MSc

An investigation entitled “Soil carbon dynamics in a rice based cropping system” was carried out with the objective of assessing and comparing the soil carbon dynamics in different rice based cropping systems viz., rice-rice-maize (R- R-M), rice-rice-bhindi (R-R-B) and rice-rice-daincha (R-R-D) in a lowland riverine alluvium. The investigation was undertaken at College of Agriculture, Vellayani and Integrated Farming System Research Station (IFSRS), Karamana during 2017-
19. At IFSRS, rice based cropping sequence viz., rice (Virippu) - rice (Mundakan)
- maize+ bhindi+ daincha has been practicing since 2011-12. The study was undertaken during three seasons of 2018-19 as three experiments.
For experiments 1 (Virippu) and 2 (Mundakan), medium duration rice variety, Uma was cultivated. Surface soil (0-15 cm) samples were collected after each crop harvest and analyzed for soil physical, chemical and biological properties and carbon fractions. The results were compared with that of rice- rice- fallow (R- R-F) sequence. All the carbon fractions (total organic carbon [TOC], particulate organic carbon [POC] - >53 and <53 µm) except labile carbon [LC] slightly increased after Mundakan crop. On an average, TOC and POC increased by 5.9 and
5.24 per cent, respectively. There was not much variation in soil bulk density (BD), EC, available K, Ca, Mg, S, Fe, B and microbial count during both the seasons. The pH slightly increased after Mundakan, with the highest value for R-R-F (5.14) and R-R-B (5.22) sequences during Virippu and Mundakan, respectively. Available N, P, Mn, Zn, Cu, microbial biomass carbon (MBC) and enzyme activities (dehydrogenase, urease and acid phosphatase) declined nominally after Mundakan season. The lowest enzyme activities were recorded in R-R-F sequence, during both the seasons.
Both paddy grain and straw were analyzed for major, secondary and micronutrients. R-R-D sequence had recorded the highest grain N (1.98 per cent) content after Virippu. K content was also highest for R-R-D sequence, after both Virippu (1.09) and Mundakan (1.42) seasons. Highest P and Ca contents were obtained with R-R-M and R-R-B sequence, respectively, after both the seasons.


Micronutrient content of grain and straw were almost the same in all the systems, after both Virippu and Mundakan, Economic yield was highest for R-R-B (5.21 t ha-1) and R-R-D (4.09 t ha-1) sequence after Virippu and Mundakan, respectively.
During the summer season (experiment 3), field was divided into three equal parts and planted with maize (var. Co 6), bhindi (var. Manjima) and daincha (var. TN local), respectively. Daincha at 50 per cent flowering stage and maize leaves after crop harvest were incorporated into the field. Soil samples were drawn at 15 cm intervals up to 105 cm depth and analysed for major soil parameters and carbon fractions. Plant samples were analysed for major, secondary and micro nutrients. Economic and biomass yield of R-R-B and R-R-M sequence were 9.21 and 25.1 t ha-1 and 3.6 and 7.89 t ha-1, respectively. Biomass yield of R-R-D sequence was
22.5 t ha-1.

The cropping systems significantly influenced soil carbon dynamics. R-R- D sequence recorded 10, 14, 13 and 22 per cent increase in TOC, LC, POC and MBC, respectively, up to 105 cm depth compared to that of R-R-F sequence at the end of cropping cycle. Enzyme activities (dehydrogenase, urease and acid phosphatase) showed significantly higher positive correlation with the carbon fractions. Among the weather parameters, atmospheric temperature (both minimum and maximum) was found to be positively correlated with soil carbon dynamics. The role of crop residue incorporation in improving soil carbon dynamics was clearly understood from the carbon management index (CMI) value, which was highest in R-R-D (120.56) followed by R-R-M (109.31), R-R-F (106.79) and R-R- B (72.81).
Except BD and available Fe and Mn, all the other soil parameters showed a declining trend with soil depth. BD ranged from 1.19 to 1.62, 1.21 to 1.73, 1.17 to
1.72 and 1.2 to 1.65 g cm-3 for R-R-B, R-R-M, R-R-D and R-R-F systems,
respectively. Available Fe and Mn increased upto 30 cm depth and thereafter declined. R-R-D system resulted in the highest soil available N, K, Ca, Mg, S, B, MBC, urease, acid phosphatase, TOC, LC and POC upto 30 cm depth. pH and EC increased by 9.3 and 77 per cent, respectively in R-R-M system. Available P and


Ca in the surface layer declined in R-R-B and R-R-M systems compared to R-R-F. Available Fe and Mn contents were reduced in the all systems except R-R-F. The R-R-B system had the highest available Zn and Cu compared to R-R-F (8 and 13 per cent, respectively). Microbial load, enzyme activities and MBC were negligible beyond 75 cm depth. Daincha recorded the highest N (1.47) and P (1.13) content, while the highest K (1.96), Ca (0.85) and Mg (0.21 per cent) were in bhindi.
The study revealed that, among the different rice based cropping systems, R-R-D system influenced soil carbon dynamics most significantly followed by R- R-M, thereby indicating the role of crop residue incorporation particularly, green manures in improving the soil carbon sequestration potential and consequently soil health.