Organic carbon dynamics in interrows of Vitis vinifera with different years of soil use and management

This study aimed to evaluate the effects of soil use and management on organic carbon stocks in the interrows of Vitis vinifera with different years of cultivation. A vineyard located in Rio Grande do Sul state, with 4, 9 and 16 years of management, was evaluated. Soil samples from different layers, 0.0-0.10, 0.10-0.20, 0.20-0.30, 0.30-0.40, 0.40-0.50 and 0.50-0.60 m, were collected on wheel tracks, between wheel tracks and under the planting rows. As a reference, samples were collected in native grassland of the region. Organic matter content, bulk density and soil porosity were evaluated. Carbon stock was determined according to the correction of the sampled thickness. The results were subjected to analysis of variance and means were compared by Tukey test at 0.05 probability level. Soil management caused alterations in its structure and organic matter content, observed mainly in the first 30 cm, regardless of the area studied. Highest values of bulk density were observed on the tractor wheel tracks, followed by under the planting row and between wheel tracks. Alteration in soil cover for vineyard planting led to a reduction in the carbon stock, compared to the native grassland. The area under management for 4 years showed higher carbon stocks between the wheel tracks, whereas the areas under 9 and 16 years of management showed highest values in the wheel tracks.


Introduction
Agricultural production has led to changes in soil quality, and soil structure is the factor directly related to such changes. Reduction in porosity and consequent increase in bulk density has caused a reduction in soil organic matter contents, contributing to the emission of gases, due to the release of CO2. According to Diekow et al. (2005) and Novais et al. (2007), the type of management adopted is a decisive condition in the alteration of soil carbon stocks and may result in the increase of these stocks in cultivated areas to values higher than those found in the same soil under native vegetation. Deforestation and vegetation burning, associated with the conventional soil tillage, with intense utilization of agricultural machinery, compromise soil properties and notably contribute to climate changes (Bernoux et al., 2005;Rangel & Silva, 2007) There is scientific consensus that terrestrial ecosystems are important for C cycle and that the soil is considered as its largest reservoir (Silva & Medonça, 2007). According to Sleutel et al. (2006), C sequestration in the soil depends on factors such as vegetation cover, management practices and classes of soil. Bayer and Mielniczuk (2008) consider that the following elements are among the abiotic factors are related to retention and/or accumulation of C in the soil: chemical management, climate, soil mechanics and soil capacity to protect organic matter (OM). Silva et al. (2006) point out that biotic compartments such as the edaphic fauna (micro, meso and macro) are important reservoirs of soil OM. Preservation and/or supply of plant material on soil surface, as well as the reduction in its turning, are strategies used to increase carbon storage in the soil, improving its quality, in addition to benefiting the environmental quality (Viana et al., 2016).
In the different land use systems, C is released by the vegetation after its cutting and burning and by the soil after the frequent tillage operations. On the other hand, the use of management practices can potentially mitigate and reduce C emissions, by sequestrating, capturing and maintaining it for as long as possible in the biomass, soil and oceans (Rosa-Filho et al., 2014). Alterations in land use, therefore, are closely related to organic matter dynamics and the biogeochemical cycle of the elements, modifying the production capacity of these environments (Brown et al., 1994). The content of organic carbon in the soil under different systems provides important information to assess soil quality.
Conversion of soil cover for the production of perennial crops causes alterations in soil quality, and organic carbon content is the most management-sensitive indicator. However, studies related to organic carbon dynamics in areas under perennial crops, especially observing the sites influenced by the traffic of tractor tracks, compared to sites not influenced, are scarce. In view of the above, this study aimed to evaluate the effects of use and management on organic carbon stocks in interrows of Vitis vinifera with different years of cultivation.

Methodology
The study was carried out in a vineyard (Vitis vinifera, var. Pinot noir) situated in the municipality of Encruzilhada do Sul, at 30° 31' 37" South, 52° 31' 6" West of Greenwich and 348 m of altitude. The climate of the region is classified as humid subtropical (Cfa) with hot summer and mean annual temperature and rainfall of 17.4 °C and 1,533 mm, respectively. Pinot Noir, grafted on 1103 Paulsen, cultivated with 4, 9 and 16 years of planting on Ultisol Xerults (EMBRAPA 2018), distributed at spacing of 2.5 m between rows and 1.3 m between plants, supported by a trellis system at three positions: planting row (CPL), wheel tracks (WT) and between wheel tracks (BWT), in the layers of 0.0-0.10, 0. 10-0.20, 0.20-0.30, 0.30-0.40, 0.40-0.50 and 0.50-0.60 m. Soil samples were also collected in a native grassland of the region (umbric Dystrophic Red Argisol), as a reference. The physical characteristics of the soils in the evaluated areas are described in Table 1. Table 1. Particle-size distribution, particle density (PD) in the 0. 00-0.10, 0.10-0.20, 0.20-0.35, 0.30-0.40, and 0.40-0.60  The difference between the weights of the samples at saturation and after application of a 6 kPa suction on the table tension was used to determine soil macroporosity (Ma) (pores with diameter larger than 0.05 mm), whereas the difference between the weights of the samples after 6 kPa suction application and after drying in oven at 105 ºC was used to determine soil microporosity (Mi) (pores with diameters between 0.05 and 0.0002 mm). Soil total porosity (TP) was obtained by the sum of these two attributes. Soil bulk density (BD) was determined by the ratio between the mass of soil dried in oven at 105 ºC and ring volume.
Organic matter content (OM) was estimated based on soil organic carbon content (OC), determined by the modified Walkley-Black wet combustion method, described by Tedesco et al. (1995). For that, 0.5 g of soil were weighed and transferred to a 500-mL Erlenmeyer flask and then mixed with 10 mL of 1N K2Cr2O7 and 20 mL of concentrated H2SO4. The solution was shaken for 1 minute and left to rest for 30 minutes. Then, 200 mL of distilled water, 10 mL of concentrated H3PO4 and 1 mL of diphenylamine indicator were added and the solution was titrated using 0.5N (NH4)2FeSO4.10H2O.
Soil organic carbon stocks were determined using the equivalent soil mass method proposed by Ellert et al. (1995), according to the following equation:
Since soil compaction leads to the collection of a sample smaller than required for the study, soil thickness was corrected using the equations proposed by Conceição (2006).

Tadd/sub = ((Mref-Mtreat) x 0.0001 ha m -2 ) / BD (2)
Where: Tadd/sub = soil thickness to be added to or subtracted from a certain layer of the treatment to attain the corresponding value of the equivalent mass (m); Mref = equivalent soil mass of the respective layer in the reference area (Mg ha -1 ); Mtreat = equivalent soil mass of the respective layer of the treatment (Mg ha -1 ); BD = bulk density (Mg m -3 ).

Tc = T + Tadd/sub (3)
Where: Tc = corrected thickness (cm); T = thickness (cm); Tadd/sub = soil thickness to be added to or subtracted from a certain layer of the treatment to attain the corresponding value of the equivalent mass (m). OCS = organic C stock at certain depth (Mg ha -1 ); OC = total organic C content at the sampled depth (g kg -1 ); BD = bulk density (g cm-3); Tc = corrected thickness of the considered layer (cm).
The experimental design was randomized blocks, analyzed as split-split plots, considering the reference area and the three years of management after planting the vines as main plots (NG,4,9 and 16 years), three sampling sites as subplots (BWTbetween wheel tracks, WTin the wheel tracks and CPLplanting row) and six layers as sub-subplots (0-10, 10-20, 20-30, 30-40, 40-50 and 50-60 cm), in three replicates. All attributes were subjected to analysis of variance and means were compared by Tukey test at 0.05 probability level.

Results
Based on the results for the analysis of soil structure, it is possible to note that, in general, there was a significant statistical difference in macroporosity (Ma), microporosity (Mi), total porosity (TP) and bulk density (BD), in relation to the three cultivation periods after years of planting the vines (4, 9 and 16 years), the sampling sites between wheel tracks (BWT), wheel tracks (WT) and planting row (LP), and depth (Table 2).
In all studied sites ( there is no management aiming to change soil structure, such as ploughing, which allows for a natural bulk density, due to the rearrangement of mineral particles. According to the results obtained for soil organic matter (OM), there was significant statistical difference between all studied areas. The native grassland showed the highest values, between 2.97 and 2.37 g kg -1 . In relation to the managed areas, the highest values were observed in the area with 9 years of management (1.70 to 2.90 g kg -1 ), followed by the areas with 4 and 16 years of cultivation, in which the values ranged from 1.30 to 2.73 g kg -1 and from 1.33 and 2.37 g kg -1 , respectively. Lower  According to the organic carbon stocks in the soil (Figure 1), in general, there was significant statistical difference between the years of cultivation and the reference area, sampling sites and depth. The native grassland, due to the lack of change in its cover and in its use and management, showed the highest carbon stocks in the soil, 11.80 Mg ha -1 . Regardless of the years of cultivation and sampling position, the highest carbon storage was found within the first 0.30 m, with values ranging from 1.18 to 2.78 Mg ha -1 .  Considering the years of planting, the area with 9 years of cultivation had the highest mean value (9.19 Mg ha -1 ), followed by the areas with 4 and 16 years of management, with mean values of 8.90 and 9.10 Mg ha -1 , respectively. Lower values in the area with 4 years were observed because this area is still affected by the turning performed during soil tillage, which led to accelerated oxidation of organic matter.
In relation to the sampling sites, the highest values were observed in the wheel tracks of the tractor (WT), with mean value of 9.37 Mg ha -1 , followed by the positions between wheel tracks (BWT) and planting row (CPL), with mean values of 8.52 and 8.30 Mg ha -1 , respectively. This can be attributed to the higher values of microporosity at this site, which favored the temporary mitigation of C among soil particles. However, with continuous movement of tractor wheels, the soil may lose its capacity for C storage and sequestration in its structure.

Discussion
Soil tillage is the activity with greatest influence on physical attributes because the application of dynamic loads by tractor wheels and agricultural implements on the soil produces stresses at the soil/tire and soil/implement interfaces in surface and subsurface, compacting the different layers of the soil (Wiermannn et al., 2000;Vieira & Klein, 2007). According to Spera et al. (2004), the conversion of natural ecosystems to agricultural activities, completely aimed at commercial production, leads to changes in soil physical properties which are usually unfavorable to plant development. Braida et al. (2010) observed that soil susceptibility to compaction can be altered by organic matter accumulation, but soil structure and its effects associated with water retention, cohesion and bulk density determine its magnitude.
According to Reichardt and Timm (2004) and Libardi (2005), medium-textured soils should have microporosity ranging from 60 to 70% and from 30 to 40% of the total porosity, respectively, in the first 30 cm, which was not observed in the present study. In general, up to this depth the soil showed macroporosity between 7 and 30% of the total porosity and microporosity between 69 and 63% of the total porosity. This reflects the effects of soil compaction, regardless of use and years of management.
The results of soil bulk density were similar to others found at different sampling positions in medium-textured soils in the interrows of vineyards in Santa Catarina (Valois et al., 2014), and in other perennial crops such as coffee (Gontijo et al., 2008) and citrus (Santos et al., 2014) subjected to intense traffic of machines, in which the increase in bulk density resulted in reduction in the total volume of pores (TP). According to Araujo-Junior et al. (2011), this result can be attributed to the soil tillage systems. The values found were considered as critical for a good development of crops in the first 0.30 m of a mediumtextured soil (Durigan & Timossi 2002;Reichert et al., 2003).
According to Rosa (2009), such alteration in soil structure leads to reduction in aeration and water infiltration in the soil, due to the decrease in macroporosity. On the other hand, the increase in microporosity favors the retention of the small volume of water which percolates in the soil profile. However, in some situations, this water is not necessarily available to plants because there are situations in which the diameter of these pores is greatly reduced and water may remain retained with very high energy, thus being unavailable to plants (Klein et al., 2009;Toigo 2010).
Higher values of OM in surface were observed because this layer has greater accumulation of material on soil surface, microorganisms and higher presence of roots of cover plants, especially in the native grassland and in the area with 9 years of management, due to the roots of the vines, which contributed to the cycling of nutrients, aggregation and water storage (Alcântara et al. 2000, Boer et al. 2007).
The conversion of the type of cover, with turning of the soil to plant grape seedlings, caused alteration in C stock, evidenced by the values found in the native grassland and the areas with different soil uses and managements. Similar results were found by Stürmer et al. (2011), evaluating C stock dynamics in deforested and revegetated areas in Southern Brazil.
According to the literature, substituting native pasture and natural forests for agricultural systems of cultivation may lead to a reduction in soil C content, primarily in response to the acceleration in the decomposition rate caused by the cultivation of the soil and greater aeration and physical exposure of OM to decomposer microorganisms (Zinn et al., 2005).
Soil C stock can be altered with higher or lower intensity, depending on the agricultural system adopted, and is one of the most sensitive attributes to the transformations triggered by management. Inadequate management practices may lead to rapid decline in these stocks, contributing to the increase in carbon dioxide (CO2) emissions to the atmosphere (Freixo et al., 2002).
It is possible to observe that the soil under the wheel tracks showed the highest C stocks, mainly in the areas with 9 and 16 years of cultivation. This can be attributed to the fact that, with the modification in soil structure, caused by compaction, the organic matter was closely related to the micropores of the soil.

Conclusion
Soil management caused alterations in soil structure and organic matter content, especially in the first 30 cm, regardless of the area studied; Highest bulk density values were observed in the tractor wheel tracks, followed by the planting row and between wheel tracks; Alteration in soil cover to plant the vineyard led to a reduction in the carbon stock, compared to the native grassland; Carbon stocks were heterogeneous in the vineyard interrows at the sites WT, BWT and CPL; The area with 4 years showed higher carbon stocks between the wheel tracks, whereas the areas with 9 and 16 years showed higher values in the wheel tracks of the tractor.

Agradecimentos
Aos colegas e servidores da Universidade Federal do Rio Grande do Sul pelas contribuições para com este estudo e aos órgãos de fomento CAPES e CNPq pelo auxílio financeiro.