Microwave and millimeter wave signals reflectance of soil carbon content

Agricultural and forestry biomass can be converted to biochar through pyrolysis gasification, making it a significant carbon source for soil. Applying biochar to soil is a carbon-negative process that helps combat climate change, sustain soil biodiversity, and regulate water cycling. However, quantifying soil carbon content conventionally is time-consuming, labor-intensive, imprecise, and expensive, making it difficult to accurately measure in-field soil carbon's effect on storage water and nutrients. To address this challenge, for the first time, we report on extensive lab tests demonstrating non-intrusive methods for sensing soil carbon and related smart biochar applications, such as differentiating between biochar types from various biomass feedstock species, monitoring soil moisture, and biochar water retention capacity using portable microwave and millimeter wave sensors and machine learning. The datasets provide details on the microwave and millimeter wave reflectance signals. We validated our quantification method using supervised machine learning algorithms by collecting real soil mixed with known biochar contents in the field.

Methods
The microwave sensor operated at a sampling frequency of 10.24 GHz, and each sample was measured 10 times to mitigate random error. Similarly, the millimeter-wave sensor, functioning within the 62 GHz to 69 GHz frequency range, replicated the microwave sensor's sampling instances to diminish error probabilities. We established 150 sampling points within the specified frequency band, setting the resolution bandwidth (RBW) at 100 kHz to balance sweep time efficiency and noise reduction. Additionally, samples were consistently positioned less than 2 centimeters beneath the direct millimeter-wave radar sensor for measurement consistency.

Soil samples were obtained from almond orchards and carefully filtered to remove large particles like stones. The biochar used was derived from a variety of sources, including 100% orchard prunings, walnut shells, pure wood, almond shells, and pinewood, designated as samples 1 through 5, respectively. In subsequent experiments, the soil was evenly spread to a weight of 50 grams, and the biochar was allocated at 2 grams, constituting 20% of the soil's weight. The materials were thoroughly combined prior to testing. For the millimeter-wave experimental setup, we prepared new soil and biochar mixtures with variable ratios ranging from 0% to 100% biochar content.

Additionally, the microwave data encapsulated a total of 8192 attributes per signal, whereas the millimeter-wave data comprised 1024 attributes.