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USDA-ARS Study Delves into Relationship Between Biochar, Soil and Crop Growth

December 16, 2011 |


Biochar has been around for thousands of years. Pre-Columbian Amazonian natives first created it by burning their agricultural waste in pits that they covered with soil. It is believed that they intentionally used the resulting biochar to increase soil productivity in constrained or infertile soil.

Today, the USDA’s Agricultural Research Service is studying how the use of biochar – which can be produced from wood or plant material as well as manure – can be used to enhance soil fertility and remediate contaminated soils.

In progress across multiple locations, initial indications are that adding biochar to agricultural soils could make soils more fertile and improve nutrient and water retention. It can also be a long-term, underground storehouse of carbon. Slow decomposition of carbon in biochar underground makes only a minimal contribution to carbon dioxide emissions, according to an ARS news article.

ARS’s biochar field research began back in November, 2007 when National Laboratory for Agriculture and the Environment (NLAE) scientists in Ames, Iowa began the first of six multi-year field studies at sites around the country. Their aim was to assess how biochar affects soil quality and the productivity of corn crops.

The NLAE team added biochar derived from hardwood biomass to 24 plots of corn – a total of nearly 8 acres. Twelve plots received nearly 8,800 pounds of biochar per acre and 12 received nearly 16,000 pounds per acre. No significant difference in either crop yield or soil quality was seen from either.

Other ARS field and lab studies in Idaho, Kentucky, Minnesota, South Carolina and Texas showed that biochar made from hardwood biomass could improve the structure of soils and increase the ability of sandy soils to retain water, but soil fertility results were more variable.

The results confirmed that biochar characteristics vary widely depending on the feedstock used to make it, the amount of time it spends in the pyrolyzer (essentially, an industrial strength kiln that heats agricultural waste, woody biomass, and other organic matter to elevated temperatures in the absence of oxygen in order to produce biochar), the temperature used during pyrolysis process, the moisture content of the biomass feedstock and other factors.

There are a host of other variables that ARS researchers are trying to sort through and better understand. Differences in structure, particle size, surface area, porosity, pH and the availability of biologically active compounds can all affect results, the ARS researchers have found.

“Now we’re studying how crops respond to soils that have been amended with biochar made from corn stover,” NLAE researcher Doug Karlen stated. “We didn’t see a significant response when we amended an acre with 8 tons of biochar made from hardwood, so now we’re amending fields with as much as 50 tons of corn stover biochar per acre.”

“We need to make sure that the biochar will actually improve the condition of the soil where it is being used,” said Jeff Novak, who coordinates ARS’s multi-location study. “We want to ensure that the correct biochar is applied to the right soil so that we avoid decreasing soil quality.”

Designer Biochars

Novak and colleagues at the ARS Coastal Plains Soil, Water, and Plant Research Center in Florence, South Carolina are working with other scientists to manufacture “designer biochars” that have properties tailored to remediate specific soil deficiencies.

Novak led a lab study to investigate the characteristics of different biochars and find out which ones could improve the sandy soils found on the Carolina coastal plain and the silt loams (soil made up of sand, silt and clay-sized particles) of the Pacific Northwest, which are derived from volcanic ash and windblown sediment known as loess.

The team produced biochars from peanut hulls, pecan shells, poultry litter, switchgrass and hardwood waste products. They pyrolyzed these materials by burning them in low oxygen at different temperatures to produce nine different types of “designer biochars.” These were then mixed into one type of sandy soil and two silt loam soils at a ratio of about 20 tons per acre. The soils were leached with water every month.

After four months, the team found that biochar produced from switchgrass and hardwoods increased soil moisture storage in all three soils, but biochar made from the other biomass sources did not. The soils with the greatest moisture content turned out to be those amended with switchgrass biochar that had been produced via high-temperature pyrolysis, which had soil moisture contents between 3%-6% higher than a control soil sample. The switchgrass biochar also reduced soil acidity. Biochar made from poultry litter resulted in greatly increased levels of phosphorous and sodium in the soils.

The soils treated with switchgrass biochar could retain water an additional 1 to 3.6 days for a soybean crop in Florence, SC, and could increase soil availability for crops grown in the Pacific Northwest’s silt loam soils between 0.4-2.5 days.

Biochar and Manure

Researchers at the Animal Waste Research Unit in Bowling Green, Kentucky are continuing to research, produce and evaluate “designer biochars,” testing varieties of biochar and animal waste in the region’s karst limestone soils, which tend to possess alkaline. They’re studying how the combination of biochar and poultry manure is affected by microorganisms and nutrients in the manure, and how the combination affects biochar’s efficiency in improving soil quality and corn yield. They’re also studying the potential for the biochar process to mitigate the emission of greenhouse gases, including methane and nitrous oxide, as well as carbon dioxide.

ARS researchers in Idaho are looking into how three different soil amendments – biochar, manure and a biochar-manure mix – affect soil quality and crop growth in the region’s calcareous soils, which, like karst limestone soils are rich in calcium carbonate.

Results after a first year of study showed no real improvement in nutrient levels aside from an increase in manganese, an essential plant nutrient, and a slight increase in total organic carbon. Soils amended with manure showed the same.

Combining manure and biochar enhanced their effects. The total increase in manganese was greater in soils where a mix of the two had been applied than that which would have been obtained by just adding the manganese increase from biochar to the manganese increase from manure.

Alarmingly, the fields amended with biochar showed a 31% crop yield decrease, along with a 33% reduction in nitrogen uptake. Sulfur uptake was reduced by 7%. The third year trial will help determine if the 2010 results were just a fluke or if they warrant further investigation.

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