Feature Photo by Roman Synkevych on Unsplash

Why we built it

Soil respiration (aka soil mineralizable carbon) is a measurement that consists of rewetting air-dried soil, a short-term incubation (1-3 days), and then measuring the CO₂ emitted by soil microorganisms during the incubation period. Soil respiration is considered a strong indicator of soil health, affecting the carbon and nutrient cycling capacity of the soil, and relating strongly to microbial activity. Due to its value as a soil health indicator, soil respiration is a recommended component of soil health assessments by both the USDA and the Soil Health Institute.

Our Sci became interested in new technologies to scale up soil respiration measurements while operating the Bionutrient Institute (BI) lab from 2018-2022. Our goal in operating the BI lab was to identify low-cost lab methods, enabling organizations to analyze crop and soil samples at sufficient scale to better understand the links between crop management, soil health, and nutritional outcomes in crops. We explored the available options for testing soil respiration, sending samples to commercial laboratories and soil respiration kits. Unfortunately, we found that these options were prohibitively expensive to run at a scale of hundreds to thousands of samples, with commercial labs charging $25-30 per sample and DIY kits like Solvita requiring consumable costs of $15+ per sample, not to mention the up-front costs of purchasing the kits.

We decided to develop the SAVR Kit (Soil Assessment via Respiration) to lower per sample costs and enable higher throughput. The SAVR kit includes numerous features to reduce the variability of respiration measurements and improve data comparability.

• The kit includes an incubator to ensure a consistent incubation temperature.
• Soil addition uses a 15-cc volume scoop and an integrated scale measures the weight of the soil and calculates the amount of water required.
• An integrated water pump precisely adds the amount of water required to the sample.

How it works

A detailed SOP for the SAVR kit is available here. In short:

1.      We use 300 ml syringes with modified valves as incubation vessels.

2.      We add a constant volume of soil (15-cc) to a one-ounce plastic cup, which sits on the syringes’ rubber piston. The piston is held upright in a 3D printed cassette with space for 7 syringes. These cassettes allow for easy transfer of soil samples into and out of the incubator and between each measurement.

3.      The kit’s integrated scale measures the weight of the soil and calculates the amount of water needed to reach 55% water-filled pore space. That amount of water is then automatically pumped onto the soil in the plastic cup.

4. Once the water is added, the syringe tube is placed over the sample, enclosing it inside the syringe and a baseline CO₂ reading is taken. A motor automatically forces air out of the syringe and into the measurement chamber. After approximately 45 seconds the CO₂ concentration in the chamber will be within 99% of the CO₂ concentration in the syringe and the readings will be recorded using SurveyStack’s hardware integration.

5.      After baseline sampling, the valve is closed, sealing the syringe and the samples are placed in the kit’s incubator. The kit’s incubator is fabricated out of a low-cost insulated food carrier with heating and cooling elements added so that the soils can be incubated at temperatures between from 20℃ to 35℃, depending on the measurement protocol, to ensure consistent incubation conditions.

6.      After the prescribed incubation period (in the BI lab we used a 24 hour incubation) the syringes are removed from the incubator and measured again.

7.      SurveyStack scripts calculate the final CO₂ evolution from the soil in µg C per g soil, correcting for the actual incubation time and soil weight, and push the results to SurveyStack database, where they are available for download or via the API.

How well it works

When designing and validating a new instrument, a lot of testing and iteration is involved. A detailed write-up of the many experiments, repeatability tests, and validation work is available here and here. Once we completed testing of the individual components of the SAVR kit and finalized a measurement process, we used soils from the BI lab to evaluate how well SAVR helps to analyze a large set of soil samples.

We measured more than 500 soil samples, from 0-10 and 10-20 cm depths, that were submitted by BI grower partners from a wide range of environments, soil types, and management practices. The study was a large-scale observational study, making traditional statistical tests such as analysis of variance (ANOVA) difficult. However, when plotting soil respiration against total organic C (LOI-C), soil respiration increased more rapidly when using the carbon building practices of no-till and cover-cropping compared to not using cover crops or practicing tillage (see below).

Whats next

We are currently finalizing some improvements to make device shipping and assembly easier, kits will be available soon! Additionally, we are looking to identify partners to conduct broader testing across different use cases. If you are interested in the SAVR kit, please contact us at info@our-sci.net.