Long Term Fertilization Impact On Soil Organic Carbon, Crop Yield and CH4 and CO2 Production in a Tropical Aeric Endoaquept Planted to Rice-Rice Cropping System Under Flooded Conditions.

Yield decline or stagnation in long-term rice growing systems under flooded conditions and its relation to soil organic carbon (SOC) fraction dynamics and gaseous-C losses are not well-explored. In a field study, impact of long-term fertilization to a tropical aeric endoaquept under continuous rice growing in a rice-rice cropping system under flooded conditions on SOC fractions and soil aggregates size distribution was investigated. The experiment consisted of intensive rice cropping for 39 years in a rice-rice-fallow sequence with unfertilized (control), compost (5 Mg.ha^-1.y^-1), compost (5 Mg.ha^-1.y^-1) + N (80 kg.ha^-1) and compost (5 Mg.ha^-1.y^-1) + inorganic fertilizer (NPK @80:40:40 kg.ha^-1) as treatments. At a soil depth of 0-20 cm, highest TOC was found under compost + NPK treatment (0.82%) followed by compost (0.75%), compost + N (0.72%), and control (0.64%) in that order. Irrespective of treatments, small macro-aggregates (0.25-2 mm) dominated aggregate size distribution (42-63%), followed by micro-aggregates (0.053-0.25 mm) at 16-39%. Active C fractions, such as microbial biomass carbon (MBC) was significantly higher under compost treatment (206.6 µg C g^-1 of soil) followed by compost + NPK treatment. On the contrary, readily mineralizable carbon (RMC) was highest under compost + N treatment. In an incubation study spread over 90 days, CO₂ and CH₄ productions were significantly higher at high temperature (45⁰C) than at lower temperature (35⁰C) and increased with moisture content of soils from 40% WHC to submergence. CO₂ production increased with days of incubation up to 90 days (0.14 to 1.23 mg CO₂-C g^-1 dry soil), whereas, CH₄ production was highest at 40 days (27.7 ng CH₄-C g^-1 dry soil) and decreased thereafter. Among the various treatments, significantly higher gaseous-C production was noticed in compost + NPK treatment, in spite of it supporting higher yield.