Is soil carbon mostly root carbon? Mechanisms for a specific stabilisation

Abstract

Understanding the origin of the carbon (C) stabilised in soils is crucial in order to device management practices that will foster C accumulation in soils. The relative contributions to soil C pools of roots vs. shoots is one aspect that has been mostly overlooked, although it appears a key factor that drives the fate of plant tissue C either as mineralized CO 2 or as stabilized soil organic matter (SOM). Available studies on the subject consistently indicate that root C has a longer residence time in soil than shoot C. From the few studies with complete datasets, we estimated that the mean residence time in soils of root-derived C is 2.4 times that of shoot-derived C. Our analyses indicate that this value is biased neither by an underestimation of root contributions, as exudation was considered in the analysis, nor by a priming effect of shoot litter on SOM. Here, we discuss the main SOM stabilisation mechanisms with respect to their ability to specifically protect root-derived SOM. Comparing in situ and incubation experiments suggests that the higher chemical recalcitrance of root tissues as compared to that of shoots is responsible for only a small portion, i.e. about one fourth, of the difference in mean residence time in soils of root-derived vs. shootderived C. This suggests that SOM protection mechanisms other than chemical recalcitrance are also enhanced by root activities: (1) physico-chemical protection, especially in deeper horizons, (2) micrometer-scale physical protection through myccorhiza and root-hair activities, and (3) chemical interactions with metal ions. The impact of environmental conditions within deeper soil layers on root C stabilisation appear difficult to assess, but is likely, if anything, to further increase the ratio between the mean residence time of root vs. shoot C in soils. Future advances are expected from isotopic studies conducted at the molecular level, which will help unravel the fate of individual shoot and root compounds, such as cutins and suberins, throughout soil profiles.

Carbono e nitrogênio nas frações granulométricas da matéria orgânica do solo, em sistemas de culturas sob plantio direto

“…Raízes de alfafa constituem-se numa importante fonte de MO particulada, que é protegida fisicamente no solo (Balesdent & Balabane, 1996;Rasse et al, 2005). Ao atuarem na formação e estabilização de macroagregados, as raízes em troca recebem a proteção física, reduzindo assim sua taxa de mineralização (Balesdent & Balabane, 1996;Rasse et al, 2005).…”

“…Ao atuarem na formação e estabilização de macroagregados, as raízes em troca recebem a proteção física, reduzindo assim sua taxa de mineralização (Balesdent & Balabane, 1996;Rasse et al, 2005). Além disso, a proteção física da MO particulada é mais eficiente em sistemas cujo tempo de permanência da cultura no campo é maior e que adicionam mais resíduos orgânicos pelas raízes (Janzen et al, 1998), como ocorre no sistema com alfafa.…”

“…Porém, é importante ressaltar que houve tendência a maiores estoques de C e N da MO associada aos minerais no sistema AL-MI. Essa tendência devese à maior adição de resíduos orgânicos pela raiz (Quadro 1), cuja estabilização desses compostos orgânicos pode estar seguindo os mecanismos para C rizodepositado, apresentados por Rasse et al (2005), proporcionando ligeiro aumento em relação aos demais sistemas.…”

“…Assim, somente com o aporte contínuo de resíduos orgânicos pelos sistemas de culturas é que há a manutenção e as maiores proporções dessa fração no solo (Baldock & Skjemstad, 2000;Balesdent et al, 2000;Six et al, 2002a;. As raízes também podem ter interferido nessa variação na proporção de MO particulada no solo entre as camadas, principalmente em sistemas com gramíneas, uma vez que há grande volume de raízes que permanecem nas primeiras camadas do solo (Balesdent & Balabane, 1996;Rasse et al, 2005).…”

Soil aggregate fraction-based 14C analysis and its application in the study of soil organic carbon turnover under forests of different ages

“…While the vegetation type is considered to be a primary factor for different plots having different SOC turnover times, the sources for the soil organic matter must also play an important role. Compared with above-ground litterfall, root debris input forms a greater contribution to the formation of stable soil carbon pool [65][66][67][68][69]. Therefore, the root-to-shoot ratio of plant debris incorporated into the soil is an essential controlling factor for the formation and stabilization of soil organic matter [70].…”