Inspection Services Blog

Research Update: Biochar as a soil amendment

Note: This is part of a Research Update series that highlights projects funded by the California Department of Food and Agriculture (CDFA) Fertilizer Research and Education Program (FREP) annual grant program.

Project Title: Soil biochar amendment to improve nitrogen and water management

Project Leaders: Suduan Gao and Dong Wang (United States Department of Agriculture, Agricultural Research Service, San Joaquin Valley Agricultural Sciences Center)

Project Status: Complete

Overview: This FREP-funded project highlights the effects of biochar on adsorption capacity for ammonium and nitrate, nitrogen (N) transformation rates, and soil water retention. This project also investigated the effects of various biochar application rates and irrigation rates on crop response and N fate under field conditions in California’s Central Valley.

Background: Biochar is a carbon-rich material produced under high temperature and limited or no oxygen, which has been shown to improve soil quality and mitigate some environmental contamination. However, observed biochar effects across studies are highly variable, with many showing no significant reductions or even increases in N loss to the environment.

The principal investigators (PI) of this project hypothesized that adsorption is one of the important mechanisms to increase N retention and plant-available N in the soil. Adsorption is the process by which a solid, in this case biochar, holds onto ions or molecules of a gas, liquid, or solute. Thus, increasing adsorption of N molecules could increase N uptake or nitrogen use efficiency (NUE) and reduce leaching and gas emissions. This project was designed to examine the mechanisms by which biochar interacts with N and investigate the effects on plant growth and the fate of N fertilizers under field conditions.

ApproachPIs conducted both laboratory and field studies to characterize biochar products made from seven feed stocks, determine adsorption capacity for major mineral N species onto biochar, and evaluate effects of biochar on N transformations in the soil. Biochar products with high adoption potential were selected and tested in two field experiments (onion and processing tomato) with varying levels of irrigation, amendments (biochar and manure), and application rates.

Application of biochar to the experimental plots (left) and onion field trial (right). Photo credit: Dr. Suduan Gao.

Results: Laboratory data showed that biochar has some capacity to adsorb ammonium (little for nitrate), but the ability to retain N was insignificant (<10% total N applied) because of pH effects and quick transformation of ammonium to nitrate. Data from the onion field experiment indicated that there were no significant biochar or irrigation (between 75% and 100% levels) treatment effects on total ammonia volatilization. Soil pore water N data showed that nitrate accumulation highly depended on irrigation level during the growing season and the accumulated nitrate was mostly leached by winter precipitation (Figure 1).

Figure 1. Nitrate concentration in soil pore water collected from two soil depths during onion growing season.

Field trials provided some evidence that irrigation, soil depth, and irrigation × soil depth interaction had significant impact on the soil N data (data not shown in this blog) but biochar and any interactions with biochar had no significant effects for both 2016 and 2017 growing seasons. Due to large variations in the data, statistical analysis did not show any significant effects of irrigation, biochar, or their interaction on nitrate leaching (Figure 2).

Figure 2. Nitrate leachate captured from the onion field from different irrigation levels and biochar treatments. The collection period was from 5 January ‒ 7 September 2018. Error bars are standard deviation of the mean (n=3).

Biochar treatment had no significant effect on fresh onion bulb yield but irrigation and interaction with biochar significantly affected the yield (Table 1). In 2016, the yield at 50% irrigation level, regardless of biochar treatment, was significantly lower than those at 75% and 100% irrigation with no significant differences between the 75% and 100% irrigation levels. In 2017, irrigation with biochar treatment showed a similar trend, but the 50% irrigation showed a higher yield than biochar treatments. The high-biochar treatment at 100% irrigation level produced significantly higher yield than some of the treatments at 75% irrigation level.

Table 1. Fresh onion bulb yield for the first two growing seasons (2016 and 2017).

† Means following by different letters in the same column indicate significant differences (p<0.05).

In the processing tomato field trial, biochar had no significant effects on crop yield, biomass, N uptake, ammonia volatilization, and N leaching loss. N Fertilization required to produce the regional average yield of 110 Mg ha-1 (49 tons/acre) for tomatoes was 263 kg N ha-1 (235 lbs. N /acre) at 70% NUE. According to the PIs, fertilization rates should be based on target yield, soil storage, and NUE. The data suggest that treatment effects may be larger following application of biochar amendments, but the effects diminish over time.

Conclusions: Based on both lab and field studies, soil amendments with biochar did not show any direct link to improving yield, increasing N uptake or use efficiency, or reducing environmental losses such as ammonia volatilization or nitrate leaching. However, the PIs recommend that biochar can be promoted as a conservation practice to replace agricultural burning to sequester carbon and sustain soil productivity. They also mention that current commercial biochar products may be too expensive for growers to buy; however, several in-situ, low-cost methods could be explored.

For additional information about the California Department of Food and Agriculture’s FREP-funded projects on research and education regarding the agronomically safe and environmentally sound use of fertilizer in California, please visit: for more information about the annual FREP Grant Program. for details about current and completed FREP-funded projects, as well as a searchable database that aims to make the research available, understandable, and convenient for growers to implement.

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