Biochar Addition to the Loam Soil of Soybean Fields Can Reduce Trifluralin Usage in Velvetleaf Control ^†

Abstract

Biochar, made by the pyrolysis of various organic materials such as plants, can amend soil physicochemical properties and improve the efficiency of pre-planted incorporated (PPI) herbicides. The excessive consumption of PPI herbicides results in environmental predicaments; improving the PPI herbicides’ efficacy by changing the soil’s biological properties might solve this problem. Trifluralin, a PPI, is recommended against annual broadleaf weeds such as velvetleaf in soybean fields. In the present study, the treatments included normal soil (NS) (sand 30% + silt 35% + clay 35%) and manipulated soil (MS) (sand 27% + silt 32% + clay 32% + biochar 9%). Two blocks of NS received the recommended dose (RD) (1.7 L/ha) (NS + RD) and a reduced dose (1.2 L/ha) (NS + ReD) of trifluralin. Meanwhile, the block of MS was exposed to the reduced dose (MS + ReD) of trifluralin. Two days after the herbicide treatments, the seeds of Abutilon theophrasti were sown. Then, seven days after sowing, the growth of the weeds was monitored and the weed control percentage was calculated using the arcsine model based on observed data. The results showed that the NS + RD was the most effective treatment in velvetleaf control (100%), followed by MS + ReD (93.5%) with no significant difference. The NS + ReD treatment resulted in 81% weed control showing significant differences with the NS + RD and MS + ReD. Thus, it seems that biochar acts as a neutral buffer and decreases the necessity of PPI herbicide application in soybean fields. Biochar application can potentially reduce soil contamination, weed resistance, environmental pollution, and the adverse effects of PPI herbicides on the soil microbial population.

1. Introduction

Biochar, a carbon-rich material, is produced by the pyrolysis of biomass under limited oxygen [1]. It is well-documented that biochar acts as a soil amendment through increasing the cation exchange capacity, water retention, microbial activity, nutrient availability, and sequestering toxic heavy metals [2]. Nevertheless, the role of biochar in ameliorating the physiochemical properties of soils [3] should be considered as a soil improver in agricultural ecosystems, especially in crop protection.

Soybean fields usually contain a complex of grass and broadleaf weeds such as velvetleaf (Abutilon theophrasti) from the Malvaceae family that can reduce the final yield of soybean [4]. For weed control in these agroecosystems, chemical herbicides are commonly used. Trifluralin, a pre-planted incorporated (PPI) herbicide, is recommended against annual grasses such as A. theophrasti in soybean fields [5]. This herbicide belongs to dinitroanilines that act as an inhibitor of microtubule synthesis [6]. Excessive usage of PPI herbicides causes strong adsorption in soil particles with negligible leaching [7] and then has adverse effects on the soil microbial population, such as a reduction in fauna diversity [8], soil and environment contamination, and especially groundwater pollution [9]. Thus, according to the PPI herbicide’s behavior in soils, it seems that changes in the soil’s physicochemical characteristics can solve the abovementioned problems by improving the efficiency of PPI herbicides and reducing their consumption.

The present study aimed to evaluate biochar potential to reduce the need for trifluralin in velvetleaf control through changing the biological conditions in the loam soil of soybean fields.

2. Materials and Methods

The effect of biochar application in soybean fields in order to control velvetleaf was assessed in three blocks and three independent biological replicates in the randomized complete block design. Two blocks were normal soil (NS) with loam soil containing sand (30%), silt (35%), and clay (35%) with pH = 7.55 and EC equal to 1.99 dS·M⁻¹. Another block had a manipulated soil (MS) that contained sand (27%), silt (32%), clay (32%), and biochar (9%) with pH = 7.99 and EC equal to 1.46 dS·M⁻¹. All of the blocks were prepared in 3 m × 3 m terraces.

Two blocks of NS received the recommended dose (NS + RD) (1.7 L/ha) and reduced dose (NS + ReD) (1.2 L/ha) of trifluralin (Trifluralin 48% EC, Ariashimi Company, Iran), respectively. The soil block of MS received only a reduced dose of herbicide (MS + ReD). All herbicide treatments were accomplished in a pre-planted form combined with soil at 5 cm depth when soil humidity was 20%. Two days after herbicide treatments, the authenticated and uniform seeds of A. theophrasti were sown.

The growth of seedlings was monitored for six weeks after sowing for data collection. Then, observed data were changed to numerical percent with inverse trigonometric functions (ArcSin X). The Shapiro–Wilk’s test and Levene’s test were used for normalization and equality of variances, respectively. Finally, the one-way ANOVA followed by the Tukey test (p < 0.05) was subjected to comparison means.

3. Results and Discussion

Results showed that velvetleaf control in NS + RD was 100%, followed by MS + ReD with 93.5% control and no significant difference (F = 9.326, p = 0.092). Meanwhile, the velvetleaf control was measured to be 81% in NS + ReD, having significant differences with NS + RD (F = 1.753, p = 0.013) and MS + ReD (F = 2.159, p = 0.031), respectively. According to the observed results, the biochar addition to the loam soil of soybean fields can reduce herbicide consumption. Previous studies have reported biochar’s ability on soil amendment and its impact on the fate and effects of herbicides in soil [10]. Due to its higher organic carbon content and specific surface area, biochar acts as the most efficient sorbent for herbicides in the soil [11]. Thus, it is suggested that the biochar addition to the loam soils of soybean fields can improve the trifluralin efficiency in velvetleaf control by magnifying herbicide persistence that leads to decreasing herbicide application.

It is worth mentioning that the biochar addition to the soil can also enhance the adsorption of herbicides by altering their mobility which leads to decreased herbicide leaching in soil [12]. Hence, to all appearances, a low dose usage of trifluralin in the loam soil of soybean fields can be related to the biochar role in high adsorption of herbicide molecules to soil particles. Consequently, biochar application leads to promoting trifluralin performance in velvetleaf control. The issue seems to be even more serious when taking groundwater pollution into account. On the other hand, decreasing the soil microbial communities’ exposure to the PPI herbicides can be added to the benefits of biochar addition to the soil [13].

4. Conclusions

Biochar is a valuable soil amender that can help to reduce PPI herbicides in soil which profoundly contributes to gaining sustainable agriculture and improving environmental health. Finally, biochar application in the long term supports the soil microbial population and reduces soil contamination, environmental pollution, and weed resistance as well.