The amount of soil carbon added today as Biochar may increase by the same amount again six years from now
Parameters:
From December 2019 the research site was a sloping silty clay loam soil in southwestern Iowa IA (41°18′29″N, 95°10′19″W). There is a mean annual precipitation of 939 mm and mean annual temperature of 9.3°C. The dominant soil series were Exira silty clay loam (fine-silty, mixed, superactive, mesic Typic Hapludolls; <7% slope) and Marshall silty clay loam (fine-silty, mixed, superactive, mesic Typic Hapludolls; 1% slope).
Biochar was applied in late October 2011 at an average rate of 9.3 Mg/ha, dryweight. It was incorporated to 15 cm soil depth by chiseling followed by disking. The biochar was derived from mixed wood (Quercus, Ulmus, and Carya spp. woodchips with particle sizes 0.1–2,000 mm) and produced using an augur bed gasification process at 600°C (ICM, Inc.). The biochar had a pH of 8.8, and consisted of 29% ash, 16% volatile matter, 55% fixed C, 63% total C, 2.7% total H, 0.6% total N, 0.06% total P, and 0.86% total K on a dry-weight basis.
What happened?
This study has found that the amount of biochar added today can cause as much soil carbon again to form under the normal soil conditions. This finding is contrary to the previous thinking that the amount of soil carbon added in this way would decrease over time.
The paper, which is approximating the amount of stable C in biochar at 60%-80%, describes increase in soil C through 'direct' and 'indirect' effects. Where the direct effect is the stable biochar added and the indirect effect is the reduction in mineralization of the soil and/or fresh crop residues, a process known as negative priming.
Some reminders of priming
Positive priming: when biochar accelerates the decomposition of soil (reduced long-term C accumulation).
Negative priming: Biochar could promote C accumulation through absoption and physical protection of organic C, inducing the negative priming effect. It can increase with an increase in soil water, biochar pyrolysis temperature and soil clay content.
The largest changes were recorded at 0-5cm depth, where biochar infused soil organic carbon increased from 24 g/kg to 29.68 g/kg where Corn was grown, 23.12 g/kg where Switchgrass was grown and 26.09 g/kg to 38.76 g/kg where Low-diversity grass was grown between 2011 and 2017.
At 5-15cm soil carbon carbon increased from 12.90 g/kg to 18.32 g/kg where Corn was grown, 19.50 to 20.25 where Switchgrass was grown and 16.9 to 21.48 where Low-diversity grass was grown in the same time period.
At 15-30cm soil carbon increase from 8.24 g/kg to 9.19 g/kg where Corn was grown, 15.56 g/kg to 17.72 g./kg where Switchgrass was grown and 14. 63 g/kg to 15.61 g/kg where Low-diversity grass was grown in the same time period.
At 30-60cm depth that the trend does not continue.
Key point
The initial values contain both soil carbon that was occurring in the soil and Biochar added soil. So, the increase compared to Biochar added is nearly double that added after 6 years (14.07 Mg soil C/ha vs. 7.25 Mg biochar C/ha)
Other changes
At 9.3 Mg/ha biochar did not affect bulk density, MWD of water-stable aggregates, columetric water content, or plant available water or water infiltration at any depth.
The biochar added and perennial grass crops didn't chance N concentration, soil PH or CEC. Perennial bioenergy crops had a significant effect on soil aggregate stability, water retention and plant available water.
Sources:
Paper by Humberto Blanco-Canqui (1), David A. Laird (2), Emily A.Heaton (2), Samuel Rathke (3) and Barat Sharm Acharya
Department support from:
(1) Department of Agronomy and
Horticulture, University of Nebraska,
Lincoln, NE, USA
(2) Agronomy Department, Iowa State
University, Ames, IA, USA
(3) Department of Environmental Science,
University of Arizona, Tucson, AZ, USA