Biochar has mixed effects on prairie restoration

Background/Question/Methods

Biochar is a carbon-rich material that is similar to charcoal that is produced when biomass is burned in the absence of oxygen. There are many potential benefits in applying biochar to agricultural soils, such as increased carbon sequestration, improved soil fertility, and increased yield. Biochar, however, is typically fine-textured and up to 50% of the material can be translocated during application. Natural systems, particularly grassland remnants in agricultural regions, are likely to be exposed to biochar if use becomes wide spread and there are few studies of biochar effects in natural systems.   We evaluated this potential scenario by using biochar in a prairie restoration in the Loess Hills region of Iowa.  We established 15 4-m² treatment plots in a former brome-dominated grassland.  Within each of five replicate blocks we randomly assigned biochar treatments [0 kg m^-2, 1.43 kg m^-2 (1% of soil volume to 10 cm) and 4.25 kg m^-2 (3% of soil volume to 10 cm)].  The biochar (Royal Oak) was produced from wood biomass. Following biochar incorporation we planted a seed mix containing 30 tall grass prairie species. We analyzed the 3 years of plant and soil data using a repeated mixed linear effects model.

Results/Conclusions

Biochar had limited effect on the plant and soil communities in this restoration and in some cases, there appears to be threshold between 1 and 3%.  Soil total carbon (C) increased with biochar (416.0 ± 20, 643.2 ± 63, 848.1 ± 10.2 g m^-2; p<0.001), there was no change in total soil nitrogen (N). Soil pH was significantly more alkaline in the 3% biochar treatment (7.93 ± 0.03, 7.88 ± 0.07, 8.04 ± 0.06; p<0.01). Gravimetric water content (%) and soil nutrients (Mehlich III extraction) P, K, Ca, and Mn, plant P, K, C:N composition, and peak aboveground and belowground biomass did not change with biochar. Root colonization by mycorrhizal fungi was reduced by the biochar in the first year only. The 3% biochar treatment significantly increased plant species richness (8.9 ± 2.1, 8.4 ± 1.6, 12.5 ± 1.3; p<0.01) and the proportion of peak aboveground biomass that was planted (10.1 ± 5%, 10.9 ± 4.7%, 24.6 ± 9.8%; p<0.01). In this experiment we find that biochar is relatively neutral to the development the prairie community, while also increasing soil carbon sequestration.  Biochar qualities, however, vary considerably between products and more testing on perennial communities is necessary before widespread application.