Plant–Soil Bioelectrochemical System-Based Crop Growth Environment Monitoring System

Abstract

This study presents the design and implementation of a crop environmental monitoring system powered by a plant–soil bioelectrochemical energy source. The system integrates a Cu–Zn electrode power unit, a boost converter, a supercapacitor-based energy management module, and a wireless sensing node for real-time monitoring of environmental parameters. Unlike conventional plant microbial fuel cells (PMFCs), the output current originates partly from the galvanic effect of Cu–Zn electrodes and is further regulated by rhizosphere conditions and microbial activity. Under the optimal external load (900 Ω), the system achieved a maximum output power of 0.477 mW, corresponding to a power density of 0.304 mW·cm⁻². Stability tests showed that with the boost converter and supercapacitor, the system maintained a stable operating voltage sufficient to power the sensing node. Soil moisture strongly influenced performance, with higher water content increasing power by about 35%. Theoretical calculations indicated that Zn corrosion alone would limit the anode lifetime to ~66 days; however, stable output during the experimental period suggests contributions from plant–microbe interactions. Overall, this work demonstrates a feasible self-powered crop monitoring system and provides new evidence for the potential of plant–soil bioelectrochemical power sources in low-power applications.

1. Introduction

Agriculture, as a fundamental sector of the worldwide economy, is closely linked to food security, agricultural supply, energy efficiency, and ecological protection. Precise monitoring of crop growth environments has become essential for improving yield, ensuring quality, and optimizing cultivation strategies. By collecting real-time data on temperature, humidity, light intensity, soil water content, and pH, agricultural managers can plan farming activities scientifically, create optimal growth conditions, improve productivity and economic benefits, while reducing pest risks and resource waste.

As shown in Figure 1, a typical agricultural monitoring system consists of sensors, data acquisition and processing units, wireless communication modules, and a power supply unit. Most current systems rely on either batteries or external power: the former suffer from limited lifetime and high maintenance costs, while the latter are constrained by wiring complexity and geographic conditions, and any outage interrupts monitoring. In remote areas, large farmlands, or greenhouse clusters, frequent battery replacement or wiring installation is labor-intensive, costly, and may delay agricultural decision-making.

Plant microbial fuel cells (PMFCs), a branch of microbial fuel cells (MFCs), have attracted growing attention for their potential in green energy and environmental monitoring [1,2,3]. In a typical PMFC, root exudates serve as organic substrates oxidized by electroactive microbes at the anaerobic anode, while electrons are transferred through an external circuit to the air cathode for oxygen reduction, thereby achieving bioenergy conversion [4,5,6]. Previous studies have shown that PMFCs hold promise in wastewater treatment, environmental sensing, and powering low-power IoT devices [7,8,9].

However, conventional PMFCs commonly employ carbon felt or graphite electrodes, which involve high cost, low current density, and complex coupling with energy management circuits [10,11]. To overcome these limitations, some studies have explored metal electrodes (e.g., Cu, Zn) to simplify electrode fabrication and enhance initial output voltage [12,13]. Such systems are sometimes referred to as plant–soil galvanic systems. Their output primarily relies on the galvanic effect of the metal electrodes, but it may also be influenced by the rhizosphere environment and soil microbial activity. Nevertheless, systematic studies on energy harvesting, long-term stability, and power management of such systems remain scarce.

Existing research mainly focuses on the principles and laboratory-scale performance of PMFCs [3,14,15], whereas studies on Cu–Zn electrode systems under long-term operation, soil moisture sensitivity, and integration with boost converters and storage modules are still lacking. These gaps limit their application in low-power scenarios.

Based on this, the objectives of this study are: (1) to construct a crop monitoring system powered by a Cu–Zn plant–soil bioelectrochemical energy source; (2) to evaluate its output performance and stability under different soil moisture conditions; (3) to estimate Zn anode consumption through theoretical calculations and explore the coupling between galvanic effects and plant–microbe interactions; and (4) to compare the results with existing PMFC studies to assess the potential of this system for low-power applications.

2. Materials and Methods

2.1. System Architecture

The system was designed in a modular fashion, consisting of an energy harvesting and management module, a control and data processing module, an environmental sensing module, and a wireless communication module (Figure 2). The energy module included a Cu–Zn plant–soil bioelectrochemical power source, a low-voltage startup boost converter, a supercapacitor storage unit, and a voltage monitoring circuit. The control and processing module was based on an ultra-low-power microcontroller (MSP430FR5969 LaunchPad, Texas Instruments, Dallas, TX, USA), responsible for energy scheduling, sensor operation, data caching, and duty cycle management. The sensing module employed a digital temperature–humidity sensor (SHT30, Sensirion AG, Staefa, Switzerland) connected via I²C and encapsulated for waterproofing to ensure long-term stability in field conditions. The wireless module was a low-power Bluetooth unit (HC-08, sampling rate 1 Hz, JinEr Electronic, Shenzhen, China), enabling short-range data transmission and fast sleep/wake switching to minimize standby consumption.

2.2. System Implementation

The system followed a “power generation–energy management–control–sensing/communication” architecture (Figure 3).

Hardware: The Cu–Zn electrode system and plant platform were assembled, followed by construction of the energy circuit using an LTC3108 boost converter (Analog Devices, Norwood, MA, USA) and a supercapacitor for harvesting and storage. The SHT30 sensor was interfaced to the MCU via I²C, while the HC-08 Bluetooth module was connected via UART. The MSP430FR5969LP served as the core controller for voltage monitoring, task scheduling, data acquisition, and transmission.

Software: A modular design was adopted, integrating an energy threshold detection and scheduling strategy. The MCU remained in deep sleep and was awakened only when the capacitor voltage reached a preset threshold, after which data acquisition and transmission were executed before returning to sleep.

2.3. Experimental Setup and Materials

Electrodes: The anode was Zn wire (0.5 mm diameter, 1 m length, 99.9% purity), and the cathode was Cu wire (0.5 mm, 1 m, 99.9%). Electrodes were arranged in parallel at 15 cm spacing, fixed by insulated supports. The Zn anode was buried 2 cm deep and the Cu cathode 20 cm (Figure 4).

Plant: Potted rose (Rosa chinensis) purchased locally was cultivated for four weeks before use. Roses were chosen due to their active root system, stable growth in pots, and continuous root exudation.

Soil: Horticultural substrate (pH 6.8, conductivity 0.35 mS/cm) was air-dried and sieved before use. Soil moisture was maintained within preset ranges (see Section 2.5).

Environment: Experiments were conducted at 25 ± 2 °C, under natural light (12 h light/12 h dark), with ambient humidity at 60 ± 5%.

2.4. Energy Management Circuit

The energy management module linked the Cu–Zn source to system components, boosting and stabilizing low, fluctuating voltage outputs for reliable supply (Figure 5). It employed an LTC3108, capable of startup at 20 mV, with integrated boost conversion, multiple outputs (VOUT, VSTORE, VSYS), and power allocation logic.

A 1:100 step-up transformer was coupled at the input with a 220 µF capacitor for filtering. A 470 µF capacitor was added at the output for buffering transient currents. A 0.1 F capacitor was used at VSTORE for storage, replaceable with a 10 F/2.7 V supercapacitor for cold-start or high-power tasks.

When VSTORE reached a threshold, the MCU was awakened to execute sensing and communication tasks, then returned to sleep. Voltage sampling was handled by the MCU’s ADC. This design leveraged LTC3108’s ultra-low startup voltage and multiple outputs, combined with MCU-based threshold control, for efficient energy harvesting, regulation, and allocation. All optimization and validation were conducted in this work.

2.5. Testing Methods

Data acquisition: Voltage and current were measured using a digital multimeter (Keithley 2700, ±0.003%), sampling at 1 Hz. All experiments were repeated three times, and averages reported.

Polarization: Open-circuit voltage (OCV) was recorded, followed by programmable resistors from 1500 to 100 Ω (100 Ω steps). Voltage and current were recorded at steady state, and power calculated to plot polarization and power curves (Figure 6).

Power density: Calculated as

$$P_D={\displaystyle \frac{P}{A}}$$

(1)

where P is power and A is the Zn anode–soil contact area (0.5 mm diameter, 1 m length, ~1.57 cm²).

Stability: A 12 h test was conducted at 900 Ω load, with voltages logged every 30 min to compute current. No watering, fertilization, or pruning was performed. ±5–10% perturbations were applied to mimic bio-fluctuations.

Soil moisture: Three controlled levels were set: low (25%), medium (50%), and high (75%). Soil water content was calibrated by gravimetric method (105 °C drying, ±2% error). During tests, water addition maintained target weight, with a capacitive soil sensor (model SM150T, Delta-T Devices, Cambridge, UK, ±3%) providing real-time monitoring.

Task execution: When capacitor voltage reached the threshold, the system awakened and executed one full cycle: sensor activation, data acquisition, and Bluetooth transmission. Capacitor voltage was recorded before and after each task, and energy consumption calculated. Four task cycles were tested per device.

3. Results and Discussion

3.1. Polarization Performance and Power Density

As shown in Figure 7, the open-circuit voltage (OCV) of the device under steady operation was ~0.72 V. With decreasing external resistance, output voltage decreased and current increased, showing typical polarization behavior. At 900 Ω load, the maximum power reached 0.477 mW, corresponding to a power density of 0.304 mW·cm⁻².

Compared with previous reports, this value is higher than several typical PMFCs. De Schamphelaire et al. reported ~0.15 mW·cm⁻² in rice PMFCs [16], and Wetser et al. obtained ~0.25 mW·cm⁻² under controlled indoor conditions and highlighted the influence of the rhizosphere environment on power generation [17]. Zhu et al. emphasized how microbial community dynamics can impact output [18]. Therefore, while Cu–Zn systems are not strictly PMFCs, biological factors still play a role. Performance enhancement in this work may result from: (1) the higher initial potential of Cu–Zn electrodes; (2) rhizosphere processes sustaining electrode activity; and (3) optimized electrode placement reducing internal resistance.

These results indicate that although not a strict PMFC, the system achieves power levels comparable to or higher than many PMFCs, sufficient for low-power monitoring.

3.2. Stability and Electrode Mechanism

As shown in Figure 8, a 12 h stability test under 900 Ω load showed an output of ~0.31 V with ±5% fluctuation. The difference from the ~0.45 V maximum point in polarization curves can be attributed to transient capture in short-term scans, while stability tests reflect long-term steady state influenced by soil moisture and microbial dynamics.

Theoretical estimation suggested a Zn anode lifetime of ~66 days if all current arose from corrosion. However, stable output within the experimental period implies additional contributions from the plant–microbe environment. Previous studies showed that root exudates and microbial metabolites (e.g., organic acids) can slow Zn passivation [19], while facultative anaerobes may assist in cathodic reduction [20]. Thus, the stability likely results from combined galvanic and biological processes.

Electrode placement in this study (Zn anode at ~2 cm, Cu cathode at ~20 cm) differs from the conventional “anaerobic anode/aerobic cathode” PMFC mode. This configuration ensured active rhizosphere contact for Zn and reduced oxygen interference at the Cu cathode, lowering internal resistance. Though not classical PMFC, it was advantageous for Cu–Zn systems.

3.3. Effect of Soil Moisture

As shown in Figure 9, soil moisture strongly influenced performance. At low moisture (25%), OCV and maximum power were lowest; at medium (50%), performance improved; at high (75%), maximum power increased by ~35% compared to medium.

This can be explained by: (1) higher water content lowering ion migration resistance and improving conductivity; (2) sufficient moisture enhancing root exudation and microbial activity. Previous studies also confirmed that PMFCs and soil cells show enhanced output in moist conditions, but instability in drought or oversaturation [17,19].

These results highlight that suitable soil moisture is essential both for plant growth and for stable power generation. Future applications should include soil water management to optimize energy harvesting.

3.4. Task Execution Capability

To assess practical feasibility, task execution tests were conducted. When the supercapacitor voltage reached the threshold, the control unit executed one cycle: (1) sensor wake-up; (2) data acquisition; (3) Bluetooth transmission.

As shown in

Table 1. Experimental Data Analysis.

Cycle	Task Execution Time	Output Voltage (Start → End)	Serial Reception Status	System Status
1	08:25:34	3.31 V → 2.88 V	Complete Data	Normal Execution
2	08:47:47	3.27 V → 2.91 V	Complete Data	Normal Execution
3	09:10:20	3.32 V → 2.87 V	Complete Data	Normal Execution
4	09:35:09	3.29 V → 2.90 V	Complete Data	Normal Execution

, four consecutive cycles showed distinct charge–discharge behavior. Capacitor voltage reached ~3.3 V before tasks, dropped by ~0.3–0.4 V after, and then recovered. The results confirm stable support for periodic sensing and communication.

Since charging rate determined task intervals, frequency was limited by source output and conditions (e.g., soil moisture). Thus, although per-task consumption was small, energy accumulation and scheduling enabled sustained operation of low-power wireless nodes.

3.5. Comparison with Existing Studies and Application Potential

Typical PMFCs report 0.1–0.25 mW·cm⁻² [19], with optimized designs reaching 0.3–0.35 mW·cm⁻² [19]. This study achieved ~0.304 mW·cm⁻² with a Cu–Zn system, a relatively high value, indicating potential from metal–biological coupling. Compared with purely galvanic soil cells, the system showed longer stability, likely due to rhizosphere and microbial modulation.

At the application level, most PMFC studies focus on cell performance, with limited exploration of integration with energy circuits [21]. Here, coupling with a boost converter and supercapacitor enabled periodic sensor tasks, demonstrating feasibility in crop monitoring.

Limitations remain: stability beyond Zn corrosion lifetime requires validation; task frequency is limited by soil moisture and input; and metal consumption raises environmental and economic concerns. Future work should explore eco-friendly electrodes and energy optimization to enhance sustainability.

In summary, this study demonstrates the potential of plant–soil bioelectrochemical systems for low-power monitoring and offers design insights for system optimization.

4. Conclusions

This study designed and implemented a crop environmental monitoring system powered by a Cu–Zn-based plant–soil bioelectrochemical source. The system integrated a boost converter and supercapacitor, enabling periodic powering of wireless sensor nodes and data transmission. Under experimental conditions, it achieved a maximum power output of 0.477 mW at 900 Ω, corresponding to a power density of ~0.304 mW·cm⁻². Stability tests confirmed sustained voltage output, with significant differences observed under varying soil moisture conditions; high moisture enhanced power output by ~35%. Theoretical estimation suggested a Zn anode lifetime of ~66 days if all current is derived from corrosion. However, stable output during the experimental period indicates that the plant–microbe environment may contribute to maintaining electrode activity. Longer-term operation is needed to validate this mechanism. Task execution tests demonstrated that the system can reliably support periodic sensing and communication tasks, confirming its practical feasibility. Compared with existing reports, the system achieved relatively high power density and superior stability to most non-plant soil cells, highlighting the value of plant–soil interactions in extending power supply duration.

In summary, although not a strict PMFC, the system shows promising potential for low-power environmental monitoring and IoT applications.