We investigated the effects of charcoal under flooded (anoxic) rice cultivation at low and high fertilizer levels during 2 y in the Maranhão lowlands, eastern periphery of Amazonia. Two applications (at onset of first and second year) of 15 Mg ha–1 of fine (< 2 mm) charcoal derived from the endocarp of the babassu (Attalea speciosa Mart.) palm nut had little influence on soil fertility, rice growth, yield, and nutritional status. Exception to this were negative impacts of charcoal on first‐year N availability, with lower sub‐superficial soil NH$ _4^+ $ availability paired with lower rice tissue N and a responsiveness of grain yields to (mainly N‐) fertilization following charcoal application. This N‐limitation effect was, however, limited to the first year and—though statistically significant—without agronomic relevance. The most consistent charcoal effect on flooded‐soil fertility was the strong increase in K availability in the second year, at low and to a lesser extent at intermediate, but not at high fertilizer level. Low K concentrations of our charcoal exclude the possibility of direct K inputs via charcoal, suggesting other indirect mechanisms for K availability increases. Methane fluxes in the second year were significantly reduced (–43.8%) by charcoal application, charcoal‐induced reductions were stronger under high‐ (–47.3%) than under low‐fertilizer regime (–26.0%). Thus, charcoal could be a valuable tool for reducing methane emissions associated with intensely fertilized flooded rice, without significantly affecting grain yields.

No abstract

Combining existing traditions of rice and of fish in an integrated rice−fish (IRF) culture is a promising strategy for sustainably increasing land productivity and diminishing the need for external inputs in smallholder agriculture of Amazonia. This study evaluates the potential of IRF for weed control in irrigated rice production. It was conducted from August 18 to December 5, 2008 in the Maranhão lowlands in the eastern periphery of Amazonia. We compared weed communities in four 3-year-old IRF and four adjacent ‘conventional’ irrigated rice (CIR) fields at 20 and 40 days after transplanting (DAT), at the mid-vegetative stage and at the onset of flowering, which served as an indicator of potential grain yield. Rice–fish fields contained differing mixtures of herbivore and omnivore fish species totaling 4000 fish per ha or 1.7 fish per m3. Total weed density was reduced in the IRF system, particularly early in the season, the most critical stage for rice development. The integration of fish into irrigated rice cultivation affected weed species composition, with fish-weeding preferentially reducing monocotyledonous Cyperaceae, one of the more aggressive and problematic weed families in this region. Monocot weed density was negatively correlated with rice aboveground biomass at 40 DAT rice. Although floristic similarity between IRF and CIR fields was low, the impacts of IRF on weed species diversity and weed species richness were not significant. Thus, IRF was not associated with a simplification of the weed community. We conclude that fish-weeding may substitute for manual or chemical weeding in irrigated rice agriculture, an important consideration for resource-poor smallholder agriculture in environmentally sensitive riverine or delta areas of eastern Amazonia.