The effect of biochar on the growth of agricultural weed species
Biochar, a carbon-rich residue similar to charcoal, has been proposed as a soil amendment to improve soil quality and increase crop yields while simultaneously mitigating climate change by the sequestration of carbon. The beneficial effect of biochar on crops may extend to weed species and, although it is well known that weeds reduce crop yields, there is little published research on the effect of biochar on agricultural weed species. In a series of greenhouse and growth chamber experiments, three questions were addressed. First, how does nitrogen interact with biochar produced from a single feedstock to affect weeds? Second, how do differences in biochar feedstock affect root growth and root system architecture? Finally, how do differences in biochar feedstocks affect weed and crop growth? In the first experiment, three common weed species, barnyardgrass (Echinochloa crus-galli L. Beauv.), large crabgrass (Digitaria sanguinalis L. Scop.), and redroot pigweed (Amaranthus retroflexus L.), were grown to maturity under greenhouse conditions using a factorial design with biochar (0 and 2% of the soil dry weight) and nitrogen (0 and 14 g N m-2) treatments. Nitrogen increased barnyardgrass and redroot pigweed total dry weight and large crabgrass panicle dry weight. Biochar increased barnyardgrass height by 22% and total dry weight by 47% but did not affect root : shoot biomass partitioning. Biochar reduced redroot pigweed height by 30% but increased branch dry weight by 95%. Finally, biochar increased large crabgrass shoot dry weight by 34% but reduced root dry weight 30% suggesting that biochar allowed large crabgrass to partition more biomass to shoots than roots. In the second experiment, we examined the effects of two types of biochar on large crabgrass root system architecture using a rhizobox mesocosm. Root growth of large crabgrass varied with the type of biochar used; however, biochar did not affect total plant dry weight. The high-nutrient biochar increased above-ground dry weight and the low-nutrient biochar increased below-ground dry weight when compared to plants grown in the unamended soil. When given a choice between unamended and biochar-amended soil, large crabgrass roots grew preferentially in the biochar-amended soil, regardless of biochar type. In the final experiment, we examined the effect of two types of biochar on the growth of two crop and two weed species grown to maturity under greenhouse conditions. Biochar increased the growth of both crop species suggesting that the incorporation of biochar, especially high-nutrient biochar, into temperate agricultural soils may increase crop yields. However, biochar also increased the growth of both weed species, which may complicate current weed management practices. Overall, this research suggests that biochar has the potential to alter root system architecture and to increase the growth of common weed species. Biochar may therefore exacerbate weed problems in agricultural systems.
Gibson, Purdue University.
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