Evidence of a strong coupling between root exudation, C and N availability, and stimulated SOM decomposition caused by rhizosphere priming effects

Abstract

Increased temperatures and concomitant changes in vegetation patterns are expected to dramatically alter the functioning of northern ecosystems over the next few decades. Predicting the ecosystem response to such a shift in climate and vegetation is complicated by the lack of knowledge about the links between aboveground biota and belowground process rates. Current models suggest that increasing temperatures and rising concentrations of atmospheric CO 2 will be partly mitigated by elevated C sequestration in plant biomass and soil. However, empirical evidence does not always support this assumption, as elevated temperature and CO 2 concentrations also accelerate the belowground C flux, in many cases extending to increased decomposition of soil organic matter (SOM) and ultimately resulting in decreased soil C stocks. The mechanism behind the increase has remained largely unknown, but it has been suggested that priming might be the causative agent. Here, we provide quantitative evidence of a strong coupling between root exudation, SOM decomposition, and release of plant available N caused by rhizosphere priming effects. As plants tend to increase belowground C allocation with increased temperatures and CO 2 concentrations, priming effects need to be considered in our long‐term analysis of soil C budgets in a changing environment. The extent of priming seems to be intimately linked to resource availability, as shifts in the stoichiometric nutrient demands of plants and microorganisms will lead to either cooperation (resulting in priming) or competition (no priming will occur). The findings lead us on the way to resolve the varying response of primary production, SOM decomposition, and release of plant available N to elevated temperatures, CO 2 concentrations, and N availability.

Short-term carbon input increases microbial nitrogen demand, but not microbial nitrogen mining, in a set of boreal forest soils

“…In contrast to our hypothesis, we found that non-symbiotic soil microorganisms adjusted to a short-term increase in C availability not by accelerating the mobilization of N from SOM polymers such as proteins, but rather by using the already available N more efficiently. Considering that (1) C effects on microbial processes are typically restricted to soils of initially low C availability (e.g., Bengtson et al 2012;Wild et al 2016), and that (2) these soils are typically characterized by an excess of N, and by low N use efficiency (e.g., Mooshammer et al 2014a;Wild et al 2015; see also Supplementary Table S1), an increase in microbial N use efficiency might be a common adjustment to enhanced soil C availability and microbial N demand. Although our findings do not rule out that an increase in plant-soil C allocation, such as expected with rising atmospheric CO 2 concentrations and temperatures, can promote the release of available N from SOM polymers, they show that such an effect can be counteracted at least in the short term by an increase in microbial N use efficiency, reducing soil N availability and aggravating plant N limitation, but also mitigating soil N losses, e.g., by nitrate leaching and denitrification.…”

“…Support for a positive effect of plants on N mining is provided by a range of studies that show higher activities of N-targeting enzymes, gross ammonification rates and soil N losses in rhizosphere than in non-rhizosphere soil (Brzostek et al 2013;Holz et al 2016;Kieloaho et al 2016;Koranda et al 2011;Weintraub et al 2007;Zhu et al 2014), a stimulation of N-targeting enzymes by artificial root exudates (Meier et al 2017), and positive correlations between rates of root exudation, SOC and (gross or net) N mineralization in experiments with intact plants (Bengtson et al 2012;Yin et al 2013Yin et al , 2014. The strongest support for a stimulation of microbial N mining by increased plant C input comes from experiments where plants were exposed to elevated CO 2 concentrations.…”

High organic inputs explain shallow and deep SOC storage in a long-term agroforestry system – combining experimental and modeling approaches

“…In the alleys and below the maximum rooting depth of crops, there are no direct inputs of FOC, but OC is transported in these deep layers due to transport mechanisms. However, further studies could explore the impact of the priming effect formalism on the estimation of its intensity by using explicit microbial biomass, for instance (Blagodatsky et al, 2010;Perveen et al, 2014).…”

“…Several authors showed that they could induce strong priming effects (Bengtson et al, 2012;Keiluweit et al, 2015), but root exudates are also a source of labile carbon, potentially contributing to stable SOC (Cotrufo et al, 2013). These opposing effects of root exudates on SOC should be further investigated, especially concerning the deep roots in agroforestry systems.…”

Physiological shifts in the microbial community drive changes in enzyme activity in a perennial agroecosystem

“…In these cases, turnover rates of the microbial biomass increase but microbial activity associated with decomposition remains stable (Kuzyakov et al 2000). At other times increased microbial activity can lead to faster rates of SOM decomposition (''priming'') and subsequent release of plant available N (Bengtson et al 2012;Dijkstra et al 2009;Jingguo and Bakken 1997;Phillips et al 2010) as a result of increased predation on bacteria or competition for N by plants, for example (Clarholm 1985;Griffiths and Robinson 1992;Kuzyakov et al 2000).…”