The application of biochar (biomass-derived black carbon) to soil has been shown to improve crop yields, but the reasons for this are often not clearly demonstrated. Here, we studied the effect of a single application of 0, 8 and 20 t ha

Black carbon (BC) is an important pool of the global C cycle, because it cycles much more slowly than others and may even be managed for C sequestration. Using stable isotope techniques, we investigated the fate of BC applied to a savanna Oxisol in Colombia at rates of 0, 11.6, 23.2 and 116.1 t BC ha À1 , as well as its effect on non-BC soil organic C. During the rainy seasons of 2005 and 2006, soil respiration was measured using soda lime traps, particulate and dissolved organic C (POC and DOC) moving by saturated flow was sampled continuously at 0.15 and 0.3 m, and soil was sampled to 2.0 m. Black C was found below the application depth of 0-0.1 m in the 0.15-0.3 m depth interval, with migration rates of 52.4 AE 14.5, 51.8 AE 18.5 and 378.7 AE 196.9 kg C ha À1 yr À1 ( AE SE) where 11.6, 23.2 and 116.1 t BC ha À1, respectively, had been applied. Over 2 years after application, 2.2% of BC applied at 23.2 t BC ha À1 was lost by respiration, and an even smaller fraction of 1% was mobilized by percolating water. Carbon from BC moved to a greater extent as DOC than POC. The largest flux of BC from the field (20-53% of applied BC) was not accounted for by our measurements and is assumed to have occurred by surface runoff during intense rain events. Black C caused a 189% increase in aboveground biomass production measured 5 months after application (2.4-4.5 t additional dry biomass ha À1 where BC was applied), and this resulted in greater amounts of non-BC being respired, leached and found in soil for the duration of the experiment. These increases can be quantitatively explained by estimates of greater belowground net primary productivity with BC addition.

Nutrient leaching in highly weathered tropical soils often poses a challenge for crop production. We investigated the eff ects of applying 20 t ha −1 biochar (BC) to a Colombian savanna Oxisol on soil hydrology and nutrient leaching in fi eld experiments. Measurements were made over the third and fourth years after a single BC application. Nutrient contents in the soil solution were measured under one maize and one soybean crop each year that were routinely fertilized with mineral fertilizers. Leaching by unsaturated water fl ux was calculated using soil solution sampled with suction cup lysimeters and water fl ux estimates generated by the model HYDRUS 1-D. No signifi cant diff erence (p > 0.05) was observed in surface-saturated hydraulic conductivity or soil water retention curves, resulting in no relevant changes in water percolation after BC additions in the studied soils. However, due to diff erences in soil solution concentrations, leaching of inorganic N, Ca, Mg, and K measured up to a depth of 0.6 m increased (p < 0.05), whereas P leaching decreased, and leaching of all nutrients (except P) at a depth of 1.2 m was signifi cantly reduced with BC application. Changes in leaching at 2.0 m depth with BC additions were about one order of magnitude lower than at other depths, except for P. Biochar applications increased soil solution concentrations and downward movement of nutrients in the root zone and decreased leaching of Ca, Mg, and Sr at 1.2 m, possibly by a combination of retention and crop nutrient uptake.

No abstract

Black carbon (BC), the residue from burning with insufficient oxygen supply, is assumed to be very stable in the environment. Here we present a simple model for BC movement and decomposition in soils based on the assumption that BC consists of two fractions with different turnover time, and that BC can move in the environment as well as decompose. Decomposition rate was calibrated against laboratory data, whilst a recent field experiment was used to calibrate losses from downward movement through the soil profile. Losses by erosion are still poorly quantified, but mass balance indicates that they may be one of the most important fluxes. The model was able to acceptably predict CO 2 production from BC as well as BC left in the soil at the end of the experiment, although BC in the subsoil was underestimated. The model was sensitive to erosion rate (varied ±50%), moisture and temperature response function on a 100-year time scale. The model was not sensitive to the decomposition rate of the stable pool on a 100 year time scale, but it was very sensitive to that on a millennial time scale. Implications and directions for future research are discussed.

Soils of the lowland tropics in the central Brazilian Amazon are generally highly leached, acidic and nutrient-poor. Charcoal, combined with other soil amendments, might improve fertility but this, in turn, could lead to increased weed problems for agricultural production. This experiment was conducted to assess weed pressure and species composition on plots receiving various inorganic and organic soil amendments, including charcoal. Additions of inorganic fertilizer, compost and chicken manure resulted in increases in weed ground cover of 40, 22 and 53%, respectively, and increases in species richness of 20, 48, and 63%, respectively. When chicken manure was applied, dominance by a few weed species was reduced, such that different species were more evenly represented. Although charcoal additions alone did not significantly affect weed ground cover or species richness, a synergistic effect occurred when both charcoal and inorganic fertilizers were applied. The percentage ground cover of weeds was 45% within plots receiving inorganic fertilizer, 2% within plots receiving charcoal and 66% within plots receiving both amendments. Improvements in the fertility of nutrient-poor soils of the tropics might increase weed pressure and make the development of effective weed management strategies more critical. These effects on weed populations were observed nearly 2.5 years after the addition of charcoal, chicken manure and compost, and > 1 year after the last application of inorganic fertilizer.

No abstract

Field trials were conducted on Amazonian Dark Earth soils in the Manaus region, Amazonas, Brazil to assess the composition and impact of weedy vegetation on maize yield. Soil fertility among the Dark Earth varied considerably with differences largely attributable to past-use history. Consequently, maize yield and weed pressure varied among field locations, reflecting these differences in soil fertility in addition to differences in weed reservoirs such as seedbanks. Maize yield in weeded plots was as much as 63 times greater on Dark Earth (0.55 t ha À1) than on corresponding adjacent soil (0 t ha À1 ), and location averages varied from 0 to 3.15 t ha À1 for Dark Earth. The percentage ground cover of weeds in weedy plots was up to 45 times greater on Dark Earth (65-99%) than on corresponding adjacent soil (2-89%), and species richness was up to 11 times greater on Dark Earth (4-14 species) than corresponding adjacent soil (1-8 species). The relative proportion of annual and leguminous weeds was 32 and 17% greater, respectively, on Dark Earth than adjacent soil, and vegetative sprouting of plants was more common on sites that had been used less intensively in the past. In general, a similar weed community was observed on the different Dark Earth sites, including many species typically associated with environments that have been highly disturbed by human activities, such as Cyperus spp., Phyllantus niuri, and Croton lobatus. Seedlings from a greater number of species emerged from forested Dark Earth seedbanks (2.1 per flat) than from forested adjacent soil (1.2 per flat). The total number of emerged seedlings was greater for Dark Earth seedbanks (9.1 per flat, 1,365 m À2 ) than adjacent soil (2.2 per flat, 330 m À2), however the species observed were not likely to be problematic for cropping. #