Inactivation of Weedy Rice Using 915 MHz Microwaves with Soil Physicochemical Property and Microbiome Retention

Abstract

There is a growing demand for alternative low cost and sustainable weed management technology suitable for aerobic and organic farming. This study evaluates 915 MHz microwave heating as a potential non-chemical approach for managing weedy rice while assessing its impact on soil physicochemical properties and selected microbial groups. Microwave power levels of 10, 20, and 30 kW were applied to soil at depths of 2.5, 8.9, and 15.2 cm under controlled laboratory conditions. Weed emergence was quantified using the total germinability index (TGI), and soil physicochemical and microbial responses were analyzed in separate experiments. TGI decreased significantly with increasing microwave power and decreasing soil depth, ranging from 0.84 (10 kW at 15.2 cm) to 0 (20 kW at 2.5 cm and 30 kW at 8.9 cm). For 8.9 cm soil depth, energy levels between 176 and 265 kJ/kg resulted in 80–100% emergence suppression, while treatment of 15.2 cm soil at 30 kW for 30 s (188 kJ/kg) reduced TGI by approximately 80% and germination by 64% relative to control. Soil physicochemical properties showed minimal changes, with values remaining within agronomically acceptable ranges. Total bacterial abundance was not significantly affected, whereas ammonia-oxidizing archaea and bacteria were reduced following treatment. These results indicate that microwave heating can effectively suppress weedy rice emergence under controlled conditions, primarily through thermal effects. However, TGI reflects emergence suppression and does not distinguish underlying mechanisms such as lethality, injury, or dormancy. Additionally, limitations including low replication, lack of depth-matched controls, and limited spatial temperature measurements should be considered. Further field-scale studies are needed to validate performance, optimize energy requirements, and assess long-term soil impacts.

1. Introduction

Rice is the third major cereal produced globally (502.98 million metric tons of milled rice) in 2022/23 behind corn and wheat [1]. In 2022, the U.S. was ranked amongst the top five largest exporters of rice [2] with almost 50% of U.S. rice produced in the state of Arkansas [3]. Rice farmers in Arkansas face many challenges that lead to lower profitability, including extreme weather conditions (mainly precipitation and high-nighttime temperature), crop management practices (irrigation timing and amount, herbicide and fertilizer application type, amount, and timing), and post-harvest drying and storage practices. Managing weeds is considered one of the most problematic factors and often leads to huge financial losses [4].

Common weeds found in Arkansas rice production include Echinochloa spp., weedy relatives of rice or weedy rice (Oryza spp.), Leptochloa spp., and Cyperus spp. [5]. Because of its similarity to cultivated rice, weedy rice is the most devastating and most difficult-to-control [6]. It has been demonstrated that weedy rice in the United States may absorb 60% of all applied N after it is established in fields, leading to a reduction of up to 80% in rice yields [7]. A case study by Durand-Morat et al. [6] to understand the monetary impact of weedy rice in the U.S. rice production system found that from 2002 to 2014, losses in a moderate infestation scenario total 5.7 million tons of rice with a $457 million economic impact per year. The ensuing production loss in the U.S. is sufficient to feed 12 million people per year.

Rice weeds including weedy rice are primarily managed with flooding and herbicide use. Herbicides like Clearfield^® (CL) are considered as one of the most effective for controlling weedy rice and used in U.S. rice fields with CL rice cultivars. Similarly, in 2018, Provisia rice cultivars that were used in rotation with CL cultivars were effective in managing weedy rice [6]. Crop rotation of rice with soybean was also used as an effective tool in the U.S. [8]. However, herbicide resistance of weedy rice is a current threat to the established CL rice production system to manage weedy rice in the U.S. [8]. Herbicide resistance of weeds is a global problem and despite having at least 11 herbicide modes of action used in rice, there are about 170 unique cases of weed resistance globally, which involve resistance to multiple herbicide modes of action in each population [9]. Transplanting rice into flooded fields is another weedy rice management practice that is effectively deployed for long in Asian countries [6]. U.S. rice production typically uses direct seeding followed by flooding, which promotes weedy rice emergence; however, maintaining extensive flooding after seeding helps suppress its growth effectively. However, there is a gradual but steady shift to dry direct seeding with aerobic rice culture (row rice production) to preserve water [10]. This has aggravated the weedy rice problem along with most other rice weeds. Against this backdrop, the need for novel tools for integrated weed management is critical.

Field soil irradiation using microwaves (MW) can be a feasible and herbicide-free way to achieve weed seedbank depletion. Most past studies have shown that the pre-emergent MW irradiation of the soil was effective in reducing weed infestation [11,12,13,14]. Applying controlled MW energy to weed seeds can cause cellular damage, leading to a reduction in seed germination and viability [15]. Microwave soil heating is majorly governed by dielectric heating mechanisms in which dipolar rotation and ionic conduction generate volumetric heat within moist soil matrices [15,16]. The penetration depth at 915 MHz depends on soil dielectric constant and loss factors, which are strongly influenced by moisture content and bulk density [10,17]. Compared with 2.45 GHz systems, 915 MHz provides greater penetration depth in high-moisture soils, making it more suitable for subsurface weed seedbank management. Field-scale microwave applicators have been previously explored for soil pasteurization and weed control [15,18,19,20], though quantitative links between energy density, thermal dose, and biological outcomes remain limited.

The implementation of MW treatment for weed seed control offers several advantages: (1) environmental safety: unlike chemical herbicides, MW energy does not leave behind residual toxic substances, making it a more environmentally friendly weed control method; (2) support for organic agriculture: the technology could replace the less effective organic herbicides used in organic crop production, and enable successful nonchemical weed management; (3) targeted application: MW can be directed precisely to weed-infested areas, minimizing collateral damage to non-target species such as soil microbes; and (4) soil conservation: MW treatments can be applied without disturbing the soil structure, thus preserving the existing soil microbiota. Although the application of microwaves offers these advantages, successful implementation requires extensive testing to optimize the MW energy levels for different weed species, seed bank depths, soil physicochemical properties, and environmental parameters including air temperature, humidity, and wind speed. In addition, the recommendations need to factor in impacts of the MW treatments on soil nutrients and beneficial soil microbes.

Microwave treatments were effective in inactivating the germ or reproductive part of seeds in soil [21]. Previously, at a depth of 2.5 cm and a soil moisture content of 6.5%, Wayland [22] treated wheat and radish seeds in situ and found that increasing energy density had a greater effect on reducing seed germination capacity than increasing exposure time. Furthermore, Menges and Wayland [23] contrasted post-emergent herbicides (methazoal, propachlor, and perfluidone) for weed control in onion production with MW energy densities between 450 and 7200 kJ m⁻². Their data indicated that MW radiation (3600 kJ m⁻²) significantly reduced weed growth. In addition, MW caused less crop damage (18%) than herbicide spray (85%). The effects of MW on oat and weed seeds in soil at various moisture levels were investigated by Barker and Craker [24]. They concluded that after 120 s of irradiation, soil temperature reached 90 °C and weed seed relative emergence ranged between 0 to 15%.

To date, no research has been done on utilizing 915 MHz MW to test pre-emergence weedy rice eradication on Arkansas rice soil. Also, the impact of MW treatment on soil nutrients as well as microbiota is not well understood. Therefore, the objectives of this study were to understand the (1) impact of different levels of MW power on weedy rice germination at various soil depths and (2) impact of MW power levels on the treated soil physicochemical properties and microbiome.

2. Materials and Methods

2.1. Soil Sampling

Weedy rice infested soil (soil collected from an area that does have a history of weedy rice infestation) was collected from a farm in Humnoke, Arkansas. The soil was Perry clay with physicochemical properties as mentioned in Table 1. All tests were performed in the Fayetteville agricultural diagnostic laboratory.

Table 1. Physicochemical properties of soil sample collected for experiments.

Properties	Soil Sample
pH	6.7
EC (μmhos/cm)	529.0
P (mg/kg)	31.2
K (mg/kg)	275.3
Ca (mg/kg)	3810.2
Mg (mg/kg)	977.6
S (mg/kg)	36.2
Na (mg/kg)	123.4
Fe (mg/kg)	335.5
Mn (mg/kg)	366.2
Zn (mg/kg)	3.0
Cu (mg/kg)	2.7
B (mg/kg)	1.4
Percentage Loss on Ignition	5.8
Total Nitrogen (%)	0.2
Total Carbon (%)	2.9

The soil samples were placed in containers with lids and further sealed with plastic wrap, then conditioned to approximately 35% soil moisture content (wet basis) to mimic soil moisture in rice fields during sowing. Moisture content was measured using the ASTM Standard D2216-19 oven drying method using a 105 °C oven temperature for 24 h [25].

2.2. Microwave Heating and Soil Temperature Measurement

The industrial microwave system (Model No. AMT2448.05, Industrial Microwave Systems, AMTEK, Cedar Rapids, IA, USA) used for the study is a 915 MHz microwave tunnel dryer configured with a conveyor belt. This system is designed to deliver high-power microwave energy for industrial-scale applications. The 915 MHz microwave frequency was chosen for its superior penetration depth and efficiency in heating moist soils compared to the more surface-limited 2.45 GHz systems [26]. While 915 MHz requires larger equipment, its ability to uniformly heat deeper soil layers makes it better suited for large-scale applications like soil treatment, where deeper energy distribution is essential.

The key components of the system include a 75 kW generator, a 1.2 m long drying oven, and an integrated console for operation and control. The drying chamber itself is a metallic enclosure with dimensions designed to enhance multimode operation. The chamber width (1 m) and height (1 m) are sufficiently large to support a broad range of modes, which contributes to minimizing uneven heating. Antenna configuration followed the manufacturer’s standard industrial design. No electromagnetic modeling, coupling analysis, or optimization was performed. Antenna polarization and impedance matching were preconfigured by the manufacturer.

Soil to be treated was transferred to a polypropylene tray, and a 10 cm by 20 cm pile with the desired height (soil depth) was made before treatment. Soil temperature during the treatment was measured using a fiber optic cable temperature datalogger (Neoptix Reflex-4, Québec, Canada). Two fiber optic cables were placed within the soil pile relatively towards the top and bottom center. Soil surface temperature was also measured just after microwave treatment using a thermal imaging camera (Model: FLIR E 60, FLIR Systems AB, Täby, Sweden). Treatment duration was fixed to 30 s. After treatment, soil was mixed using a spatula and allowed to cool for 2 min before bagging in labeled plastic bags. After cooling, samples were then tested for germination, physicochemical attributes and microbial analyses.

2.3. Experimental Design

Microwave power and soil depth were the factors tested for total germinability index, soil physicochemical and microbial properties. Table 2 below shows the factors and levels used in the design of experiments. Three independent experimental datasets were generated:

Table 2. Experimental factors for each set of experiments. Experiments were conducted independently; factors were not combined in a single design.

Experiments	Factors	Constants
Germination/temperature analyses	Microwave power (10, 20 and 30 kW), and soil depth (2.5, 8.9 and 15.2 cm)	Exposure duration of 30 s
Soil physicochemical analyses	Microwave power (10, 20 and 30 kW), and soil depth (2.5, 8.9 and 15.2 cm)	Exposure duration of 30 s
Soil microbial quantification	Microwave power (10, 20 and 30 kW), and exposure duration (30, 60, and 90 s)	Soil depth of 15.2 cm

(i)

Microwave power × soil depth (fixed exposure duration of 30 s) for temperature and germination analyses;

(ii)

Microwave power × soil depth (fixed exposure duration of 30 s) for soil physicochemical analyses;

(iii)

Microwave power × exposure duration (fixed soil depth of 15.2 cm) for soil microbial quantification.

These experiments were conducted independently using separately collected soil samples. Statistical analyses were performed independently for each dataset.

The untreated control (1.3 cm depth) was used as a reference for baseline germination, soil physicochemical properties, and soil microbial analyses but is not depth-equivalent to treated samples. Therefore, comparisons between treated and control soils should be interpreted qualitatively rather than as direct depth-controlled contrasts. This represents a fundamental limitation of the experimental design. Complete randomization was done before experimentation to reduce any manual error and bias. This study used two replicates per treatment, balancing logistical constraints with robust statistical methods. Future studies with additional replicates could further enhance reliability.

2.4. Greenhouse for Germination Analyses

Soil samples after microwave treatment were subjected to tests for the analysis of seed germination. A greenhouse was used in which the air temperature ranged from 23 °C to 37 °C. Samples were placed in 5 cm deep containers (10 cm wide and 20 cm long) with drainage holes at the bottom. The second container was used for 15.2 cm deep samples to allow ease in seed germination. The containers were soaked in water to achieve full saturation and then drained to remove gravitational water. This would represent the ‘field water holding capacity’ of the container. Weed emergence was assessed over two weeks (14 days). Emerged weeds were counted on Day 7 and Day 14; seedlings were removed after each count, and the soil in each pot was thoroughly mixed at Day 1 and Day 7 to facilitate germination of all viable, non-dormant seeds. All emerged species were identified visually and counted as germinated. The normalization was performed to account for variability in the initial weed seed density among soil samples collected from the field. Because naturally infested soil may contain uneven seedbank distribution, direct comparison of raw germination counts could introduce bias. The Total Germinability Index (TGI) rescales germination values between 0 (minimum observed germination) and 1 (maximum observed germination), enabling relative comparison of treatment effects independent of initial infestation variability. The TGI metric reflects cumulative emergence but does not differentiate embryo lethality from sublethal injury or dormancy induction. Future studies incorporating tetrazolium viability assays or controlled germination kinetics would clarify physiological mechanisms. The TGI was calculated using Equation (1).

$$TGI={\displaystyle \frac{G_i-G_{min}}{G_{max}-G_{min}}}$$

(1)

where

G_i is the total number of germinated seeds of the ith sample,

G_min is the minimum total number of germinated seeds in all samples, and

G_max is the maximum total number of germinated seeds in all samples.

2.5. Soil Physiochemical Properties and Microbial Quantification

Separate sets of experiments were conducted to understand the impact of microwave treatments on soil physicochemical properties and microbes without affecting germination results. These samples were collected separately a day before the samples used for germination analyses to avoid any bias due to storage duration. The same field was used to source these samples as used in germination tests. Samples were transported in an insulated box from the field to the laboratory and were stored immediately in a −20 °C freezer and thawed before the experiments. After each microwave treatment, 10 g subsamples for physicochemical properties and microbes were taken after mixing treated soil samples with a spatula. Subsamples were transferred to the −20 °C cooler immediately, and after all experiments were completed, all subsamples were sent to the University of Arkansas’ Fayetteville Agricultural Diagnostic Laboratory, Fayetteville, AR for soil physicochemical attributes analyses and the soil microbial testing laboratory for microbe quantification, respectively. The physicochemical properties measured include soil pH, electrical conductivity, soil composition (silt, clay and sand), macronutrients (P, K, Mg, S, Na, Ca) and micronutrients (Fe, Mn, Zn, Cu and B). For soil microbe quantification, total bacteria count was measured to give an idea of the overall impact of treatment on microbes, and ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB) were chosen to be measured for their role in denitrification. Quantitative PCR (qPCR) was performed to quantify total bacteria (16S rRNA gene) and ammonia-oxidizing archaea (AOA) and bacteria (AOB) using amoA gene targets. Genomic DNA was extracted from 0.25 g of soil using a commercial soil DNA extraction kit following the manufacturer’s protocol. qPCR reactions were conducted in 20 µL volumes containing SYBR Green master mix, gene-specific primers, and template DNA under standard thermal cycling conditions (initial denaturation at 95 °C followed by 35–40 amplification cycles). Primer sets and conditions were selected based on established literature. Standard curves were generated using serial dilutions of known gene copy numbers, with amplification efficiencies between 90–105% and R² > 0.98. To assess potential PCR inhibition, selected samples were analyzed at multiple dilutions. Gene copy numbers were expressed per gram of dry soil.

2.6. Data Analysis

Data obtained from the experiments were subjected to analysis of variance (ANOVA) using JMP Pro 17 statistical software (JMP Pro 17, SAS Institute, Cary, NC, USA) with complete randomization (F-test). The normality of data and homogeneity of variance were examined. The ANOVA was used to see the effect of main factors on the total germinability index, soil physicochemical attributes, and soil microbial data. Tukey’s HSD was used to compare the levels of each factor. The level of significance for the analysis was set at a 95% confidence level. The control soil samples at a fixed soil depth of 1.3 cm were used as a numerical reference with no statistical comparison to treated soil samples.

3. Results

3.1. Impact of Microwave Energy on Soil Temperature

Table 3 shows the soil temperature as a function of microwave power and soil depth. For a constant soil depth of 2.5 cm, as power increased from 10 kW to 30 kW, soil temperature increased from 77.5 °C to 95.5 °C. Similar increments were observed for 8.9 cm and 15.2 cm soil depths. Increasing power at constant soil depth increased the specific energy supplied to the soil (Table 3). This led to more friction in water molecules within the soil, generating heat and resulting in higher temperatures. However, with the increase in soil depth at a particular power level, overall soil temperature reduced. This reduction is attributed to the lower specific energy that the soil received.

Table 3. Specific energy and soil temperature for each 30 s microwave treatment.

Microwave Power (kW)	Soil Depth (cm)	Soil Volume (cm3)	Soil Mass (kg)	Specific Energy (kJ/kg Soil)	Soil Temperature (°C) *
10	2.5	500	1.5	200	77.5
10	8.9	1780	3.4	88	40.9
10	15.2	3040	4.8	63	38.6
20	2.5	500	1.5	400	92.3
20	8.9	1780	3.4	176	53.6
20	15.2	3040	4.8	125	41.3
30	2.5	500	1.5	600	95.5
30	8.9	1780	3.4	265	70.0
30	15.2	3040	4.8	188	58.3

* Mean value determined after 30 s from data recorded near surface and at bottom of soil pile on the tray.

Microwave penetration depth is a key factor influencing the effectiveness of soil heating. At 915 MHz, the penetration depth depends on the dielectric properties of the soil, which are influenced by moisture content, temperature, and bulk density [27]. Soils with higher moisture content, such as those used in this study (35% wet basis), exhibit greater attenuation of microwave energy due to increased dielectric losses.

For 10 kW, 20 kW, and 30 kW, the soil temperature was reduced from 77.5 °C to 38.6 °C, 92.3 °C to 41.3 °C, and 95.5 °C to 58.3 °C as the soil depth increased from 2.5 cm to 15.2 cm, respectively. These temperature variations are consistent with the calculated specific energy values and illustrate the effect of microwave energy dissipation with depth. The initial moisture content further contributed to this gradient, as water molecules absorbed a significant portion of the microwave energy, limiting its penetration.

Soil surface temperature after a 30 s treatment also showed similar trends for temperature changes with varying microwave power and soil depth (Figure 1). Surface thermal images and two internal temperature measurement points were used for a relative comparison of temperature increase; spatial uniformity was not quantified. These observations align with theoretical principles and explain the patterns of germination and total germinability index (TGI) observed in the study. Surface-dominant heating patterns at greater depths, such as 15.2 cm, are consistent with prior studies on microwave heating in moist soils [17]. Optimizing microwave power and duration could improve heating uniformity and penetration depth for practical field applications.

Figure 1. Soil surface temperature after 30 s microwave treatment (A) 15.2 cm at 10 kW, (B) 15.2 cm at 20 kW, (C) 15.2 cm at 30 kW, (D) 2.5 cm at 30 kW, (E) 8.9 cm at 30 kW, and (F) 15.2 cm at 30 kW. Soil was placed on the tray in a rectangular arrangement of 10 cm by 20 cm. These images are for replicate 1. Panels (A–C) helps in visualizing changes in soil surface temperature with increasing microwave power, whereas panels (D–F) helps in visualizing changes in soil surface temperature with increasing soil depth.

3.2. Impact of Microwave Energy on Total Germinability Index (TGI)

The microwave power and soil depth had a significant effect on the TGI (Table 4). TGI ranged from 0.84 (10 kW for 15.2 cm soil depth) to 0 (20 kW for 2.5 cm soil depth or 30 kW for 8.9 cm soil depth) (Figure 2). TGI increased as soil depth increased, and power decreased. The control with no microwave treatment showed TGI of 1 that was numerically higher than that of any other treated soil. The minimum TGI for 2.5 cm, 8.9 cm and 15.2 cm of soil depth were 0, 0, and 0.18 attained at 20 kW, 30 kW and 30 kW power treatment for 30 s, respectively. In terms of specific energy, farmers can inactivate weedy rice completely for a soil layer of up to 2.5 cm using 400 kJ/kg or lower. Even 200 kJ/kg showed promise to do a complete inactivation as 10 kW treatment was not significantly different from 20 kW in terms of TGI (Table 3 and Figure 2). For 8.9 cm of soil, 176 to 265 kJ/kg resulted in complete inactivation (Table 3 and Figure 2). For 15.2 cm soil, more than 188 kJ/kg specific energy is needed to achieve 100% inactivation. Reduced TGI reflects overall emergence suppression and should not be interpreted as direct evidence of seed lethality, as alternative mechanisms such as sublethal injury or dormancy were not assessed.

Table 4. Statistical analysis (F-test) to determine the significance of factors on the total germinability index (TGI) of soil after microwave treatment.

Factors	Levels	p-Value
Microwave power (kW)	10, 20 and 30	0.0009 *
Soil depth (cm)	2.5, 8.9, 15.2	0.0003 *
Replication	1 and 2	0.3533

* Signifies statistical significance at 95% level of confidence.

Figure 2. Total germinability index of microwave-treated soil determined after 14 days in greenhouse. Bars not connected with a common letter are statistically different.

3.3. Impact of Microwave Energy on Soil Physicochemical Properties

Soil physicochemical properties along with p-value showing the treatment impact on them are reported in Table 5. Although several parameters (pH, EC, K, Mn) showed statistical significance, absolute magnitudes of change were small and remained within agronomically acceptable ranges. No consistent directional degradation trend was observed. In general, soil pH, phosphorus, calcium, magnesium, sulphur, sodium, iron, zinc, copper, boron, percentage loss on ignition, total nitrogen, and total carbon were properties that were not changed as compared to non-treated soil. Properties that changed significantly were soil pH due to power levels, electrical conductivity due to change in soil depth, potassium due to power levels and soil depth, and manganese due to soil depth (Table 5). All these differences as indicated by p values seem to be with only a few treatments and control. These differences can also occur due to climate change, land use and land management [28]. Repeated seasonal microwave exposure could alter soil aggregation, organic matter mineralization rates, and microbial succession. Long-term field trials are needed to assess cumulative impacts on soil structure and ecosystem services.

Table 5. p-value showing impact of microwave power and soil depth on soil physicochemical properties and mean values of properties in soil treated for different levels of microwave power and soil depth. Exposure duration was fixed to 30 s for these tests. This soil physicochemical properties dataset was generated in a separate experiment from germination and microbial analyses.

Soil Physicochemical Properties	p-Value *		Mean Values of Soil Physicochemical Properties for Treated Soil (MW Power-Soil Depth)
	MW Power	Soil Depth	10 kW-2.5 cm	10 kW-8.9 cm	10 kW-15.2 cm	20 kW-2.5 cm	20 kW-8.9 cm	20 kW-15.2 cm	30 kW-2.5 cm	30 kW-8.9 cm	30 kW-15.2 cm	Control #
Soil pH	0.0083 *	0.8148	6.6 b	6.6 b	6.6 b	6.5 b	6.7 ab	6.7 ab	6.8 a	6.8 a	6.8 a	6.7 ab
Electrical Conductivity (μmhos/cm)	0.0873	0.0307 *	465.0 b	500.0 ab	516.0 ab	497.5 ab	519.5 ab	567.0 a	510.5 ab	498.0 ab	517.5 ab	529.0 ab
Phosphorus (mg/kg)	0.2354	0.6804	31.8 b	34.0 ab	33.7 ab	35.0 a	34.6 ab	32.8 b	34.8 ab	34.0 ab	34.2 ab	31.2 b
Potassium (mg/kg)	0.0459 *	0.0138 *	263.8 b	292.7 ab	296.0 ab	264.1 b	262.9 b	315.1 a	259.3 b	258.5 b	266.0 b	275.3 b
Calcium (mg/kg)	0.6229	0.3466	3724.5 a	3909.9 a	3854.5 a	3807.2 a	3753.2 a	4071.3 a	3969.2 a	3852.4 a	3921.1 a	3810.2 a
Magnesium (mg/kg)	0.7329	0.4565	969.5 a	1054.9 a	989.5 a	951.0 a	936.1 a	1062.4 a	1023.4 a	982.8 a	1023.8 a	977.6 a
Sulphur (mg/kg)	0.5758	0.4557	30.6 a	37.4 a	34.4 a	39.5 a	35.5 a	41.0 a	50.4 a	31.9 a	30.9 a	36.2 a
Sodium (mg/kg)	0.7734	0.5843	121.8 a	132.2 a	123.6 a	126.1 a	119.1 a	136.7 a	134.4 a	123.6 a	130.5 a	123.4 a
Iron (mg/kg)	0.5742	0.5728	371.9 a	312.0 b	349.0 b	336.0 b	342.8 b	323.9 b	316.4 b	324.6 b	344.5 b	335.5 b
Manganese (mg/kg)	0.1021	0.0151 *	168.6 ab	287.8 a	349.6 a	63.0 b	246.2 ab	181.0 ab	123.1	187.3 ab	274.7 a	366.2 a
Zinc (mg/kg)	0.4121	0.7018	3.0 a	3.2 a	3.2 a	3.0 a	3.1 a	2.9 a	3.3 a	3.1 a	3.0 a	3.0 a
Copper (mg/kg)	0.6767	0.3384	2.1 a	3.4 a	2.8 a	2.4 a	2.7 a	2.9 a	3.1 a	3.0 a	2.8 a	2.7 a
Boron (mg/kg)	0.7832	0.1711	1.8 a	1.5 a	1.5 a	1.7 a	1.6 a	1.7 a	1.6 a	1.7 a	1.6 a	1.4 a
Percentage Loss on Ignition	0.4149	0.2857	6.1 a	5.4 a	6.2 a	6.3 a	5.9 a	5.9 a	5.6 a	5.5 a	5.9 a	5.8 a
Total Nitrogen (%)	0.6445	0.5574	0.3 a	0.2 a	0.3 a	0.3 a	0.2 a	0.3 a	0.2 a	0.2 a	0.2 a	0.2 a
Total Carbon (%)	0.4922	0.5354	3.0 a	2.6 a	3.0 a	3.2 a	2.9 a	2.9 a	2.7 a	2.7 a	2.8 a	2.9 a

* p-values < 0.05 are considered to show statistical significance. ^# control samples were non-microwave treated soil with a fixed soil depth of 1.3 cm. Standard deviations as measured by duplicates were not mentioned in the table for better readability. However, differences in the same row were calculated based on standard deviations as an outcome of LS means student’s t test. Values connected in the same row with common letters were not statistically different.

3.4. Impact of Microwave Energy on Soil Microbes

The soil total bacteria, ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB) results are mentioned in Table 6. p values show there was no significant impact of microwave treatments on the levels of total bacteria and AOA; however, for AOB, there was significance. Looking at the mean values of total bacteria, the only significant difference as compared to the control was that of soil treated with 10 kW power for 30 s. This could be due to the gentler increase in soil temperature after 10 kW and 30 s treatment that accelerated the growth of bacteria. For AOA, most of the treatments except 10 kW for 30 s showed significantly lower AOA values as compared to the control. Auyeung et al. [29] and Waghmode et al. [30] mentioned in their study that with heating soil, AOA numbers reduced significantly. For AOB as well, similar reductions were observed where the sample after all treatment showed significant reduction as compared to the control. However, these changes are expected just after heating, and soil microbial community numbers can be enhanced naturally or with fertilizer use as reported by past studies [31,32,33,34]. Functional recovery following field application requires further longitudinal assessment.

Table 6. p-value showing impact of microwave power and treatment duration on total bacteria, ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB), and mean values of these in soil treated for different levels of microwave power and treatment duration. Soil depth was fixed to 15.2 cm for these tests. This microbial dataset was generated in a separate experiment from germination and physicochemical analyses.

Soil Microbes	p-Value *	Mean Values of Soil Microbes for Treated Soil (MW Power-Duration)
	Treatment (MW Power-Duration)	10 kW-30 s	10 kW-60 s	10 kW-90 s	20 kW-30 s	20 kW-60 s	20 kW-90 s	30 kW-30 s	30 kW-60 s	30 kW-90 s	Control #
Total bacteria (copies per g of soil)	0.0767	4.1 × 108 a	3.5 × 108 ab	2.7 × 108 abc	0.7 × 108 c	1.7 × 108 bc	2.1 × 108 abc	2.6 × 108 abc	2.0 × 108 abc	0.8 × 108 c	1.4 × 108 bc
AOA (copies per g of soil)	0.1313	2.7 × 107 ± ab	2.4 × 106 ± bc	4.2 × 105 c	1.1 × 104 c	1.7 × 104 c	8.9 × 104 c	4.4 × 104 c	4.1 × 105 c	2.9 × 105 c	3.0 × 107 a
AOB (copies per g of soil)	<0.0001 *	1.7 × 108 b	3.5 × 107 b	3.7 × 107 b	4.2 × 107 ± b	2.4 × 107 b	2.0 × 107 b	2.7 × 107 b	1.8 × 107 b	9.7 × 106 b	9.1 × 108 a

* p-values < 0.05 are considered to show statistical significance. ^# control samples were non-microwave treated soil with a fixed soil depth of 1.3 cm. Standard deviations as measured by duplicates were not mentioned in the table for better readability. Values connected in the same row with common letters were not statistically different.

4. Discussion

We found that 915 MHz microwave heating significantly reduced weedy rice emergence under controlled laboratory conditions, with effectiveness strongly influenced by specific energy and soil depth. The total germinability index (TGI) ranged from 0.84 (10 kW at 15.2 cm) to 0 (20 kW at 2.5 cm and 30 kW at 8.9 cm), demonstrating strong suppression at higher energy inputs and shallower depths. For 8.9 cm soil depth, energy levels between 176 and 265 kJ/kg resulted in near-complete emergence suppression, while at 15.2 cm depth, 188 kJ/kg reduced TGI by approximately 80%. These trends are consistent with the observed temperature increases (up to ~95 °C at shallow depths), which fall within known thermal thresholds for seed inactivation, supporting thermal effects as the dominant mechanism.

While TGI provides a normalized measure of emergence, it does not distinguish the underlying physiological mechanisms of reduced germination. The observed reductions likely reflect a combination of thermal lethality and other responses such as sublethal injury or dormancy induction. Therefore, results should be interpreted as effective emergence suppression rather than definitive seed inactivation. Additional assays would be required to differentiate these mechanisms.

Microwave treatment had limited impact on most soil physicochemical properties, with observed changes remaining within agronomically acceptable ranges and showing no consistent degradation trend. This suggests that short-duration microwave exposure may preserve key soil properties under the tested conditions. However, some statistically significant differences were observed, and their long-term implications require further investigation.

Microbial results indicated that total bacterial abundance was not significantly affected, whereas ammonia-oxidizing archaea (AOA) and bacteria (AOB) showed reductions following treatment. Given the role of these groups in nitrogen cycling, these changes suggest that specific functional microbial processes may be temporarily affected. However, prior studies indicate that microbial communities can recover over time, and this requires validation under field conditions.

Several limitations should be considered when interpreting these findings. The use of two replicates limits statistical robustness, and the absence of depth-matched controls restricts direct quantitative comparisons. Additionally, temperature measurements were limited to two internal locations and surface imaging, and spatial heating uniformity was not quantified. As such, the results should be viewed as proof-of-concept under controlled conditions.

Overall, this study demonstrates that 915 MHz microwave heating can effectively suppress weedy rice emergence while maintaining general soil physicochemical stability in the short term. These findings support the potential of microwave-based weed management, though further work is needed to validate field-scale performance, optimize treatment parameters, and evaluate cumulative impacts on soil systems.