**1. Introduction**

Soil is considered the largest carbon pool, accounting for about 2500 Gt, followed by the atmospheric (760 Gt) and biotic (560 Gt) pools [1]. Soil organic carbon (SOC) includes plants, roots, fungi, and microbial residues and accounts for more than 60% of global total soil carbon [1]. The understanding of weed-removal practices, including herbicides application

**Citation:** Malamataris, D.; Pisinaras, V.; Babakos, K.; Chatzi, A.; Hatzigiannakis, E.; Kinigopoulou, V.; Hatzispiroglou, I.; Panagopoulos, A. Effects of Weed Removal Practices on Soil Organic Carbon in Apple Orchards Fields. *Environ. Sci. Proc.* **2023**, *25*, 25. https://doi.org/10.3390/ ECWS-7-14185

Academic Editor: Luis Garrote

Published: 14 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Correspondence: d.malamataris@swri.gr; Tel.: +30-2310798790 (ext. 216)

and mulching, which affect soil carbon balance dynamics, is crucial for yield-potential preservation, terrestrial ecosystem conservation, and food security [2,3]. Sustainable weed managemen<sup>t</sup> constitutes a challenge for ensuring agricultural productivity since in the case of improper management, infestations of grass and broadleaf weeds could reduce crops yield and quality. The level of weed and crop competition depends on soil and environmental conditions, weed density, and agricultural practices, including crop row spacing, planting density, and the presence of competitive cultivars [4].

Herbicides constitute the primary tactic to address weed control and are widely used, mainly because they are easily applied and improve the cost/benefit ratio of the agronomic business. However, their repeated use may evolve weed resistance to multiple mechanisms of herbicide action [5] and result in herbicide-resistant weed growth in the case of some cultivations [6]. Moreover, their systematic use may adversely affect soil hydraulic properties. Nowadays, attempts are being made to eliminate the use of herbicides in favor of environmental protection and the appearance of herbicide-resistant weed population avoidance.

Mulching is widely recognized as one of the most efficient weed and soil managemen<sup>t</sup> techniques in terms of soil structure improvement, soil moisture evapotranspiration reduction, soil water holding, and soil erosion mitigation, all of which support plant growth [7,8]. The effectiveness of mulching depends on the selected technique, the soil characteristics, and the type of crops residue, while the benefits are usually developing slowly and observed after a long period, which may exceed six or more growing seasons [9]. Mowing and mulching techniques seem to increase SOC content and carbon (C) storage, thus reducing greenhouse gas emissions [10], although relevant studies regarding mulching effects on SOC concentration present contradictory results [8,11,12]. This uncertainty relies on complex and labile soil carbon fractions, including light organic carbon, dissolved organic carbon, particulate organic carbon, and easily oxidizable organic carbon, which present a high spatiotemporal distribution based on local climate, soil texture, and soil-management conditions, as well as a large effect on soil biochemical processes, nutrients, and carbon cycling [7].

The purpose of this study is to evaluate the effects of herbicides and mulching technique application on soil organic carbon in low sloping apple orchard fields.

### **2. Materials and Methods**

#### *2.1. Study Area and Soil Analysis*

The Agia basin in central Greece, where the Pinios Hydrologic Observatory was established in 2015, covers an area of 52.5 km<sup>2</sup> [13]. Based on the Digital Elevation Model produced by the Hellenic Cadastre S.A. at 5 m resolution, the Agia basin spans altitudes varying from 94 to 1520 m a.s.l. (Figure 1). Land use includes forested, agricultural, and urbanized areas. Agriculture, which is characterized by land fragmentation leading to small land parcels in the order of 1–1.5 ha on average, is the dominant socio-economic activity. Apple and cherries orchards form the key cultivations, the irrigation needs of which are covered by groundwater. Considerable spatio-temporal differentiation of groundwater distribution results in seasonal availability deficiency at specific high productivity zones. During such periods, groundwater abstraction depths are considerable, which, along with the increased energy costs, make irrigation managemen<sup>t</sup> and water saving essential.

### *2.2. Field Experiment*

Soil texture in the study area is classified as loam (68.38%), sandy loam (24.86%), silt loam (5.00%), silty clay loam (1.15%), sandy clay loam (0.42%), and clay loam (0.18%). Soil also presents a pH of 6.94, 7.05, and 7.06; 42.66%, 42.97%, and 41.09% sand; 38.42%, 38.14%, and 37.16% silt; and 18.88%, 18.85%, and 21.70% clay, at 0–5 cm, 5–15 cm, and 15–30 cm depth, respectively, based on field measurements conducted in July 2020 along with data derived from the SoilGrids system [14].

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Field studies were conducted at the beginning of the previous growing season, i.e., 2022 in the Agia water basin. In particular, soil samples were collected at 12 points in different orchard parcels; half of them by applying herbicides and the rest by implementmulching technique (Figure 1). The area of parcels ranges from 1.8 to 19.1 ha with averagevalueof7.2ha.Sampleswerecollectedat2differentdepths,i.e.,0–10cmand

collected

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 cm, samples analyzed. Organic carbon content was determined using a wet combustion technique, according to the Wakley–Black method [15]. Soil samples were air-dried, and the fraction of fine earth (<2 mm) was used for the analysis. In total, 1 g of soil was transferred into a 500-mL widemouth Erlenmeyer flask; 10 mL of 1 N K2Cr2O7 and 20 mL of concentrated H2SO4 were added, and the flask was swirled until soil and reagents were mixed. After 30 standing, 200 mL of deionized water was added to the flask, while 3–4 drops of diphenylamine were used as an indicator for the titration with 0.5 N FeSO4.

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**Figure 1.** Study area and location of field parcels under study.

### **3. Results and Discussion**

Figure 2 presents high in-field variability of soil organic carbon in terms of both weed removal practice and soil depth. The difference between the mean SOC in fields applying herbicides and mulching was also found to be statistically significant, as indicated using the student's *t*-test method. SOC was found to be 31% and 7% higher for the fields applying mulching compared to those applying herbicides for depths 0–10 cm and 10–30 cm, respectively. The results confirm the importance of mulching application in SOC fractions, particularly in the upper soil layers [7,16].

**Figure 2.** Soil organic carbon (%) for the two different weed removal practices at two different soil depths.

Increased SOC documented in the fields where mulching was applied provides multiple assets to the farmers that have adopted it. Soil fertility significantly improves, especially

when SOC increase relates to crop residue retention [17]. Soil water holding capacity is also positively affected, despite the fact that the latest literature suggests that actual improvement is rather limited [18]. Precision agriculture and environmentally friendly pest and weed managemen<sup>t</sup> are adopted techniques in the framework of the Green Deal and the Farm to Fork policies that call for a 50% reduction in agrochemicals used in the European Union Member States by the year 2030. Hence, compliance with these regulations and the environmental targets set to call for the systematic adoption of techniques such as mulching is necessary. Additionally, the progressive increase in agrochemicals as a result of the energy crisis and the declining number of licensed drastic compounds for use in the member states is a strong incentive to convert to nature-based solutions such as mulching.
