Management and environmental controls of belowground carbon allocation in terrestrial ecosystems
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The carbon sequestration function of ecosystems that global climate mitigation efforts increasingly count on, depends on the preservation of existing ecosystem carbon pools, and facilitating carbon uptake processes while hindering the release (REF). While conceptually straightforward, accurate quantification of changes in soil carbon is not trivial, the data on the dynamics of soil carbon are limited, and represent a major uncertainty in carbon cycle models. The high spatial variability, and scaling assumptions undermine the value of direct measurements, and such estimates can reliably detect changes in soil C pool only when measurements are highly replicated and the sampling interval is sufficiently long (decades, rather than years; Bellamy et al., 2005; Don et al., 2011; Maia et al., 2010; Xie et al., 2007; Yan et al., 2011). In contrast, flux-based assessments of soil carbon inputs and losses can identify consistent trends at annual scale (Noormets et al., 2015). Recent years have seen significant progress in our understanding of the magnitude and controls of microbial processing of soil carbon and its contribution to overall forest carbon balance (Bracho et al., 2018; Kuzyakov, 2010; Kuzyakov et al., 2000; Kuzyakov and Gavrichkova, 2010; McElligott et al., 2016; Templeton et al., 2015). Similarly, recent developments in spectrometry have shown that the pathways and chemical identity of carbon inputs to soil can influence its persistence and half-life in soil (Strukelj et al., 2013; Strukelj et al., 2012). Still, uncertainties remain about the pathways and dynamics of carbon inputs to soil.Belowground carbon dynamics remain a significant uncertainty in ecosystem and Earth System Models (De Kauwe et al., 2014; Friedlingstein et al., 1999). The models' ability to capture interannual dynamics in the carbon cycle, response to extreme weather events and disturbances all hinge on realistic representation of allocation of assimilated carbon to different plant compartments, and their subsequent entry to and processing in the soil. Given that allocation is difficult to measure directly with existing tools, indirect approaches like the ratios of carbon pools (e.g. root:shoot ratio) and total belowground carbon flux (TBCF; Giardina and Ryan, 2002; Raich and Nadelhoffer, 1989) have been used to estimate the plant allocation patterns. In addition to the difficulty of quantifying changes in belowground carbon pools (due to their high spatial variability of soil C content), difficulty of isolating the highly dynamic fine roots, and the variableallocation to and use from the non-structural storage carbohydrate pool (Richardson et al., 2015) further complicate the assessment of plant carbon allocation. Yet, recent progress in our understandingof fundamental plant allometry (Chen et al., 2013; Chen et al., 2014) and soil CO2 efflux partitioning (Bracho et al..........