average, growth period is considered. However, during

average,
the total amount of carbon translocated into the soil by cereal and pasture
plants is approximately the same (1500 kg C/ha), if the same growth period is
considered. However, during one vegetative period, cereals and grasses allocated
below the ground are about 1500 and 2200 kg C/ha respectively (Kuzyakov and
Domanski, 2000). This accounted for nearly 5 to 21% of all photosynthetically
fixed carbon transferred to the rhizosphere through root exudates and ranged
from 20 to 50% of plant biomass.

Rhizosphere
priming effect

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Microbes
play a major role in association with rhizosphere mass and their interaction
within and outside accelerate mineralization process in enhancing soil nutrient
status. Acceleration or retardation of decomposition of soil organic carbon
caused by root activity is termed rhizosphere priming effect (Merino et al.,
2015).

An
increase in atmospheric CO2 increases photosynthesis, hence root
exudation (Drigo et al., 2008). Carbon input from plant to soil through root exudation
is one of the major sources of available carbon for microorganisms (Luo et al.,
2014). Exudates from living roots stimulate a quick response of soil microbes
with acceleration of native soil organic C mineralization, the so-called rhizosphere
priming effect. Soil microbial activities are driven primarily by readily
available or labile C provided by root turnover and root exudates influxes
(Dijkstra et al., 2013).

Root
biomass and soil carbon

All
the organic carbon found in the soil is primarily plant derived. The two main
sources of carbon in the soil are: (1) accumulation of soil organic matter due
to the humification after plant death and (2) root exudates and other
root-borne organic substances released into the rhizosphere during plant growth
as well as sloughing of root hairs and fine roots by root elongation. The first
mode of carbon sequestration is well documented (Kumar et al., 2006), but
carbon sequestration by plant roots is still under investigation. CO2
fixed by crop plants and its translocation into the roots is a simultaneous
process. Figure in the annexure reveals the pathway of
carbon transfer from biosphere towards the rhizosphere (Paul and Clark, 1996).

Carbon
is added in the soil system by plant roots through root death, root exudates
and root respiration. It is difficult to quantify the contribution of these
three separately. Metabolically active respiring roots are responsible for
exudation and respired CO2. However, non-metabolically active roots
release carbon in its soluble form, which is termed as lysis. The mechanism of
root death provides a greater insight into the understanding of carbon
sequestration into the soil (Jones, 1998).

There are three states
of root death:

(1) Non-apoptotic
death, where metabolic activity immediately ceases and all the carbon and
nutrients from the roots enter into the soil (e.g. in mechanically damaged
roots).

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