A climate gradient approach elucidates mechanisms in the regulation of soil organic matter turnover in the Eastern Mediterranean

Background/Question/Methods

The turnover of soil organic matter (SOM) is controlled by complex underlying biotic and abiotic interactions, related to the physico-chemical characteristics of the soil. While the role of climate in the regulation of SOM turnover is manifested through its constrains on both primary production and decomposition kinetics, the mechanisms of climatic regulation of SOM turnover are largely unknown. Therefore, the modeling and forecasting of SOM turnover rates across climate types and seasons is unapproachable. Understanding drivers and mechanisms of SOM turnover at local and regional scales, across a wide range of ecosystems, is a precondition for determining whether soil behaves as a source or sink of atmospheric CO₂under climate change scenarios. The major objective of the current research was to gain insight into feedback mechanisms operating across boundaries of physical, chemical and biological soil properties and that control SOM turnover in Eastern Mediterranean ecosystems. In contrast with local observations along temporal lines or spatial surveys of discrete ecosystem, we hypothesized that a continuous gradient of defined climatic conditions (from a humid-Mediterranean to an arid sub-climate; 245-km north-south transect) along which the dynamics of abiotic and biotic soil properties are spatio-temporally analyzed could reflect a regulatory scheme of SOM turnover.  

Results/Conclusions

A model of SOM turnover dynamics in the regional (Eastern-Mediterranean) level is proposed, based on the following ecological principles: (1) Rainfall enhances soil weathering and texture fineness which, in turn, positively affect water holding capacity, vegetation and SOM content; (2) Climate aridity level and the associated intensities of solar radiation and dry-wet cycle frequency enhance SOM lability (increased dissolved-to-total organic carbon ratio (DOC/TOC)) via photodegradation and microbial pulse-oriented activity; (3) Microbial basal respiration (MBR) and biomass (MB) positively depend on both TOC and DOC pools along both geographic and seasonal axes; (4) Carbon-use efficiency decreases (elevated qCO₂) with increasing aridity level at both the geographic and seasonal scales; (5) Organic carbon availability (DOC/TOC) and the microbial quotient (qCO₂) exhibit an abiotic-biotic feedback relationship along both geographic and seasonal lines; and (6) As negative correlations between qCO₂ and total soluble nitrogen levels were recorded at both the geographic and seasonal scales, it is speculated that the degree to which the soil microbial community is nitrogen-limited acts as a complementary trigger for SOM mineralization. Thus, the typical nitrogen limitation and relatively high carbon availability in arid soil ecosystems interact in triggering an intensified soil catabolic activity.