SOM and microbes - what is left from microbial life in soils

Soil organic matter (SOM) is the basis for many soil functions and plays an important role for soil fertility and mitigation of global change. Recently, novel analytical tools have been adopted and significant progress has been made in the field of SOM characterisation and elucidation of SOM processes. The results obtained led to the perception of SOM as a continuum of plant and microbial residues at different stages of decay rather than newly synthesised macromolecules. There is increasing evidence that microbial residues make a large contribution to SOM. Here, we review processes involved in SOM formation and turnover. Plant-derived material is processed by microorganisms and transformed into microbial biomass and finally necromass. The latter is persistent in soil, mainly by its spatial organisation and by interactions with soil minerals. SOM formation therefore is embedded in the triangular relationship between soil, plants and microorganisms. Critical flux controlling factors in this process chain are the energy content and the availability of plant-derived carbon to the microorganisms, their carbon use efficiency, which determines the yield of biomass produced per substrate consumed, and the effectivity of stabilisation of the necromass. These factors depend on microbial abundance and metabolism as well as on environmental factors. Microbes and microbial communities are thus both drivers and substantial contributors to SOM dynamics in soil. This improved understanding offers various options to assign properties and processes in soils to processes of living organisms, which was previously not possible. Mechanistic insight into the carbon flow from plant material input through the microbial foodweb to microbial necromass stabilisation and finally to SOM will be the basis for future improvements of SOM models. These improved models will be the basis of knowledge-based land management options for sustainable soil use

Restructuring of a Peat in Interaction with Multivalent Cations: Effect of Cation Type and Aging Time

<div><p>It is assumed to be common knowledge that multivalent cations cross-link soil organic matter (SOM) molecules via cation bridges (CaB). The concept has not been explicitly demonstrated in solid SOM by targeted experiments, yet. Therefore, the requirements for and characteristics of CaB remain unidentified. In this study, a combined experimental and molecular modeling approach was adopted to investigate the interaction of cations on a peat OM from physicochemical perspective. Before treatment with salt solutions of Al³⁺, Ca²⁺ or Na⁺, respectively, the original exchangeable cations were removed using cation exchange resin. Cation treatment was conducted at two different values of pH prior to adjusting pH to 4.1. Cation sorption is slower (>>2 h) than deprotonation of functional groups (<2 h) and was described by a Langmuir model. The maximum uptake increased with pH of cation addition and decreased with increasing cation valency. Sorption coefficients were similar for all cations and at both pH. This contradicts the general expectations for electrostatic interactions, suggesting that not only the interaction chemistry but also spatial distribution of functional groups in OM determines binding of cations in this peat. The reaction of contact angle, matrix rigidity due to water molecule bridges (WaMB) and molecular mobility of water (NMR analysis) suggested that cross-linking via CaB has low relevance in this peat. This unexpected finding is probably due to the low cation exchange capacity, resulting in low abundance of charged functionalities. Molecular modeling demonstrates that large average distances between functionalities (∼3 nm in this peat) cannot be bridged by CaB-WaMB associations. However, aging strongly increased matrix rigidity, suggesting successive increase of WaMB size to connect functionalities and thus increasing degree of cross-linking by CaB-WaMB associations. Results thus demonstrated that the physicochemical structure of OM is decisive for CaB and aging-induced structural reorganisation can enhance cross-link formation.</p></div

A comparison of cation uptake with respect to the amounts remained in treatment solution in aluminium (SP-Al), calcium (SP-Ca) and sodium (SP-Na) treated samples at different cation addition pH.

<p>A comparison of cation uptake with respect to the amounts remained in treatment solution in aluminium (SP-Al), calcium (SP-Ca) and sodium (SP-Na) treated samples at different cation addition pH.</p

Effect of sample treatment by exchange resin followed by addition of specific cations on the major cation content with respect to different charging pH, shown for highest cation loading.

<p>Also the values for untreated (SP) and exchange resin-treated samples (SP-H) are shown.</p

Investigated soil properties with respect to the cation addition pH.

<p>(A) <i>CEC_eff</i>, (B) <i>DOC</i>, (C) <i>CA, (D) T^*, (E) T_2,fast</i> and (F) contribution of fast relaxing water molecules to the total T₂ decay, (G) contribution of Lorentzian line to the ¹H wideline and (H) Difference in the contribution of Lorentzian component after heating event. Dotted lines and dashed lines represents the average values observed for SP and SP-H samples, respectively.</p

Aging effects on WaMB transition temperature (A), Lorentzian line fraction (B) and on the amount of mobilisable water (C), expressed by the difference of the respective parameters after and before aging.

<p>Positive values indicate an increase, and negative values indicate a decrease in the parameter, respectively. Dotted line along zero shown in (C) distinguishes the positive and negative effects.</p

Langmuir fit parameters of sorption curves.

<p>Langmuir fit parameters of sorption curves.</p