Organic Carbon Storage in Waterlogging Soils in Ávila, Spain: A Traditional Agrosilvopastoral Region

Abstract

Soils play a crucial role in the protection, management, and ecological understanding of the La Moraña region, located in Ávila province, Central Spain, which has a moderate population, traditional agriculture, livestock farming, and low industrial activity, resulting in relatively low environmental degradation. The region’s soils often experience prolonged water stagnation, influencing its agronomy, ecology, and economy. This study aimed to estimate and understand the soil’s role in the C sequestration of an agrosilvopastoral system under conditions of temporary water stagnation and different land uses. The results showed that ryegrass-magaza and Pinus pinaster show more content in soil carbon sequestration storage (98.7 and 92.4 Mg per hectare) compared to the adjacent degraded rangeland (75.8 and 63.9 Mg ha⁻¹). Arenosols exhibited a higher total amount of SOC stocks. The soil profile with ryegrass sequestered more nitrogen (9.7 Mg ha⁻¹) than other land uses; moreover, Arenosols have a lower nitrogen sequestration capacity even in low-forest conditions. The study highlights significant differences in carbon accumulation due to the management practices, temporary water layers, and parent material.

1. Introduction

On a global scale, soil plays a vital role by providing food, clean water, and habitats for biodiversity, while also contributing to climate resilience. It supports cultural heritage and landscapes, but remains a fragile resource that must be carefully managed and protected for future generations [1,2]. Soil serves important functions, such as producing sufficient quantities of nutritious and safe food, feed, fiber, and other biomass for industries. It also regulates and stores water, replenishes aquifers, purifies contaminated water, and mitigates the effects of droughts and floods, thereby aiding in climate change adaptation. Additionally, soil captures carbon from the atmosphere through the process of photosynthesis, playing a key role in climate mitigation [1,2]. However, human activities, often unsustainable, can have a negative impact on soils. For this reason, it is crucial that we adopt sustainable management practices in agriculture and forestry to prevent industrial contamination. As noted by [1], there is a strong connection between soil restoration, carbon sequestration, food security, and biodiversity. Carbon sequestration in terrestrial ecosystems, particularly soils, is a crucial issue that requires the attention and understanding of soil scientists, agronomists, foresters, ecologists, and policymakers. Grasslands, in particular, are a key factor in sequestering atmospheric carbon and providing other essential benefits [3]. Grassland soils constitute a large potential reservoir of soil organic carbon (SOC), but this potential is likely to depend on grass and livestock management and soil treatments [4]. Adopting permanent grazing may improve SOC storage, microbial activity, and productivity [5] in agrosystems. On the other hand, forests are another fundamental store of carbon for living or dead biomass, and in soils under tree cover where two-thirds of the total is stored [6]. The integration of trees in agriculture conserves soil, provides food, fodder, and timber, and enhances ecosystem health and diversity. Moreover, Lorenz and Lal [2] are considered powerful tools for carbon sequestration. In line with this, Aryal et al. [7] point out the influence of silvopasture systems on carbon capture in Mexico, reporting a significant increase in soil organic carbon content (45–54%) under tree pasture systems compared to open pasture. Therefore, it is not surprising that there is a growing body of literature on the carbon sequestration potential of agroforestry, including studies by Saha et al. [8], Cardinael et al. [9], Feliciano et al. [10], and Shi et al. [11], Rumpel and Chabbi [12], among others.

Ávila is a province in Spain known for its well-preserved natural environment, which has been maintained due to various factors, such as the region’s subsistence farming practices and the mountainous landscape in parts of its territory. The La Moraña region, a site in this province, is a territory where farming practices have coexisted with flora and fauna for decades, so agriculture and nature conservation are perceived as far from being considered a source of conflict. In this region, native vegetation pasture (dominated by annual species), forest, and crops (barley or wheat, or both) constitute an important agrosilvopastoralism system.

Indeed, agriculture and livestock are the primary land uses in La Moraña. The central concept in this area is that a farm operates as a system where farmers conduct their activities to achieve production goals while also maintaining the sustainability of the land’s potential, aligning with the views of Fresco et al. [13].

This region is distinguished by its well-preserved meadows, pastures, and patches of oak forests, which offer essential ecosystem services like carbon sequestration and storage. An identifying feature of La Moraña is that many of its soils are affected by hydromorphism, meaning they experience periods of water stagnation. Huerta et al. [14] noted that Miocene and Quaternary sands form excellent aquifers with underground flows moving from south to north. These authors also observed that, near El Oso, the water table is very shallow, with an ascending vertical flow component. These wetlands are small and typically dry for part of the year, especially in summer. They are primarily recharged during the winter through direct precipitation, although during the dry summer season, they can receive contributions from groundwater.

Linkemer et al. [15] have previously highlighted the negative impact of waterlogging on crop growth. However, the management of waterlogged soils can offer valuable ecosystem services, such as carbon sequestration. Considering Amendola et al. [16] emphasize that wetland soils are a crucial part of the Global Carbon Cycle, storing approximately 20–25% of the world’s terrestrial soil organic carbon (SOC), it raises important questions about the role of soils in La Moraña in terms of carbon sequestration, particularly in a region like the one studied.

The key aims of this study are: (i) to evaluate the vertical distribution of soil organic carbon (SOC) and nitrogen stocks throughout entire soil profiles, (ii) to investigate the soil factors that influence SOC accumulation, and (iii) to examine the effects of prolonged flooding on the soils of La Moraña, as well as the dynamics and accumulation of SOC in surface soil layers under different land uses in Cambisols and Arenosols.

2. Materials and Methods

2.1. The Study Area

La Moraña region is a plain extension approximately 5 to 10 km wide, situated in the northeastern part of the Sistema Central in the province of Ávila, Spain (Figure 1). Within La Moraña, El Oso lagoon occupies a shallow natural depression with a depth of less than 1 m and a flat bottom. The lagoon is situated on Tertiary formations consisting of arkoses, which have a yellowish-brown hue and occasionally appear gray. These arkoses were deposited in the southwestern part of the Duero Basin during the mid-Miocene. Furthermore, the Tertiary layers in the area are covered by Quaternary sandy deposits, which belong to an extensive dune system [17].

There is limited literature on the unique characteristics of La Moraña, as noted by Desir et al. [17], Sanz Donaire and García Rodríguez [18], and Martín Escorza [19]. The Duero River basin, located in a semi-arid region of the Iberian Peninsula, contains numerous wetlands due to its steppe-like nature. La Moraña lies within the Ávila cereal plain, a small territory characterized by sandbanks and arkoses.

The Geological Map of Spain (MAGNA) provides a broad geological overview of the study area [20]. The surrounding landscape patterns belong to the southern Duero basin, where the flat terrain produces an endorheic phenomenon associated with a locally high water table, such as the well-known El Oso lagoon. Arkosic muds predominate in the study area, alongside areas of aeolian sands.

Within this region, small dunes of aeolian sands are present, promoting the growth of pine forests. According to Desir et al. [17], the most extensive development of these dunes occurs several kilometers to the north. These typical dunes are located directly adjacent to advancing sand fronts and generally do not rise more than 10 m above the surrounding topography. Pine species such as Pinus pinaster and Pinus pinea thrive on these dunes, although along nearby vineyards.

The area is underlain mainly by Miocene arkoses, post-Tertiary deposits such as river terraces, Pliocene coluvions, and aeolian dunes. The landscape is flat or smoothly undulating in the larger part of the area and hilly in the portion bordering the Mountain Range (Central System). Certainly, sands of aeolian origin have been deposited in some places, that form mantles and dunes, and lacustrine deposits made up of sands and shales with carbonate precipitates from the Holocene [20,21]. It is important to note that both Miocene and Quaternary deposits are naturally porous in nature and permeable, so they constitute good aquifers.

The vegetation is composed of the pine forest of Rodeno pine (Pinus pinaster) on sand; salt marsh dominated by Camphorosma monspeliaca; hydrophilic meadows around the lagoon and humid areas; dry farmland (highly anthropized); and mimetic channels with the matrix where they are inserted. Barrera et al. [22] cite taxa in La Moraña that probably constitute the first provincial appointments. Nevertheless, the study area is characterized by diverse land uses. Agriculture in the region is primarily family farming focusing on continuous barley and wheat cultivation in a rainfed system (without irrigation) and without the use of external inputs such as amendments or fertilizers. In pasture areas, the natural xerophilic vegetation serves as the main source of feed for cattle, sheep, and goats within an extensive grazing system. Secondary forest restoration areas, previously used for pasture or agriculture, are aimed at ecosystem recovery. Native forest areas with no human intervention serve as reference or control areas for comparison. From an agricultural perspective, cereal farming, particularly barley and wheat, has been practiced in the region for centuries. As a result, the natural vegetation, primarily Quercus ilex ssp. ballota forests, has been reduced to small remnants of its original extent.

According to Martín-Garcia et al. [23], the average annual temperature in the area is 12.8 °C, and the average annual precipitation is 411 mm, with a potential evapotranspiration of 731 mm. The local climate was determined based on the aridity index (AI), calculated according to the definition provided by the United Nations Environment Program [24] using Equation (1) Aridity Index:

$$AI={\displaystyle \frac{Pr}{ET}}\times 100$$

(1)

where:

AI = aridity index;

Pr = mean annual precipitation (mm);

ET = mean annual reference evapotranspiration (mm), calculated by the Penman–Monteith/FAO method;

Aridity index is as follows:

$$AI={\displaystyle \frac{411}{731}}\times 100=56.2$$

Based on this methodology, the regional climate is categorized as follows: (a) arid (AI < 20), (b) semi-arid (20 ≤ AI < 50), (c) dry sub-humid (50 ≤ AI ≤ 65), (d) wet sub-humid (65 ≤ AI ≤ 100), and (e) humid (AI ≥ 100). With an AI value of 56.2, the classification for this region is dry sub-humid.

2.2. Soil Sampling

Four geo-referenced soil profiles were excavated to a depth of 1.5–2 m for a full description according to FAO Guidelines [25], with color by Munsell [26]. Some features of the analyzed profiles are presented in

Table 1. Some features of the analyzed profiles.

Profile	Parent Material	Vegetation/ Use	Soil Name		Horizon Depth (cm)	Color Munsell
			Soil Taxonomy	WRB-FAO
1				Eutric Cambisol	Ah1 (0–20)	10YR5/2	Yellowish-brown
1	Arkoses	Ryegrass,	Aquic	(Protocalcic,	Ah2 (21–75)	10YR3/4	Dark yellowish-brown
1		grassland	Haploxerept	Oxyaquic	Bw (76–110)	10YR7/1	Light gray
1				Alkalic)	C > 111	10YR5/3	Pale brown
2				Eutric Cambisol	Ah1 (0–18)	10YR6/1	Gray
2	Arkoses	Hydrophilic	Aquic	(Protocalcic,	Ah2 (19–72)	2.5Y5/2	Grayish-brown
2		grassland	Haploxerept	Oxyaquic	Bw (73–120)	10YR7/1	Light gray
2				Alkalic)	C > 121	10YR5/4	Yellowish-brown
3		Pine forest, grassland, rattle grass	Oxiaquic	Eutric Arenosol	Ah (0–16)	10YR3/3	Dark brown
3	Aeolian sands		Xeropsamment	(Aeolic,	C1 (17–78)	10YR6/4	Light yellowish-brown
3				Humic)	C2 > 79	10YR5/4	Yellowish-brown
4				Eutric Arenosol	Ah (0–18)	10YR6/1	Gray
4	Aeolian sands	Ryegrass,	Oxiaquic	(Aeolic,	AC (19–42)	10YR5/4	Yellowish-brown
4		magaza	Xeropsamment	Oxyaquic)	Ahb (43–71)	10YR4/4	Dark yellowish-brown
4					C > 72	10YR5/6	Yellowish-brown

Soil Taxonomy = Soil Survey Staff 2014 WRB-FAO = IUSS Working Group WRB 2015.

, while Figure 2 shows the distribution of these soil profiles. These profiles were sampled separately from horizon to horizon until the parent material was reached.

In September 2013, pig-fattening slurry was applied, and ryegrass was sown the following month in October (profile 1). The ryegrass was grazed by Hereford cows using a rotational grazing system, managed by an electric shepherd. This rotational grazing took place three times between February and June.

In the pine forest, where profile 3 is located, the cows have been grazing for several years. Their trampling helps to incorporate pine needles into the soil. During periods without pasture, external inputs of fodder and straw are provided. The fodder is spread across the pine forest floor using a unifeed cart, while straw is left in bales (“straws”) near the straw barn. The cows eat from these bales but leave behind a considerable amount of straw around the area.

Eight sampling sites were selected in early August this year (see

Table 2. Some features of the analyzed topsoils samples. The depth at which the samples were taken is 0–20 cm.

Soil Sample	Land Use
1	Pine forest, grazed with free-range Hereford cows and, currently, horses
2	Cropland planted with ryegrass, used by Hereford cows with rotational grazing
3	Cropland that was planted with wheat harvested by Hereford cows with rotational grazing and, currently, goats on that plot
4	Pine forest grazed with Hereford cows only once, with little impact
5	Cropland that was planted with triticale, harvested by Hereford cows with rotational grazing
6	Soil in the immediate vicinity of the lagoon: grey impermeable and bare soil
7	Ungrazed pine forest
8	Cropland that was planted with triticale and rye, harvested by free-range Wagyu cows.

). We focused on a shallow soil depth (0–20 cm) because of its significant role in agricultural and forestry production. At each site, we collected five random samples using an auger and stored them in polyethylene bags. These individual samples were then combined to create a composite sample. After sampling, any extraneous materials such as stones and leaves were removed. The samples were dried at room temperature, crushed, sieved through a 2 mm mesh, and prepared for further laboratory analysis. Additionally, the bulk density was measured in undisturbed samples.

2.3. Laboratory Methods

Samples were air-dried, and roots, stones, and other debris were eliminated by sieving through a 2 mm mesh. Soil texture was analyzed using the Bouyoucos hydrometer method [27]. Bulk density was measured with the cylinder method [28]. pH levels were determined potentiometrically in a 1:2.5 soil-to-water suspension and in KCl (1 N) using a digital pH meter. The Kjeldahl method [29] was employed to estimate the total nitrogen available in the soils, while total nitrogen was assessed using the same method [25]. Organic carbon content was analyzed through dichromate digestion (wet oxidation technique) [30], and available phosphorus was measured colorimetrically based on its reaction with ammonium molybdate [31].

In addition, for undisturbed samples (from the 8 surface samples), bulk density was measured using the cylinder method [28]. The process involved pressing the undisturbed soil into a cylindrical metal core. Afterward, the soil was sliced at both ends with a knife and placed into a box for further analysis.

2.4. Estimation of the C and N Stocks

The C and N stocks of each horizon of each profile were determined according to FAO recommendations [32], also used by Veldkamp [33] and Batjes [34], following the Equation (2) Stock equation of the horizon:

$$Stock=(A\times T\times BD)\times (1-Fg)\times 100$$

(2)

where:

Stock C or N is stock of the horizon (Mg·ha⁻¹);

A is C (%) or N (%) content of the horizon;

T is the horizon thickness (m) of the horizon;

BD is the bulk density of the horizon (Mg·m⁻³);

Fg is the coarse fractions (>2 mm) (g/g). Since a negligible stone content was detected in the soils studied, the stone content was not taken into account in the estimation of the C and N stocks.

3. Results

The soils were Cambisols and Arenosols [35], and Inceptisols and Entisols [36], and exhibited differences among them in the physical–chemical and chemical parameters, and also in terms of the parent material nature and management type. These soils are characterized by low to very high pH values (5.63–9.10) (

Table 3. Results of the analytical data.

Soil	Hor.	Depth (cm)	BD (Mg m−3)	SOC (%)	pH	P (ppm)	N (%)	C Stock (Mg ha−1)	N Stock (Mg ha−1)
1	Ah1	0–20	1.23 ±0.07	1.16 ±0.04	6.32 ±0.11	65.9 ±0.21	0.152 ±0.01	28.5	3.7
	Ah2	20–75	1.55 ±0.09	0.13 ±0.01	8.64 ±0.12	37.1 ±0.14	0.055 ±0.01	28.1	4.6
	Bw	75–110	1.41 ±0.07	0.07 ±0.01	8.75 ±0.10	18.4 ±0.19	0.028 ±0.02	6.3	1.3
2	Ah1	0–18	1.52 ±0.06	0.98 ±0.02	8.11 ±0.11	84.3 ±0.13	0.173 ±0.02	15.8	4.7
	Ah2	18–72	1.689 ±0.09	0.53 ±0.01	9.03 ±0.09	94.8 ±0.17	0.066 ±0.01	48.3	6.0
	Bw	72–120	1.628 ±0.07	0.15 ±0.01	9.10 ±0.09	43.7 ±0.19	0.019 ±0.01	11.7	1.6
3	Ah	0–16	1.173 ±0.07	4.60 ±0.09	6.61 ±0.10	94.2 ±0.15	0.179 ±0.03	86.3	3.3
	C	16–68	1.461 ±0.06	0.1 ±0.01	6.67 ±0.10	15.3 ±0.19	0.021 ±0.01	7.5	1.2
4	Ah	0–18	1.079 ±0.07	2.49 ±0.09	6.92 ±0.09	210.3 ±0.13	0.368 ±0.03	48.3	0.7
	C	18–42	1.222 ±0.08	1.72 ±0.08	5.63 ±0.09	76.3 ±0.16	0.084 ±0.01	50.4	2.4

). There appears to be a moderate SOC content in surface horizons (0.98%), to a high SOC content under forests (4.60%) with a decrease in depth.

The soil reaction was acidic in some areas due to the organic nature of the soil, while, in others, it was alkaline due to the stagnation of water rich in bases [37]. This variation trend is observed in measurements carried out both in water and in KCl. The higher pH values in water compared to the pH values in KCl indicate the predominance of a net negative charge in all soils.

Soil profile 3 shows a high content of organic matter, with a clear tendency for this content to decrease gradually with depth (Figure 3).

Soil texture exhibits significant variability, influenced by whether the soils are formed from arkoses or sandy parent materials. Phosphorus levels range from 18.4 to 210.3 ppm, indicating a medium to high availability, while the nitrogen content shows a wide variation, from 0.019% to 0.368%.

The data about the soil organic carbon stock (SOCS) appear in Table 3, which shows significant variations concerning land use types and depth. In the upper horizons, the SOC stock in the pine forest (86.3 Mg ha⁻¹) was significantly higher than in ryegrass-magaza (48.3 Mg ha⁻¹) and lowest in grazing land (28.5 Mg ha⁻¹) and hydrophilic grassland (15.8 Mg ha⁻¹). The SOCS consistently declined with depth across all land uses, except under ryegrass-magaza due to burial processes within the profile. The SOCS values were highly heterogeneous between Cambisols and Arenosols across the horizons (Table 3). Regarding the total carbon stock, profile 4 has the highest amount, at 98.7 Mg ha⁻¹, followed by profile 3 at 93.8 Mg ha⁻¹, both of which are on sandy parent materials. In contrast, the profiles with the lowest carbon sequestration capacity are profile 2, at 75.8 Mg ha⁻¹, and profile 1, at 62.9 Mg ha⁻¹, both located on arkose parent materials.

The data gathered and analyzed indicated that the topsoil carbon sequestration of soil under Pinus pinaster was 86.3 Mg ha⁻¹ and is significantly higher than that of deteriorated rangeland. Similar results were found by Jarecki et al. [38].

The difference in SOC stock between diverse land use types narrowed with soil depth. The high SOC content in surface horizons could be attributed to the continuous addition of undecayed and partially decomposed plant and animal remains. It is also possible that the relatively high amount of SOC in the study area could be related to the adequate application of organic inputs (manure) and moderate cultivation.

Although the number of samples does not allow for robust statistical analysis, it was observed that the amount of SOC in soil increased with the amount of clay (in soils developed on arkoses). This may be related to the coating of soil organic carbon (SOC) around clay minerals, which protects it from weathering and microbial degradation, thereby helping the SOC remain in the soil for extended periods.

Regarding the nitrogen (N) stock, soils under ryegrass-grassland on arkose parent materials stored 9.6 Mg ha⁻¹, while hydrophilic grassland stored 12.3 Mg ha⁻¹. On aeolian sands, soils under pine forest stored 4.5 Mg ha⁻¹, whereas ryegrass-magaza stored 3.1 Mg ha⁻¹. As expected, a discernible trend is observed.

Pine forests, grasslands, and croplands were designated for different land uses, with ryegrass being classified separately due to its significance. The grassland soils have been covered with deep-rooted native grasses for over 15 years, while the cropland soils have been cultivated with wheat and have had only one year of fallow. Conventional reduced tillage has been employed, with significant applications of manure from livestock.

In the topsoil (0–20 cm), SOC contents range from 0.34% to 1.45% (see

Table 4. Values of organic carbon, nitrogen, C:N ratio, and bulk density in soils under different land uses. Calculated values of carbon and nitrogen storage. Depth 0–20 cm.

Soil Sample	Use	SOC (%)	N (%)	C/N	BD (Mg m−3)	C Stock (Mg ha−1)	N Stock (Mg ha−1)
1	Pine forest	1.04	0.164	6.3	1.546 ±0.05	32.0	5.0
2	Cropland	1.45	0.336	4.3	1.454 ±0.06	42.7	9.7
3	Cropland	1.25	0.259	4.8	1.437 ±0.07	35.9	7.4
4	Pine forest	0.56	0.184	3.0	1.609 ±0.08	18.0	5.9
5	Cropland	0.80	0.270	2.9	1.514 ±0.08	24.1	8.2
6	Lagoon	0.60	0.299	2.0	1.521 ±0.06	18.2	9.0
7	Pine forest	0.34	0.168	2.0	1.652 ±0.07	11.2	5.5
8	Cropland	0.73	0.262	2.8	1.501 ±0.06	21.8	2.6

), while total N varies between 0.164% and 0.336%. Variations and trends across land uses were not particularly pronounced. The highest SOC and N contents were found in cropland soils, while the lowest was detected in pine forest soils. Table 4 also shows C/N ratios ranging from 2.0 to 6.3, which we attribute to different land uses; these differences likely reflect variations in the composition of organic matter entering the soil due to different plant covers. The highest C/N ratio was observed in cropland soils and the lowest in pine forest soils. We attribute this to the greater mineralization and oxidation of organic matter in cultivated soils. The relatively higher C/N ratio in cropped soils is likely due to the addition of manure.

Regarding the carbon stock in the top 20 cm of soil (Table 4), it is observed that the highest accumulation occurs in soils with crops that have been temporarily covered with ryegrass and then grazed by Hereford cattle under controlled conditions. Soil under pine forests does not reach as high values as those in croplands, although the values are not very far apart. We hypothesize that these variations in carbon storage are possibly due to changes in land use [39], although the period has been short, and it usually takes several decades for changes in land use to become apparent [40]. Soils near the lagoon do not show significant carbon accumulation, which is interpreted as being due to hydromorphism, which acts as a limiting factor for organic carbon accumulation.

The highest nitrogen content is observed in cropland with ryegrass (Table 4), even though it has been grazed. The lowest value is found in cropland with free grazing. Pine forests show intermediate values, while the lowest values occur without grazing. This suggests that controlled grazing can enhance the nitrogen stock, whereas free grazing does not have the same effect.

4. Discussion

Nowadays, organic matter is a critical parameter for assessing certain aerodynamic properties of soils. The study results demonstrate that the SOC content in the soils under study ranges from moderate (0.98%) to high (4.60%) under forests in surface horizons. It is also observed that the SOC content decreases in deeper horizons, consistent with the findings of Frank et al. [41]. However, these results differ from those of many common soils in the region as reported by Forteza et al. [42], which typically have lower organic matter contents.

The soil organic matter content generally increases with vegetative cover and afforestation [6]. In the study area, traditional practices such as ‘rotation of pasture lands’ and ‘pasture cropping’ have been employed, which can explain the higher soil organic carbon content observed in grazing land. Thus, Bai and Cotrufo [4] state that proper grazing management can generate a high carbon gain; and other authors indicate that the adoption of permanent grazing in agrosystems can improve soil carbon storage, in addition to other benefits [5].

Comparing Cambisols to Arenosols, it is evident that Arenosols have a lower capacity for nitrogen sequestration, even when under forest cover. Dos Santos et al. [43] reported that Acrisols, Arenosols, and Regosols had higher nitrogen stocks (8.07 to 9.72 Mg ha⁻¹) compared to Ferralsols, which had lower nitrogen stocks (6.10 Mg ha⁻¹) similar to those of shallower soils (5.87 to 6.72 Mg ha⁻¹)

While soil use and management play significant roles in agricultural systems, Lal [1] highlights that climatic factors and soil characteristics can also impact the sequestration of organic carbon. For example, according to Lal and Bai et al. [1,44], clayey soils can sequester more carbon than sandy soils. Traditionally, agrosilvopastoral systems have been used as sustainable production models in La Moraña. These models can promote soil organic carbon storage [5]. The mean SOCS in the Spanish topsoil layer was 56.57 (MgC ha⁻¹) [45]. According to Rabbi et al. [46], soil carbon and nitrogen stocks rise with a higher environmental humidity.

It is expected that prolonged soil moisture in La Moraña is associated with the growth of larger vegetation types, which could contribute to increased carbon and nitrogen accumulation in the area’s soils.

In addition, the flat topography of the studied territory, associated with the soil parent material and climate, controls the hydrological flow, affecting the rates of weathering and profile development [47]. As stated by Wang et al. [48], the flat relief allows for greater water infiltration into the soil, favoring the formation of deeper profiles, with a greater potential for the accumulation of C and N.

This accounts for the relatively high clay content and the resulting water retention, which can cause stagnation. In the soil profiles, the parent material is fairly uniform, composed mainly of arkoses and sands. The influence of groundwater depends on its residence time in the subsoil. This duration cannot be annual, as the humid areas dry out in the summer. However, the effects of water stagnation are visible in the area (Figure 4).

However, according to Calvo de Anta [49], the carbon stocks gradually increase as precipitation increases, and its variability is also dependent on other factors, fundamentally, the presence/absence of active lime or active Al.

It is widely recognized that the impacts of waterlogging are associated with excess water, leading to reduction processes [50] that typically result in low soil chroma and value (<2). The lack of oxygen also reduces organic matter mineralization, leading to the accumulation of organic carbon in the soils [51]. Our findings showed that, when compared with other studies on soils in the province of Avila or the Castilla y Leon Community [42], there is a general increase in organic matter under hydromorphic conditions, which aligns with the results of Chaplot et al. [52].

This rise can be attributed to the effect of soil saturation on organic carbon stability. Saturation leads to reducing conditions, which suppress microbial activity due to the limited availability of free oxygen.

In addition, the high soil organic carbon stocks (SOCSs) observed in these soils, compared to the findings of Leifeld et al. [53], may be linked to intensive grazing practices. In this regard, San Miguel [54] pointed out that pastoral activities in grazing systems with Quercus ilex are essential for the structural integrity and function of the topsoil, influencing carbon levels. Nair et al. [55] indicated that systems involving grass and trees are more efficient in sequestering carbon than other land uses, as the secondary roots of trees gradually incorporate large amounts of carbon into the soil, leading to an accumulation of underground carbon over time.

Furthermore, the ecological context plays a vital role in understanding how livestock impacts soil properties and processes, which is essential for evaluating the soil organic carbon content through specific carbon stabilization mechanisms [56].

Nevertheless, regarding the effect of soil management, several facts can be discussed. Some scholars such as Wang et al. [57] or Parras-Alcántara et al. [39] point out that the soil carbon stock capacity depends not only on abiotic factors but also on soil use and management. West and Post [58], Puget and Lal [59], Blanco-Canqui and Lal [60], and García-Moreno et al. [61] point out that organic farmers, by not tilling, do not favor the degradation of soil aggregates; and, given that, at the same time, under this management, enhancing the crop biomass and, “in turn”, residue return, the SOC is therefore not destroyed, but rather increases.

It is worth thinking whether the increment of soil carbon storage can be a consequence of activities on management and forestation; this is well-documented in the literature [62,63]. Our finding indicated that the amount of soil carbon sequestration in the soil under Pinus pinaster (86.3 Mg ha⁻¹) was significantly higher than ryegrass-magaza (50.4–48.3 Mg ha⁻¹), grazing land (38.9–7.5 Mg ha⁻¹), and hydrophilic grassland (48.3–13.2 Mg ha⁻¹). This fact can be associated with more litter accumulation on the surface.

Currently, it is recognized that the estimation of soil organic carbon fluxes is an essential aspect of carbon sequestration [64]. Our results indicated a medium storage capacity of C and N in sandy soils when compared to the accumulation potential of these elements in global dryland soils [65]. The last authors estimated values from 50 (Mg ha⁻¹) (hyper-arid climate) to 180 (Mg ha⁻¹) (dry sub-humid climate) for the C stock and 5 (Mg ha⁻¹) (hyper-arid climate) to 22 (Mg ha⁻¹) (dry sub-humid climate) for the N stock. Oliveira Filho et al. [66] estimated C stock values (from surface to the rock) ranging from 6.81 (Mg ha⁻¹) (semi-arid climate) to 219.6 (Mg ha⁻¹) (dry sub-humid climate) and N stocks ranging from 0.65 (Mg ha⁻¹) (semi-arid climate) to 62.2 (Mg ha⁻¹) (dry sub-humid climate) for several soil classes in Brazilian drylands. According to Rodríguez-Murillo [67], the average SOCS values for soil groups in Peninsular Spain are 71.4 (Mg ha⁻¹) and 98.8 (Mg ha⁻¹) for Cambisols and Leptosols, respectively.

The soils of La Moraña are enriched in the soil organic carbon stock, likely due to factors such as the soil depth, the initial soil carbon content, and the period following the implementation of best management practices [68]. This enrichment is a testament to the careful stewardship of farmers, ranchers, and foresters in the region.

Bulk density affects organic carbon stocks, but. as noted by Calero et al. [69], clay minerals, due to their high surface area, help protect organic carbon by forming stable organo-clay complexes. Consequently, organic carbon associated with sand particles has a lesser impact. However, in the study area, soils on sandy materials exhibit higher SOC accumulation, which is linked to specific land use and management practices.

The findings indicate that soil organic carbon (SOC) is influenced by both the native SOC content and the effects of land use. Intensive agriculture, when not properly managed, has shown limited development in La Moraña. Therefore, the reduction in SOC in croplands compared to untilled soils under scrub vegetation is not significantly different. However, subsistence farming under agroforestry systems in the region has led to improvements in the soil organic carbon density in croplands.

The multi-functional benefits of SOC in food and fiber production and agricultural sustainability are well-documented. Wallace [70] highlighted numerous reasons why organic matter is crucial for crop productivity, noting it as the foundational material for soil fertility. Organic matter reduces bulk density [71], thereby improving water infiltration. Increased SOC enhances the water storage capacity and nutrient availability for crops.

Globally, stabilizing or increasing the soil organic carbon content is crucial for sustainable production. Our findings indicate that SOC stocks in the surface horizon (0–20 cm) vary depending on land use systems and soil types. Surprisingly, the total carbon storage in cropland soils was significantly higher than in pine soils. Implementing silvicultural practices, such as planting and thinning, combined with controlled cropping, can significantly enhance carbon sequestration in soils affected by waterlogging.

Although some soils are Arenosols, the total carbon storage in the forest ecosystem remains relatively high, which we attribute to the increased vegetation growth. The SOC storage in native grassland soils shows variability, which we link to differences in the vegetation cover, management practices, and soil type (Cambisol or Arenosol).

Given that pine forests are not very dense, soils with a relatively high SOC content may not have reached their carbon saturation limit and thus hold significant potential for further SOC sequestration, as previously noted by Stewart [72]. Additionally, as Jandl et al. [73] have pointed out, changes in forest structure can lead to different forest environments, affecting forest growth and overall carbon stocks.

Appropriate cover crop management is a well-known factor in enhancing soil carbon stocks [3,4,74]. Our results confirm that, with proper use and management, combining cropland with controlled grazing can positively influence soil carbon sequestration.

5. Conclusions

Soils developed on arkoses and aeolian, or both, sands in wetlands are common in La Moraña. The most outstanding properties of the soils were, in addition to the few signs of pseudogleyzation (despite being affected by waterlogging), an alkaline reaction while the SOC appears with a moderate content in surface horizons (0.98%), to a high content under forests (4.60%) with a decrease in depth. Despite the area’s distinct natural and anthropogenic influences on carbon (C) and nitrogen (N) storage, these soils have historically been under-researched. Our study provides the first assessment of soil organic carbon stocks (SOCSs) in La Moraña. Key findings include the following: Soils under Pinus had the highest topsoil C stocks, reaching 86.3 Mg ha⁻¹. Pasture soils showed the lowest SOCS values, ranging from 50.4 to 48.3 Mg ha⁻¹ horizon by horizon. Grazing land and hydrophilic grassland had even lower SOCSs, with values of 38.9 to 7.5 Mg ha⁻¹ and 48.3 to 13.2 Mg ha⁻¹, respectively. Regarding the nitrogen (N) stock, soils developed on arkoses showed higher accumulations than those on aeolian sands.

Our study concluded that hydromorphism, lithology, topography, and land use are the primary factors affecting C and N accumulation in La Moraña soils. These findings suggest that optimizing agricultural management and planning landscape designs can help modulate soil properties, reduce potential contamination issues, and ensure the sustainability of agroecosystems. The results provide valuable insights into the sustainable use of these soils based on their suitability and support capacity. Thus, the findings of this research suggest that traditional land use and management practices significantly contribute to carbon sequestration, thereby impacting climate change. Additionally, these practices affect soil quality, ecosystems, the environment, and biodiversity. However, further research is needed to assess the impacts of the continuous residue addition and the effects of balanced fertilization on intensively cultivated lands.