than a week for NHC+4and NO-3did not jeopardise sample integrity. We discuss and provide the appropriate tools to ensure researchers consider best storage practice methods when designing and organising ecological research involving assessments of soil properties related to C and N cycling.We encourage researchers to use standardised methods where possible and to report their storage treatment (i.e. temperature, duration) when publishing findings on aspects of soil and ecosystem functioning. In the absence of published storage recommendations for a given soil type, we encourage researchers to conduct a pilot study and publish their findings