PhD

The study entitled ―Exploration on the links between soil carbon storage
and root biomass and elucidation of drivers of carbon stabilization‖ was
conducted at the Department of Soil Science and Agricultural Chemistry, College
of Agriculture, Vellayani during November 2019 to September 2021 with the
objective to study the links between soil carbon storage and root biomass in soils
of different agro ecological units and to identify the key drivers of C stabilization
and NP fluxes under different management practices.
The study area comprised of three Agro ecological units (AEUs) of
Southern Kerala viz. Southern and Central Foot Hills (AEU 12), Southern High
Hills (AEU 14) and Kumily High Hills (AEU 16). The study was carried out in
three parts namely exploration on the links between soil organic C and NP pools
with root biomass in soils of different AEUs, assessment of carbon storage under
different land use system and identifying the drivers of C stabilization and field
experiments to study the effect of management practices on the link between root
and shoot biomass C and SOC and NP pools.
For the study exploration on the links between soil organic C and NP
pools with root biomass in soils of different AEUs, the study area was surveyed
and geocoded soil samples from 0-20 cm and 20-60 cm depth were collected
using core samplers. The root biomass from the soil samples were separated out
and weighed. The soil samples were analyzed for its various physical, chemical
and biological properties. For assessment of carbon storage under different land
use system and identifying the drivers of C stabilization, the most prominent land
use system of each AEU was identified and five samples were collected from each
system. The sampling size was one sq.m to a depth of 60 cm. The plants of the
same area were uprooted and their shoot and root biomass were recorded. Both
the soil and plant samples were collected and analysed for various parameters.
The field experiment in split plot design on grain cowpea – fodder maize
cropping sequence was laid out with the main plot treatments as m1: conventional
tillage, m2: deep tillage (30 cm depth) and m3: no till and sub plot treatments as s1:
POP recommendation, s2: soil test based POP, s3: organic nutrient management
(TOF-F), s4: POP + AMF, s5: soil test based POP + AMF, s6: TOF-F + AMF and
s7: absolute control. After the harvest of grain cowpea, shoot biomass were
removed and roots were retained in three replications and in the other three
replications total biomass of grain cowpea were added into the soil and left for
decomposition. After that fodder maize was raised in the field and the crop and
soil samples were collected and analysed for various parameters.
The results of the Part I revealed that the physical properties like bulk
density (BD) and gravel per cent of all the AEUs showed an increase towards
depth while the electrochemical properties showed a decrease. Among the
different AEUs, AEU 16 recorded lowest BD (1.22 Mg m-3
) and gravel per cent
(30.53 %) and had a subsoil increase of 12 per cent and 17 per cent for BD and
gravel per cent respectively. The different fractions of soil C and N showed a
decrease with depth for all AEUs. The soil total organic carbon (TOC-5.94 %)
and recalcitrant C (RC-1.64 %) content were highest for AEU 14 with a decrease
of 26 per cent and 31 per cent respectively for subsoil. The highest dissolved
organic C (DOC-54.63 mg kg -1
) and labile C (LC- 877.50 mg kg -1
) content were
for AEU 16 with a subsoil decrease of 45 per cent and 27 per cent respectively.
AEU 12 recorded lower values for C fractions which may be due to decreased
root biomass by 38 per cent and 25 per cent in surface soil and 55 per cent and 70
per cent in subsoil than that of AEU 14 and AEU 16 respectively. The root
biomass and soil C fractions were positively and significantly correlated at both
sampling depths. The highest correlation coefficients between root biomass and
soil C fractions were recorded by DOC (0.976) followed by RC (0.931) and LC
(0.975) followed by DOC (0.953) for surface and subsoil respectively. From the
regression analysis perfect fit towards linear regression model, expressed as R2
value, was highest for DOC (0.95) and LC (0.94) at sampling depths of 0-20 cm
and 20-60 cm respectively.
The different fractions of N were highest for AEU 12 and surface soil
showed an increase in total nitrogen (TN) by 6 per cent and NH4-N by 20 per
cent, NO3 – N by 18 per cent and organic N (ON) by 5 per cent than subsoil. For
soil P fractions an increase was observed with depth and AEU 12 recorded highest
values for P fractions. Among soil N and P fractions, ON and labile P (LP) were
found to be more correlated to root biomass and with higher R2
values at both
sampling depths. The MBC (26.89 mg kg -1
) and DHA (34.94 µg TPF g-1
24 hr-1
)
were highest for AEU 16 and surface soil showed an increase in MBC by 28 per
cent and DHA by 30 per cent, than subsoil.
For part II, the most prominent land use system of each AEU were
identified as rubber plantations for AEU 12 and AEU 14 and cardamom
plantations for AEU 16. The rubber plantations of AEU 14 recorded highest C
storage (434.0 t ha-1
) and lowest value was observed for cardamom plantations of
AEU 16 (329.9 t ha-1
). The soil physical properties and electrochemical properties
behaved similar to that of Part I. Cardamom plantations of AEU 16 recorded
lowest BD (0.97 Mg m-3
) and gravel content (28 %) while AEU 12 had highest
pH (5.61) and lowest EC (0.39 dS m-1
). Among the different land use systems,
rubber plantations of AEU 14 recorded highest values for soil TOC (6.72 %) and
DOC (55.16 mg kg-1
) content while cardamom plantations had highest soil LC
(910.91 mg kg-1
) and surface soil RC (1.92 %) content but subsoil RC content was
more for rubber plantations of AEU 14. In rubber plantations the root biomass
were correlated to all C fractions and more correlated to RC and TOC and in
cardamom plantations root biomass were significantly correlated to TOC (0.98)
and DOC (0.95) fractions only. A significant and positive correlation between
root lignin and soil C fractions (RC and TOC) was also observed.
The different fractions of N and P were highest for cardamom plantations
of AEU 16 and surface soil showed an increase in TN by 5 per cent, NH4-N by 14
per cent, NO3– N by 22 per cent and ON by 4 per cent than subsoil and a subsoil
increase of TP by 12 per cent, LP by 29 per cent and NLP by 11 per cent were
also observed. The shoot biomass were more correlated to soil N and P fractions
than root biomass and were more correlated to ON and TN and to TP and NLP
among soil N and P fractions respectively. A significant positive correlation
between N and P removal and soil NP pools were also obtained. The MBC and
DHA were highest for cardamom plantations of AEU 16 and surface soil showed
an increase in MBC by 25 per cent and DHA by 23 per cent than subsoil.
In the field experiment, among the various nutrient management
treatments, soil test based POP + AMF (s5) recorded the highest plant height,
shoot biomass and grain yield plant-1
(107.70 g) and TOF-F + AMF (s6) showed
highest values for root characteristics and quality parameters for grain cowpea.
Similarly for fodder maize grown under both conditions, the treatment soil test
based POP + AMF (s5) gave highest shoot biomass, fodder yield and quality
parameters while highest root biomass were recorded by the treatment, TOF-F +
AMF (s6). Among the tillage levels, the no till treatment (m3) performed best in
connection with growth, yield and quality characteristics throughout the cropping
period.
Tillage and nutrient management had significantly influenced various soil
properties. The lowest soil BD and higher WSA per cent and soil pH were
reported by the treatment TOF-F + AMF (s6) throughout the cropping sequence.
Among tillage levels, deep tillage (m2) remained superior for soil BD and pH and
no till treatment (m3) for WSA per cent respectively. The treatment, TOF-F +
AMF (s6) remained superior for soil C fractions viz., TOC, LC and RC content,
mineralizable N fractions (NH4-N and NO3-N), labile P and MBC content and
dehydrogenase activity throughout the cropping sequence. The treatment, soil test
based POP +AMF (s5) recorded higher values for NP fractions like TN, ON, TP
and non labile P (NLP). Among the tillage levels, the no till treatment (m3)
remained superior in connection with soil chemical and biological properties
especially towards the end of cropping period.
As the cropping sequence advances an improvement in soil physical,
chemical and biological properties were observed and this is mainly attributed to
the crop residue addition of grain cowpea and more improvement was observed
for total residue incorporation than root residue alone addition.
The soil C pools were highly linked to root biomass and NP pools to shoot
biomass. The root biomass and root lignin were the main drivers of C
stabilization. The treatments with AMF remained superior in various soil
properties and yield and growth attributes emphasizing the favourable role of
AMF in C storage and nutrient cycling in soils. With regard to nutrient
management, soil test based POP + AMF recorded the highest yield in cropping
sequence while organic nutrition (TOF-F) + AMF contributed more to soil
properties indicating the need for further research on nutrient translocation and
assimilation under organic nutrition. The no tilled condition with total residue
incorporation responded better than root residue alone incorporation, hinting to
the fact that more organic matter contributing practices improved the
physicochemical and biological conditions of soils favourably.