Experimental evidence for the attenuating effect of SOM protection on temperature sensitivity of SOM decomposition

Abstract

The ability to predict C cycle responses to temperature changes depends on the accurate representation of temperature sensitivity (Q 10 ) of soil organic matter (SOM) decomposition in C models for different C pools and soil depths. Theoretically, Q 10 of SOM decomposition is determined by SOM quality and availability (referred to here as SOM protection). Here, we focus on the role of SOM protection in attenuating the intrinsic, SOM quality dependent Q 10 . To assess the separate effects of SOM quality and protection, we incubated topsoil and subsoil samples characterized by differences in SOM protection under optimum moisture conditions at 25 1C and 35 1C. Although lower SOM quality in the subsoil should lead to a higher Q 10 according to kinetic theory, we observed a much lower overall temperature response in subsoil compared with the topsoil. Q 10 values determined for respired SOM fractions of decreasing lability within the topsoil increased from 1.9 for the most labile to 3.8 for the least labile respired SOM, whereas corresponding Q 10 values for the subsoil did not show this trend (Q 10 between 1.4 and 0.9). These results indicate the existence of a limiting factor that attenuates the intrinsic effect of SOM quality on Q 10 in the subsoil. A parallel incubation experiment of 13 C-labeled plant material added to top-and subsoil showed that decomposition of an unprotected C substrate of equal quality responds similarly to temperature changes in top-and subsoil. This further confirms that the attenuating effect on Q 10 in the subsoil originates from SOM protection rather than from microbial properties or other nutrient limitations. In conclusion, we found experimental evidence that SOM protection can attenuate the intrinsic Q 10 of SOM decomposition.

Labile substrate availability controls temperature sensitivity of organic carbon decomposition at different soil depths

“…First, the physical protection of SOC due to aggregation or association with soil minerals tends to increase with depth (Kögel-Knabner et al 2008;Schrumpf et al 2013), which may lead to a physical disconnect between microbes and substrates and a lower temperature dependence of microbial respiration in subsoils (Gillabel et al 2010;Conant et al 2011). Although physical protection of SOC was not directly measured in this study, MBC and total N contents may indicate the intensity of physical protection of SOC for microbial decomposition.…”

“…However, previous studies have shown inconsistent patterns of Q 10 values of soil respiration with soil depth. Increase (Lomander et al 1998;Fierer et al 2003;Jin et al 2008;Karhu et al 2010), decrease (Winkler et al 1996;MacDonald et al 1999;Gillabel et al 2010), or no changes (Fang et al 2005;Leifeld and Fuhrer 2005;Rey et al 2008) in apparent Q 10 values with increasing soil depth have been observed in different studies. Much of the variation in the apparent temperature sensitivity of SOC decomposition may be related to the fact that labile substrate availability is often unaccounted for in these studies Gershenson et al 2009;Conant et al 2011).…”

“…Therefore, accurate representation of the temperature sensitivity of SOC decomposition (often measured in Q 10 value) in Earth System Models (ESMs) is crucial because it affects our predictions of the impact of climate change on soil carbon stock (Holland et al 2000;Fang et al 2005;Foereid et al 2014). However, there is still no consensus on how the temperature sensitivity of SOC decomposition varies with biotic and abiotic factors Kirschbaum 2006;Smith et al 2008;von Lützow and Kögel-Knabner 2009;Subke and Bahn 2010;Conant et al 2011), such as soil depth (Fierer et al 2003;Rey et al 2008;Gillabel et al 2010) and labile substrate availability Gershenson et al 2009;Fissore et al 2013).…”

“…First, at ambient substrate condition (without glucose addition), increasing soil depth leads to increasing intensity of physico-chemical protection of SOC and decreasing amount of labile substrate for microbial respiration (Rumpel and Kögel-Knabner 2011;Schrumpf et al 2013). This increasing substrate limitation causes decreasing apparent temperature sensitivity of SOC decomposition with increasing depth (Gillabel et al 2010). Second, if the kinetics of soil respiration can be adequately approximated by the Michaelis-Menten equation, Q 10 of the substrateinduced respiration should be significantly higher than that of the basal respiration without substrate addition, because the canceling effect of the temperature sensitivity of K m on the temperature sensitivity of soil respiration is negligible when the substrate is saturated and much higher than K m ([C] ) K m , Larionova et al 2007;Gershenson et al 2009).…”

Comparing models of microbial–substrate interactions and their response to warming

“…Their application specifically highlights the role of extracellular enzymes during decomposition and how these constraints will further affect the release of soil organic matter as a consequence of warming. While microbial decomposition models are able to improve prediction of organic carbon stock globally and can successfully recreate litter decomposition dynamics, the long-term trajectory of a warming response needs further evaluation Hararuk et al, 2015). In particular, a positive feedback between depolymerisation and microbes can only be curbed via the longer-term adjustment of soil organic matter and therefore lead to oscillation in both microbial biomass and soil organic matter .…”