Tuesday, November 3, 2009
Convention Center, Exhibit Hall BC, Second Floor
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.