MSc.

The present study entitled “Sequestration of carbon as influenced by nutrient management practices under long term fertilizer experiments” was undertaken at RARS, Pattambi and College of Horticulture, Vellanikkara. The soil samples were collected from the plots of Permanent Manurial Trial (PMT) and AICRP on long term fertilizer experiments (LTFE) at Pattambi after the harvest of rice (Virippu crop), 2016. New Permanent Manurial Trial with dwarf indica rice variety, was started in Virippu, 1973. The AICRP on Long term fertilizer experiment was initiated in Mundakan 1997. The two field experiments where being conducted continuously in the same fields, the present Virippu crops of these experiments being in their, 89th and 39th season respectively. The objectives were set out to study the effect of long term application of fertilizers and manures on soil carbon pools, thermal stability of stored carbon and growth characteristics of plant.
The PMT consists of 8 treatments viz.,T1: 90 kg N ha-1 as cattle manure, T2: 90 kg N ha-1 as green leaf manure, T3: 45 kg N ha-1 as cattle manure + 45 kg N ha-1 as green leaf manure, T4: 90 kg N ha-1 as ammonium sulphate, T5: 45 kg N ha-1 as cattle manure + N:P2O5:K2O 45:45:45 kg ha -1, T6: 45 kg N ha-1 as green leaf manure + N:P2O5:K2O 45:45:45 kg ha -1, T7: 22.5 kg N ha-1 as green leaf manure + 22.5 kg N ha-1 as cattle manure + N:P2O5:K2O 45:45:45 kg ha -1 and T8: N:P2O5:K2O 90:45:45kg ha -1. The LTFE consists of 12 treatments viz.,T1: 50 per cent NPK (as per KAU POP recommendation), T2 : 100 per cent NPK, T3 : 150 per cent NPK, T4 : 100 per cent NPK + 600 kg ha -1 CaCO3, T5 : 100 per cent NPK, T6 : 100 per cent NP, T7 : 100 per cent N, T8: 100 per cent NPK + FYM @ 5 t ha -1to the virippu crop only, T9: 50 per cent NPK + FYM @ 5 t ha -1,T10 : 100 per cent NPK + in situ growing of Sesbania aculeata (for Virippu crop only), T11 : 50 per cent NPK + in situ growing of Sesbania aculeata (for Virippu crop only) and T12 : Absolute control (No fertilizers or manures).
In PMT, the organic nutrient management (T1) and Integrated Nutrient Management (INM) practice (T5) were equally superior in growth and productivity of the crop. However the dehydrogenase activity and microbial biomass carbon was significantly influenced by the INM practice. The physiological properties of the plant like stomatal conductance and photosynthetic rate, yield attributes such as panicle length, number of panicles per plant and number of filled grains corroborated the trend in yield. The available primary nutrient status of the post harvest soil was higher in T8 (N: P2O5: K2O 90:45:45kg ha -1). However the substitution of 50 per cent mineral N by organic manures under INM treatments improved the available nutrient status suggesting the synergistic role of organic manures.
In LTFE, the integrated nutrient package (T8) had significantly higher dehydrogenase activity and microbial biomass carbon in post harvest soil which was positively reflected on yield and yield attributes. Application of FYM had beneficial role over green manures towards organic carbon build up in soil.
In LTFE with 20 years history, it was seen that all the carbon pools (active, slow and passive) contributed towards yield whereas in PMT, with 44 years history, it was the slow pool of carbon that showed higher correlation with crop yield. Data on analysis of different carbon pools revealed that slow pool is the most predominant yield determining pool in the long run. The percentage contribution of carbon pool towards total soil organic carbon in paddy soil of the present work can be rated as passive (54%) > slow (36%) > active (10%).
An incubation study was conducted at four different temperature regimes (15, 25, 35 and 450C) using the soil collected from the plots of LTFE as well as PMT. The activation energy and the rate constants provided a good insight on decomposability of organic matter. In general, all the treatments in PMT, except T5 (25°C), T6 (15°C) and T8 (35 and 45°C) had comparable amounts of carbon decomposition. Thermal stability studies indicated that the rate of reaction decreased with increase in temperature due to the faster exhaustion of the substrates in initial days of decomposition at higher temperatures. Q10 values were also less than one in both the experiments due to the exhaustion of labile pool available for microbial decomposition. Treatments with inorganics recorded lowest activation energies indicating the instability of even recalcitrant or passive pools.
It could be summarized from the results that, the carbon build up in soil was significantly higher under organic nutrient management. But the per cent share of slow carbon pool (Mean Residence Time: 20-40 years) to total carbon was higher under INM. Moreover, the thermal stability of carbon was found to be remarkably higher under INM than sole application of organic manures or inorganic fertilizers. The study fortifies INM as a stable practice to sustain soil organic carbon and crop productivity in the context of rising temperatures.
Chemical and physical stability of carbon and its threshold carbon concept in soils of Kerala need to be explored.