by Joey Blankenship
In 2018, we set up a pot-scale experiment in the greenhouse that involved mixing UA Campus Agricultural Center (CAC) soil and manure with different application rates of Pacific Biochar simulating 8, 17, 33, 50, and 66 tons per acre in each pot. Pacific Biochar was chosen because this biochar had the 2nd highest water-holding capacity while also being relatively affordable ($162 per ton compared to $1500 per ton from Black Owl Biochar). We also included a microbially inoculated biochar that was 10% compost (by weight) in half of the pots. Our results showed that the 33 tons per acre application rate (4% biochar by soil weight) was sufficient to achieve our goal for increased soil water retention. We continue to monitor soil moisture in the pots at least twice a week and add water when moisture levels become too dry.
How will the pot-scale patterns compare to the scale of a field? How will crops respond to the presence of biochar in the soil?
Our planned wheat field experiment at the UA Campus Agricultural Center will begin in October 2019. We have allotted plot space to the inoculation experiment (see below) and will have 36 plots total (12 ft by 15 ft). The design involves treatments with biochar only (4% Pacific Biochar), microbially activated biochar (4% Pacific Biochar with inoculant to promote plant growth), and control plots without biochar or inoculant. There will be four replicates of all treatments randomly assigned to plots. Once the biochar is tilled into the soil and the Sonoran White Wheat seeds have germinated, we will set up our flood irrigation technology (flow meter, Netafim piping, pressure column). The plots will be arranged so that one-third of the plots will receive 100% of the normal irrigation rate (replicated across biochar treatments), one third will receive 75% of the normal irrigation rate, and one-third will receive 50% of the normal irrigation rate. The different irrigation rates will show us how a crop commonly farmed in the Sonoran Desert (i.e., wheat) responds to decreased irrigation in the presence of biochar, and whether augmenting the soil microbiome can help maintain crop production despite decreased irrigation. We will use Decagon moisture sensors and dataloggers to monitor soil moisture at a depth of 4 inches in each plot. With additional funding for a more extensive soil sensor network, we can monitor the moisture at more than one location on each plot and at multiple soil depths. We will record crop height throughout the growing season and measure the yield and kernel mass at the end of the growing season. We will run this experiment for multiple growing seasons to observe the effects of the single biochar application over time. Might the benefits of biochar increase over time rather than diminish over time as with compost and
other common soil organic amendments? The laboratory-scale inoculation study (see below) will help us narrow down the most effective inoculant to use before we test the microbially activated biochar under field conditions in Fall 2019.
How much fertilizer is retained in soil with different rates of biochar addition?
The scientific literature explains that biochar can increase the effectiveness of fertilizer applications by reducing the run-off of fertilizer as well as having a slow-release effect, allowing more time for plants to take up nutrients. Additionally, Dos Cabezas vineyard (see below) is interested in improving their P availability for their wine grapes and would like to quantify how biochar and phosphorus act together in soil. To answer these questions with desert agricultural soil, we added fertilizer to the greenhouse experiment described above to observe the nitrogen (N) and phosphorus (P) losses from the soil system through gaseous emissions and water leaching. We chose to use diammonium phosphate (DAP) because we aim to quantify the N and P bioavailability without adding additional carbon, which most organic fertilizers contain. The additional carbon can affect nutrient retention, which is good. However, we want to see how the biochar itself affects nutrient retention. We added DAP at the rate of 200 kg N/ha, which is roughly the total amount that a conventional farmer would add DAP to a wheat field during one growing season. We are using the Gasmet FTIR Gas Analyzer to measure gas fluxes (CO2, CH4, N2O, NO, NH3, H2O) that come from the soil, and we are collecting the water that leaches through the soil. We have a microplate spectrophotometer that we will use for measuring N and P in the collected water samples. Half of the columns contain a very small amount of compost that we added in Fall 2018 as an analog for microbially activated biochar.
Although the gas measurements are only partially complete, we are eager to see if the pots with a small amount of compost show different patterns than biochar alone. Once the gas measurements are finished, we will remove the soil from the pots to quantify effects of biochar on total C, N, and P, pH, salinity, microbial activity (using substrate-induced respiration), and surface area. We may also use an Environmental Scanning Electron Microscope to visualize the surface of the biochar particles after they have aged in soil with and without compost. The scientific literature describes different organic coatings that form on the surface of biochar over time due to different environmental conditions.
Methods for quantifying phosphorus in high pH soil are not fully established. How will we measure phosphorus in the soil solution water with a microplate spectrophotometer?
We are working to develop an accurate, consistent, and high throughput method for quantifying inorganic P in order to measure baseline conditions at all field sites, measure the differences in soil P between different biochar application rates in the greenhouse pot experiment, and to measure the effects that the inoculants have on soil P. Several methods
exist for neutral and acidic soils; the methods must be altered for high pH soils. Also, while replicability is necessary; minor chemical interferences can produce a wide variety of errors. The Duval Ecology Lab at New Mexico Tech has shared their draft SOP that we are using and amending as necessary.
It is important in the future to have a straightforward way of measuring soil P, both inorganic and organically bound. With an ICP-OES, which is in our recent budget request to TAB AG, we would have a straightforward way of accurately measuring P as well as accounting for micro nutrients. The ICP-OES nutrient and heavy metal analyzer can help us to ensure that there is no heavy metal contamination in the biochar we produce together in the future, which is an essential component for quality control with biochar products.
What is the most effective way to inoculate biochar with phosphorus-solubilizing bacteria?
We know from the scientific literature that certain microorganisms can unlock P that is already present in soil but unavailable to plants. The specific microbes are called phosphorus solubilizing bacteria (PSB). We want to inoculate biochar with PSB. We plan to add PSB species isolated from compost (e.g., Pseudomonas putida, Rahnella aquatilis, etc.) and cyanobacteria (i.e., algae) isolated from Sonoran Desert soils. By using species that are already adapted to a hot desert climate, we hope to find innovative microbial inoculants that are likely to survive and thrive in crop fields in the Desert Southwest. Additional benefits of cyanobacteria, besides unlocking P, are that they fix N naturally (i.e., consistent biological fertilizer from N2 in the atmosphere) and they produce organic glues that help bind soil particles together to resist water and wind erosion.
Will biochar boost the effectiveness of nutrient-releasing microorganisms in desert soil?
The porous structure of biochar is the ideal habitat for soil microorganisms. If microorganisms can thrive inside biochar, their population may increase. This increased microbial activity has the potential to mineralize nutrients for plants to utilize. During the next few months (March June 2019), we plan to test different inoculants in the lab, primarily using Pacific Biochar and
CAC soil. By mid-summer, we will know which inoculant we want to use in the wheat field based on which inoculant actively produces plant-available P in the soil as well as maintains an adequate microbial population for at least 3 months after inoculating soil.
Does biochar increase soil microbial activity at the field-scale?
The second exciting field-scale study is located at the Dos Cabezas Wine Works grape vineyard in Sonoita, AZ. The vineyard has three distinct soil types in close proximity, thus promoting research development on biochar products that work across a wide variety of soils. The different soil textures (clay loam and two sandy loams that differ in carbonate content) will
allow us to monitor the effects of biochar on soil functioning and crop health and productivity. On March 20, 2019, we will be amending the vineyard soil with two rates of Pacific Biochar addition: 0.75 tons/acre and 1.5 tons/acre. These are the recommended application rates from California vineyard owners that show effectiveness while remaining affordable. We will monitor soil moisture using remote sensors for several years after the biochar application to observe patterns of water retention after precipitation events and scheduled drip irrigation. We will monitor microbial activity using substrate-induced respiration every 2 months and nutrient release patterns for inorganic N and P using buried ion-exchange resin strips. An aim of the vineyard owner is to increase the P availability in the vineyard soil and be able to apply a P fertilizer without losing the nutrients to run-off. The vineyard owner’s desire to maximize P accessibility aligns with our research goal of using biochar to maximize fertilizer efficiency.
A pot-scale study was run in the greenhouse during Summer 2018 that compared different biochar and mulch feedstocks at a rate of 4% (by weight) using soil from an abandoned cropland at the North Altar Watershed Area near Three Points, AZ. Greenhouse gas and NH3 fluxes and soil moisture were recorded for 3 months. We are currently characterizing physical and chemical properties of various “homemade” biochars (pyrolyzed using our 3-gallon reactor) and mulches used in this experiment, and other commercially available biochars. The homemade biochars include mesquite wood, buffelgrass, Tank’s Green Stuff decorative woodchips, and unprocessed yard trimmings from Tank’s Green Stuff pyrolyzed at 500 °C. The commercially available biochars include Black Owl Premium Biochar, Black Owl Environmental Ultra Biochar (which held the most water), Royal Oak Biochar, True Char Biochar, Soil Solutions Biochar, and Pacific Biochar. Our objective is to create a database of biochar properties— especially for large-scale feedstock sources in the Desert Southwest—that includes salinity, total C and N, pH, specific surface area, bulk density, and water-holding capacity.