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A study entitled “Effect of long term fertilization on soil carbon dynamics in rice-rice
cropping sequence of Onattukara” was carried out with the objective of studying the effect of
long term fertilization on soil carbon dynamics and to explain the relationship between different
active pools of soil organic carbon and rice yield. The study was conducted at the permanent
manurial trial (PMT) plot at Onattukara Regional Agricultural Research Station, Kayamkulam,
during 2019-21. The experiment comprised of field experiment during Virippu and Mundakan
season, soil analysis and computation of various indices like Carbon pool index (CPI), Carbon
lability index (CLI), Carbon management index (CMI), sensitivity index (SI), critical carbon
input, carbon budgeting and plant analysis.
Field experiment comprised of 8 treatments viz., T 1 (80 kg N ha -1 as FYM), T 2 (80 kg N
ha -1 as Ammonium Sulphate), T 3 (80 kg N ha -1 as A.S + 40 kg P 2 O 5 ha -1 as Rajphos), T 4 (80 kg
N ha -1 as A.S + 40 kg K 2 O ha -1 as MOP), T 5 (40 kg P 2 O 5 ha -1 as Rajphos + 40 kg K 2 O ha -1 as
MOP), T 6 (80 kg N ha -1 as A.S + 40 kg P 2 O 5 ha -1 as Rajphos + 40 kg K 2 O ha -1 as MOP), T 7 (80
kg N ha -1 (20 kg as FYM and 60 kg as A.S.) + 40 kg P 2 O 5 ha -1 as Rajphos + 40 kg K 2 O ha -1 as
MOP) and T 8 (Absolute control) replicated thrice using rice variety Jaya. Treatment receiving 80
kg N ha -1 (20 kg as FYM and 60 kg as A.S.) + 40 kg P 2 O 5 ha -1 as Rajphos + 40 kg K 2 O ha -1 as
MOP (T 7 ) recorded the highest growth and yield. Soil samples were drawn from 3 different
depths at 15 cm interval viz., 0-15 cm, 15-30 cm and 30-45 cm and analysed for physical,
chemical and biological properties. There was no textural variation and the texture of the soil
remained as sandy loam in all the treatments during both the seasons. An improved physical
condition was noticed in T 7 with low BD (1.32 Mg C ha -1 ), high WHC (33%) and hydraulic
conductivity (1.25 cm h -1 ). The soil acidity varied from very strongly acidic to strongly acidic.
The highest content of available N, P, K and S was recorded in T 7 during both the seasons, while
T 1 (FYM) recorded the highest content of Ca and Mg. The highest content of Zn, Cu and B were
recorded in T 7 whereas Fe and Mn recorded the highest values under T 2 (Ammonium sulphate).
Deficiencies of Ca, Mg, S and B were recorded in all the treatments. Generally all the parameters
showed a decreasing trend with increasing depth except BD and pH. The highest nutrient content
in grain and straw were also recorded in T 7 . Highest BC ratio (1.61 and 1.64) was recorded in T 7
during both the seasons.
244SOC, SMBC, SMBN, SMBP, WSC, water soluble carbohydrates and dehydrogenase
activity were the highest under T 7 . T 7 recorded the highest TOC, very labile, labile, less labile
and non labile C fractions and in all the treatments active C pool dominated the passive C pool.
The contribution of active C pool to TOC accounted for 55-66%. Computed indices viz., CPI
(1.56), CLI (1.34) and CMI (209.04) were found to be the highest in T 7 . Sensitivity Index (SI)
was worked out to determine the sensitivity of each fraction to management practices and
observed that active pool, SOC, water soluble carbon and water soluble carbohydrates were the
most sensitive fractions while TOC and passive C pool were the least sensitive to management
practices. Critical carbon input to maintain the initial level of SOC stock in soil was worked out.
All the treatments recorded a positive critical carbon input which indicates that additional C has
to be given inorder to sustain the present level of SOC stock. Among the treatments T 7 recorded
the highest critical carbon input (0.470 Mg C ha -1 ) and it was due to initial high SOC stock.
Carbon budgeting was done by computing SOC stock, C build up per cent, C build up rate and C
sequestered and were the highest in T 7 followed by T 1 . Significant positive correlation was
observed between different C pools, C pools and nutrient availability, C pools and biological
activity and C pools and yield.
Present study provided an insight on how management practices affect soil C buildup and
C sequestration in rice-rice cropping sequence of Onattukara. Integrated nutrient management
(T 7 ) with application of 20 kg N ha -1 as FYM and 60 kg N ha -1 as Ammonium sulphate along
with 40 kg P 2 O 5 ha -1 as Rajphos and 40 kg K 2 O ha -1 as MOP significantly improved TOC, C
fractions, CPI, CLI, CMI, C buildup rate and yield. Application of FYM @ 8t ha -1 (T 1 ) also
improved the above mentioned parameters but the BC ratio was less (1.19 and 1.21) compared to
T 7 . Increased soil acidity and deficiencies of secondary and micronutrients were also observed in
all the treatments under study. Hence along with integrated nutrient management practices, soil
test based fertilizer and lime application including secondary and micronutrients should be given
importance to sustain the soil fertility in rice-rice cropping sequence of Onattukara.