Abstract
Subsoils are known to harbor large amounts of soil organic carbon (SOC) and may represent key global carbon (C) sinks given appropriate management. Although rhizodeposition is a major input pathway of organic matter to subsoils, little knowledge exists on C dynamics, particularly stabilization mechanisms, such as soil aggregation, in the rhizosphere of different soil depths. The aim of this study was to investigate the influence of natural and elevated root exudation on C allocation and aggregation in the topsoil and subsoil of a mature European beech (Fagus sylvatica L.) forest. We experimentally added model root exudates to soil at two different concentrations using artificial roots and analyzed how these affect SOC, nitrogen, microbial community composition, and size distribution of water-stable aggregates. Based on the experimental data, a mathematical model was developed to describe the spatial distribution of the formation of soil aggregates and their binding strength. Our results demonstrate that greater exudate additions affect the microbial community composition in favor of fungi which promote the formation of macroaggregates. This effect was most pronounced in the C-poor subsoil, where macroaggregation increased by 86 % and SOC content by 10 %. Our modeling exercise reproduced the observed increase in subsoil SOC at high exudate additions. We conclude that elevated root exudation has the potential to increase biotic macroaggregation and thus the C sink strength in the rhizosphere of forest subsoils.
Highlights
Soils represent the largest terrestrial organic carbon (OC) pool, an increase in soil OC (SOC) stocks via improved management practices has the potential to mitigate global climate change (Jobbágy and Jackson, 2000; Lal et al, 2011; Powlson et al, 2011; Stockmann et al, 2013; Lal, 2016; Minasny et al, 2017)
Microbial residues as determined by the amino sugars (AS) content strongly increased with depth from 26 to 46 μg AS C g−1 soil organic carbon (SOC) when standardized to SOC content
Macroaggregates made up only 25% of the soil mass, whereas the large microaggregate fraction (250–53 μm) clearly dominated with a contribution of more than 50%
Summary
Soils represent the largest terrestrial organic carbon (OC) pool, an increase in soil OC (SOC) stocks via improved management practices has the potential to mitigate global climate change (Jobbágy and Jackson, 2000; Lal et al, 2011; Powlson et al, 2011; Stockmann et al, 2013; Lal, 2016; Minasny et al, 2017). A major input pathway of OC to subsoils is plant roots, which provide OM in the form of rhizodeposits such as dead root cells, soluble root exudates, sloughed-off cells, or mucilage (Rasse et al, 2005; Jones et al, 2009; Rumpel et al, 2012) These root-derived compounds trigger the development of a narrow zone around the roots, which is influenced by their activity and considered as a hotspot of biological, chemical, and physical activities in soils, i.e., the rhizosphere (Hinsinger et al, 2009). Rhizodeposits and especially soluble root exudates represent an available C source for soil microbes (van Hees et al, 2005) and have frequently been observed to alter the native C mineralization rates of inherent SOC (Huo et al, 2017), a process widely referred to as the priming effect (Kuzyakov, 2010; Dijkstra et al, 2013). This effect can be either positive or negative (i.e., accelerated or reduced decomposition of inherent SOM), and several studies have found that its magnitude may vary among soil types, soil depths (Paterson and Sim, 2013), and exudate addition rates (Blagodatskaya and Kuzyakov, 2008)
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