**5. Factors affecting herbicide degradation in biochar-amended soils**

The impact on the degradation of herbicides due to their high sorption in the biochar particles depends on the rate of biochar applied to the soil. The application of different rates of application of hardwood biochar in Rhodic Ferralsol soil increased atrazine degradation by 49% (0.1% of biochar), 51% (1.0% of biochar), and 62% (5.0% of biochar) after 88 days of incubation (**Table 1**) [19]. DT50 of isoproturon in unamended Alfisol was 16 days, however, when biochar was added at 1.5 and 5%, DT50 increased to 67 and 136 days, respectively (**Table 1**) [46], i.e., the persistence of isoproturon is prolonged as the rate of biochar added to the soil increases. DT50 of fomesafen increased from 34.6 days in unamended soil to 51, 83, and 160 days in amended soils with rice husk biochar at 0.5, 1, and 2%, respectively [61]. The increased persistence of fomesafen can be explained by the higher sorb capacity of biochar and, therefore, little bioavailability of the herbicide for microbial degradation.

Pyrolysis temperature defines the physicochemical characteristics of biochars [69]. Generally, biochar produced at relatively high pyrolysis temperatures (>500°C) presents an increase in specific surface area, microporosity, and hydrophobicity, improving herbicide sorption [70]. However, even with higher herbicide sorption capacity, degradation at high pyrolysis temperatures may be more intensified than low temperatures. The addition of sugarcane bagasse biochar produced at 700°C in clay soil decreased the DT50 of metribuzin from 57 (unamended soil) to 39 days, but when biochar was produced at 350°C, DT50 went from 57 to 74 days (**Table 1**) [56]. These conflicting results could be due to the impact of ash on the alkalinity of the soil amended with biochar produced at 700°C (20.3% of ash), which increased the soil pH and improved the conditions for the degradation of metribuzin, and to the greater amount of dissolved OC from biochar produced at 350°C (3.78 mg g−1), which is more preferred by microorganisms as substrate, increasing the persistence of the herbicide. The variation in pyrolysis temperature of eucalyptus wood residue biochar affected the total hexazinone unavailable (mineralized + non-extractable residue) being higher for 850°C (46%) and 950°C

(49%) compared to biochar pyrolised at 650°C (33%) and 750°C (42%) [71]. The addition of biochar did not alter the mineralization of hexazinone, but it did reduce the bioavailability of this herbicide in the soil due to the greater amount of nonextracted residue, reducing the risk of environmental contamination [71].

Aging alters the properties of biochar, affecting the degradation of herbicides, however, these changes are not fully elucidated [72]. Glyphosate showed no variation in degradation in two tropical soils (Ultisol and Alfisol) amended with eucalyptus biochar aged [73]. The aging of soil-wood biochar mixtures (*Betula* sp. and *Piceaabies*) decreased glyphosate and diuron sorption compared to fresh biochar amended soil [18]. In addition, herbicide degradation was not affected by changes or biochar aging in the soils studied [18]. The degradation of S-metolachlor was not affected with the addition of three macadamia nutshell biochars aged [74]. The persistence of mesotrione in different soils amended with fresh and aged biochar was similar to unamended soils [75]. In contrast, the extractable amounts of picloram were 20 and 50% lower for soils amended with fresh and aged oak wood biochar, respectively, in relation to unamended soil [76]. The addition of 10% fresh biochar from the olive oil industry increased the DT50 of metribuzin from 20 (unamended soil) to 30.2 days, however, the DT50 decreased to 6.4 days with the addition of aged biochar, possibly because microorganisms in soil aged with biochar used metribuzin as a source of carbon and energy instead of the labile fraction of soil OM (**Table 1**) [55]. The effects of biochar on herbicide degradation in soils should not be generalized due to the different characteristics of biochars and the complexity of the soil system. The variation of temperature and application rate of biochar can bring different degradation responses for each herbicide studied. Furthermore, the aging of biochar in the soil can influence the bioavailability of herbicides in soil solution by altering the sorption capacity of the biochar; therefore, the conditions of pyrolysis, type of feedstock as well as aging must be taken into consideration when planning its use in agriculture and for soil remediation purposes [18].

### **6. Simultaneous use of herbicides and biochar**

In an agricultural context, the property of biochar that offers potential for herbicide sorption (environmental remediation) can also decrease the efficacy of herbicides applied to the soil, influencing their bioavailability and susceptibility to leaching and consequently their degradation [77]. The bioavailability of diuron and microbial degradation was reduced in soils amended with rice straw biochar, which decreased the effectiveness of diuron to jungle rice (*Echinocloa colona*) control [78]. The addition of wheat straw biochar to the soil inactivated the herbicides atrazine and trifluralin, resulting in increased seed germination and biomass of annual ryegrass (*Lolium rigidum*). In this study, the efficacy of the herbicides for ryegrass control was achieved when the application doses were four times higher than recommended [44]. In a bioassay with *Echinochloa colona*, injuries 9 days after planting decreased with increasing application rates of rice straw biochar indicating that sorption of clomazone increased and directly influenced the bioavailability of herbicide in the soil [79]. The control efficiency of S-metolachlor was evaluated on green foxtail (*Setaria viridis)* in soil amended with wood biochar at different application rates (0, 0.5, 1, and 2%) [80]. *S. viridis* control at the highest application rate (2%) was lower than the other application rates evaluated, however, better than the control treatments (no herbicide) [80].

The biochar applied to soil also influences the soil physicochemical properties and the improved nutritional availability of these directly impacts crop growth and consequently weed growth [81]. Soil amended with walnut shell biochar

(5 Mg ha−1) for 4 years was evaluated for weed control [82]. Weed density was dramatically higher in biochar-amended soils (60-78%) compared to unamended soil, being related to increased nutrient availability and improvements in soil physicochemical properties such as cation exchange capacity (CEC), density and porosity, increased soil aeration, and water retention. The application of 2 Mg ha−1 of cow bonechar prevented weed control by indaziflam which is related to the increase of soil fertility, especially the phosphorus and carbon content, and to the increase of pH because it is a basic material [83]. In addition, goosegrass (*Eleusine indica)* and crabgrass (*Digitaria horizontalis)* accounted for about 99.7% of the entire weed community infestation [83].

On the other hand, the decrease in efficacy depends on the characteristics of the herbicide evaluated. The dose of pretilachlor to inhibit 50% of E. colona emergence and biomass was higher in soil amended with rice-husk biochar, however, the effectiveness of pendimethalin in controlling E. colona was not influenced by the application rate of biochar [84]. The effectiveness on metribuzin in soils amended with biochar was evaluated by White Junior et al. [56]. The addition rates of biochar did not alter Palmer (Amaranthus palmeri) emergence, and it is possible that the residual activity was sufficient to reduce germination at any rate of biochar [56].

The addition of biochar to soil increases the sorption of different herbicides and reduces their effectiveness, which may result in the need for higher herbicide application rates, additional application times, or more weed control operations required [85]. Residual herbicides, applied in pre-emergence, can not provide good weed control regardless of soil type after biochar application. This does not necessarily mean that biochar should be avoided, however, when biochar is applied to the soil, management practices need to be adjusted to obtain appropriate weed control [86].

#### **7. Conclusions**

Modifying soil characteristics with biochar is a world-renowned emerging practice for either environmental and/or agronomic purposes, and the benefits these carbonaceous materials brig to the soil are clear. However, the pyrolysis conditions for biochar production directly interfere with the physical–chemical properties of the produced material, which govern the biochar-herbicide interactions. If the objective is to apply the herbicide in pre-emergence after the addition of biochar in the soil, care should be taken, as biochar can decrease or increase the persistence of the chemical product, interfering in the effectiveness of weed control over time. On the other hand, if the objective is herbicide remediation in contaminated soils, the interference of biochar in the bioavailability of the herbicide in the soil solution to increase soil microbiological diversity should be known.

#### **Acknowledgements**

The authors wish to thank the Coordination for the Improvement of Higher Education Personnel (CAPES - 88887.479265/2020-2100) and Foundation for Research Support of the State of Minas Gerais - Brazil (FAPEMIG - APQ-01378-21) for the financial support.

### **Conflict of interest**

The authors declare no conflict of interest.
