Charcoal in Amazonian paddy soil—Nutrient availability, rice growth and methane emissions

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.