INTRODUCTION

Grasslands contain approximately 20% of the world’s soil organic carbon (SOC) stocks, indicating their significant role in the global carbon cycle (Puche et al., 2019). However, soils can act as carbon sources due to anthropogenic activities such as intensive livestock grazing, agricultural practices, and other land-use changes that may lead to OC losses. Therefore, improving grassland management for livestock production could be key to enhancing OC sequestration and, consequently, mitigate climate change (Bai and Cotrufo, 2022; Soussana and Menaire, 2014; Lal, 2004).

Grassland management is generally aimed at increasing forage quantity and quality to improve livestock productivity. Nevertheless, improved management practices can also contribute to building soil OC stocks, stimulating plant growth, and protecting OC and nitrogen (TN) in soils (Dondini et al., 2023; Gebremedhn et al., 2022).

Soil carbon and nitrogen interactions play a crucial role in regulating key ecological processes such as nutrient cycling and energy flow (Sardans and Peñuelas, 2012). Adequate nitrogen availability is essential for plant growth, and thus for increasing OC inputs into soils (Dondini et al., 2023). Introducing legumes into grasslands can enhance soil carbon and nitrogen inputs by increasing root biomass, root exudation, and fine root turnover (Bai and Cotrufo, 2022). In this context, the introduction of Leucaena (Leucaena leucocephala ssp. glabrata) into grass pastures could help increase livestock productivity (Radrizzani and Nasca, 2014), enhance soil OC and TN levels (Radrizzani et al., 2011; Conrad et al., 2017; Banegas et al., 2019), and reduce greenhouse gas emissions from livestock (Franzluebbers and Stuedemann, 2009). In previous studies with Leucaena introduction in the region, Banegas et al. (2019) found an increase in OC concentration in the subsoil (20‒100 cm depth), particularly as associated-mineral organic carbon in the deepest horizon (50‒100 cm). The authors also showed an increase in TN concentration, especially in the topsoil, related to an increment of the labile organic nitrogen form.

Soil OC and TN can be fractionated into particulate organic matter (POM) and mineral-associated organic matter (MAOM). These fractions differ in their formation pathways, physical and chemical properties, and mean residence times in the soil. While POM is derived from the fragmentation of plant and microbial residues, MAOM is formed through a process where small organic molecules, leached from plant residues or released from plant roots, interact with minerals directly or indirectly through microbial activity. Microbes can assimilate these small molecules, and their dead cells, called necromass, then become associated with mineral surfaces (Bai and Cotrufo, 2022).

Understanding the response of soil OC, TN, and their respective fractions to grassland management practices is essential for assessing the potential of these soils to sequester atmospheric carbon and the mechanisms that regulate soil carbon cycling (Zimmerman et al., 2006). Due to the limited information available on the integration of tropical tree legumes into pasture systems based on megathermic grasses, we hypothesize that the introduction of Leucaena leucocephala into Chloris gayana cv. Finecut pastures will enhance soil carbon and nitrogen stocks, with a particularly significant contribution at greater soil depths. To test this hypothesis, the study aims to evaluate the quantity and vertical distribution of OC and TN content, as well as their particulate and mineral-associated fractions, in the soil profile (0–100 cm) of a 10-year-old leucaena-grass pasture compared to an adjacent pure tropical grass pasture in the Argentinean Chaco region.

MATERIALS AND METHODS

Site description

This study was conducted at Investigación Animal de la Región Chaco Semiárido (IIACS), operated by the Instituto Nacional de Tecnología Agropecuaria (INTA), located in the western Chaco region (27º11’ S, 65º14’ W; 335 masl), Argentina. The climate is subtropical sub-humid, with a dry season from April to September and an average annual rainfall of 880 mm (75% occurring from October to March). The soil type is classified as a Fluvaquentic Haplustoll (Soil Survey Staff, 1999) (table 1; fig. 2a).

Treatments

Four 1-hectare plots were established with a pasture of Urochloa brizantha (syn. Brachiaria brizantha) cv. Marandú (Brachiaria) in 1995. In that homogeneous area, in December 2009, Leucaena cv. K636 was randomly introduced into two of these four plots to evaluate the effect of Leucaena incorporation into ageing pure grass pastures. Leucaena seeds were no-till planted in double-row hedgerows (1 m between rows), with 5 m spacing between adjacent twin hedgerows (figure 1a). In November 2011, the Brachiaria pasture was reseeded with Rhodes grass cv. Finecut (4 kg/ha). Therefore, the treatments evaluated in the present study were as follows: i) Pure pasture (PP): two 1-hectare plots with Urochloa brizantha and Chloris gayana; and ii) Pastures associated with Leucaena leucocephala (LP): two 1-hectare plots with Urochloa brizantha and Chloris gayana accompanying with Leucaena. Both treatments -pure pasture (PP) and Leucaena —enriched pasture (LP)— have been rotationally grazed by Criollo cattle at variable stocking rates, depending on forage availability, from early spring (October) to late autumn (June) (fig. 1b). For most grazing periods, LP was grazed at approximately three times the stocking rate of PP to restrict the height growth of Leucaena, which resulted in overgrazing of the interrow grass (fig. 1d).

Soil sampling

Soil samples were collected from four parcels: two with pure pasture (PP) and two with Leucaena-enriched pasture (LP). Sampling took place in both pasture types in March 2019, using twelve 10-meter transects (three per parcel; six per treatment). The collection of soil samples was described in Banegas et al. (2014).

Soil samples were air-dried at 40°C and sieved through a 2 mm mesh. Organic carbon (SOC) concentration was determined using the Walkley–Black method (Nelson and Sommers, 1996), while total nitrogen (TN) concentration was measured using the Kjeldahl method (Bremner, 1965). Fractions of SOC —particulate organic carbon (POC) and mineral-associated organic carbon (MAOC)— and TN —particulate organic nitrogen (PON) and mineral-associated organic nitrogen (MAON)— were quantified following the method described by Cambardella and Elliott (1992). The particulate fraction was obtained by dispersion in 0.5% sodium hexametaphosphate, separated by particle size using sieves ranging from 2000 to 53 µm, and determined as SOC and TN. The associated fraction was determined using the same procedure but working with the fraction smaller than 53 µm (Cambardella and Elliott, 1992).

With these values the ratios C/N, POC/PON, MAOC/MAON, POC/SOC, and MAOC/SOC were also calculated.

To complement the evaluation of the treatments, the stratification ratio (SR) was calculated at depths of 0–20/20–50 cm (SR1) and 0–20/50–100 cm (SR2). This ratio relates the organic carbon (SOC) content of the surface layer to that of the subsurface layer. The rationale for this approach is that the surface layer is strongly influenced by management practices (e.g., tillage, cropping systems, fertilization), whereas the subsurface layer is less affected by such practices (Franzluebbers, 2002). According to this author, the stratification of SOC is a useful index for assessing soil quality, since surface SOC plays a key role in controlling erosion, enhancing infiltration, and conserving nutrients.

To estimate SOC and TN stocks (in tons per hectare), a volumetric conversion was applied using bulk density for the surface soil layer. The SOC and TN contents, expressed as percentages, were then converted into mass values following the approach proposed by Ellert and Bettany (1995).

Grass basal ground cover

Grass basal ground cover (the percentage of the ground covered by plant crowns of each species) was measured in October 2010 and in August 2020 to measure changes over time in botanical composition in relation to PP and LP treatments. Grass basal ground cover was estimated by recording the lengths of intercepts along a eight transects per treatment. Briefly, a 10-m measuring tape was placed on the ground and noting where plant stems or basal parts intersect the tape. The total length of these intercepts was added to calculate the percentage of ground covered by grass species (Urochloa brizantha and Chloris gayana). The total length of the intersections along the 10-m transect represents the proportion (%) of the ground surface occupied by these species.

Statistical analyses

Analysis of variance of soil fertility parameters: SOC, Particulate Organic Carbon (POC), mineral-associated organic carbon (MAOC), TN, Particulate nitrogen (PTN), mineral-associated nitrogen (ATN) and basal grass cover and mean comparisons (Tukey, P<0.05) within pastures were performed to assess the effects of leucaena introduction. Correlation analysis between basal grass cover and SOC was performed using Pearson test. All statistical analyses were carried out using InfoStat software (Di Rienzo et al., 2016).

RESULTS

Soil organic carbon (SOC) and total nitrogen (TN)

Both variables exhibited a clear vertical distribution, with the highest values observed in the 0–20 cm soil layer. At this depth, soil organic carbon (SOC) concentration was higher in PP than in LP (13.8 ± 0.45 g·kg^-¹ vs. 12.1 ± 0.75 g·kg^-¹ of soil, respectively). In contrast, at the deepest layer (50–100 cm), SOC content was greater in LP compared to PP (6.94 ± 0.22 g·kg^-¹ vs. 4.03 ± 0.20 g·kg^-¹ of soil, respectively) (fig. 3).

Total nitrogen (TN) concentrations in the topsoil (0–20 cm) were higher in LP than in PP (1.49 ± 0.05 g·kg^-¹ vs. 1.33 ± 0.05 g·kg^-¹ of soil, respectively). However, no significant differences between treatments were found in the deeper layers (20–50 and 50–100 cm).

Significant differences in the soil C/N ratio were detected at depths of 0–20 cm and 50–100 cm (table 3), with higher values in PP at the surface layer, while LP showed the highest ratio at greater depth.

Organic Carbon and Nitrogen Fractions

Significant differences in POC content were observed at the surface layer (0–20 cm), with higher values in PP (6.77± 0.52 g·kg^-¹) than in LP (5.52±0.21 g·kg^-¹). Conversely, at deeper layers (50–100 cm), POC was higher in LP (3.44 ± 0.26 g·kg^-¹) than in PP (1.81 ± 0.41 g·kg^-¹). No significant differences were detected in the intermediate layer. The POC/SOC ratio did not differ significantly across the soil profile. Approximately 49% and 39% of total SOC was associated with the labile fraction in the 0–20 cm and 20–50 cm layers, respectively, under both treatments. At 50–100 cm, this proportion was 49.6% for LP and 44.8% for PP.

MAOC did not differ significantly between treatments in the 0–20 cm layer and 20–50 cm depth (fig. 3). However, at 50–100 cm, MAOC content was higher under LP (3.50 ± 0.38 g·kg^-¹) compared to PP (2.22 ± 0.40 g·kg^-¹). The MAOC/SOC ratio showed no significant differences between treatments. At 0–20 cm, 51% of SOC was associated with MAOC, while at 20–50 cm, the proportion reached 61% for both treatments. At 50–100 cm, the MAOC/SOC ratio was 50% for LP and 55% for PP.

Nitrogen fractions (PON and MAON) were also vertically stratified across the soil profiles in both treatments. No significant differences in PON content were found between LP and PP at any evaluated depth (fig. 3). In contrast, MAON concentrations were significantly higher in PP than in LP at both 0–20 cm (0.60 ± 0.06 g·kg^-¹ vs. 0.43 ± 0.03 g·kg^-¹) and 20–50 cm (0.47 ± 0.03 g·kg^-¹ vs. 0.30 ± 0.04 g·kg^-¹), respectively.

Regarding the carbon-to-nitrogen ratios within fractions —i.e., POC/PON and MAOC/MAON— significant differences between treatments were observed at all depths. In the LP treatment, the POC/PON ratio showed wide variation across the profile, ranging from 6.05 to 20 at deeper layers. In contrast, PP exhibited more stable values, ranging from 6.63 to 9.35. For the MAOC/MAON ratio, higher values were consistently observed in LP compared to PP, ranging from 11 to 14 (table 3).

Basal ground cover of pastures

In October 2010, one year after leucaena establishment, grass basal ground cover of sown grass (Urochloa brizantha) was higher in PP than in LP, being 12.73% and 5.67% respectively (p<0.05) (table 1). In August 2020, 9 years after seeding with Chloris gayana, grass basal ground cover of the two sown grasses (Urochloa brizantha and Chloris gayana) was also higher in PP than in LP, being 19.03% and 1.26% respectively (p<0.05) (table 2). In both sampling times, LP was dominated by Cynodon dactylon (18.3% in 2010 and 27.1% in 2020) and broadleaf weeds (3.94% in 2010 and 5.19% in 2020). These results indicate not only significant differences between treatments across both years, but also notable variation between years within each treatment (table 2). A decline in grass condition over time was observed in LP, while an improvement in grass condition was evident in PP.

DISCUSSION

The vertical distribution of SOC and TN, as well as their fractions in pasture soils, showed similar patterns to those reported in a previous study (Banegas et al., 2019). Although the literature indicates that the inclusion of legumes such as L. leucocephala is expected to improve grass cover and, consequently, increase SOC and TN content in the topsoil (Conrad et al., 2017; Radrizzani et al., 2011), in this study, an increase was observed only in TN. At the soil surface, SOC content was higher under the PP treatment, which may indicate that PP retains more C than N after nine years of livestock production. Moreover, the lower SOC values at 0–20 cm in LP compared to PP suggest that although legumes like L. leucocephala are intended to enhance soil carbon dynamics, the actual outcomes are strongly influenced by management practices and existing grass cover.

This study measured changes over time in basal cover and revealed a clear decline in the condition of sown grasses in LP and an improvement in PP. The decline in grass cover in LP was attributed to the higher stocking rate applied to control the height of Leucaena hedgerows (Radrizzani and Nasca, 2014). Reduced grass cover decreases both litter input and root turnover, thereby limiting Leucaena’s potential contribution to SOC sequestration in the topsoil (Radrizzani et al., 2011). In contrast, the better grass cover observed under the PP treatment enhances litter deposition and root turnover, particularly in the topsoil, where approximately 70% of the root biomass is located within the first 30 cm of the soil profile (Banegas et al., 2020).

The stratification ratio (SR), defined as the ratio of soil organic carbon content in the topsoil to that in the subsoil, has been used as an indicator of SOC sequestration dynamics and soil quality due to its high sensitivity to changes in ecological land types, ecosystems, and management practices (Franzluebbers, 2002). We calculated this ratio for both treatments: SR1 (0–20/20–50 cm) and SR2 (0–20/50–100 cm). In the PP treatment, SR values were 2.12 ± 0.07 for SR1 and 3.43 ± 0.08 for SR2, while in the LP treatment, significantly lower SR values were obtained (1.69 ± 0.12 and 1.75 ± 0.04 for SR1 and SR2, respectively). The lower SR2 value observed in LP supports the hypothesis of reduced organic inputs to the soil surface in this treatment compared to PP, due to a decrease in grass cover and, hence, biomass production.

In both treatments, SR values increased vertically, coinciding with a reduction in SOC content from the topsoil to the subsoil (Zhang et al., 2024). Franzluebbers (2002) states that SOC SR values greater than 2 are rare in degraded conditions, and SR values for soil organic carbon and nitrogen pools above 2 could suggest an improvement in soil quality. These results highlight the importance of promoting adequate grass persistence, emphasizing the need for adaptive management strategies to optimize carbon sequestration benefits in this kind of silvopastoral system (Radrizzani et al., 2019).

SR2 values are also related to the greater SOC content at deeper layers in LP compared to PP. The SOC values observed at the 50–100 cm depth were consistent with those previously reported for these treatments (Banegas et al., 2014). Although various factors can influence SOC accumulation in deeper soil layers, the higher SOC levels under LP in this study were likely due to the deep rooting system of Leucaena, which enhances organic matter inputs at greater depths. These results indicate a sustained SOC presence below 50 cm, which may be attributed to the relative stability of SOC in subsoil layers. At such depths, SOC is less affected by management practices and exhibits slower decomposition rates, owing to the low energy yield of recalcitrant organic compounds and the physical protection provided by stable organo-mineral complexes and soil aggregates (Rolando et al., 2020). Jackson et al. (2017) also mentioned that deep soils contain approximately 68% of soil organic carbon below 30 cm of soil depth.

Like soil OC concentrations, soil TN declined with depth in both pastures, since most of the N (~90%) was bound to SOC in organic matter. These results are consistent with previous studies, which reported higher total nitrogen (TN) concentrations in the topsoil (0‒15 cm) of Leucaena–grass silvopastoral systems compared to grass-only systems (Conrad et al., 2017, 2018; Radrizzani et al., 2011). In similar analyses, differences between treatments were observed even at greater depths (up to 1 meter). However, in this study, no significant differences between treatments were found below the 0–20 cm layer. Differences between treatments were observed mainly in the topsoil and could be attributed to both the recycling of N-rich residues and biological N fixation after legume introduction. In concordance, Imogie et al. (2008) remarked that the enhancement of soil nutrient levels can be credited to L. leucocephala, which, alongside its nitrogen-fixing properties, plays a significant role in enriching the soil through the decomposition of its fallen leaves and in increasing nitrogen and organic matter content in the soil. The same authors suggest that L. leucocephala also promotes a favorable microclimate that stimulates soil microfauna activity, which, in turn, improves the soil’s water retention capacity and promotes nutrient recycling.

The C:N ratios ranged from 8.14 to 11.91 (table 3). The values obtained at different depths for the treatments reflects the dynamics of SOC and TN accumulation in each system, with greater carbon input at depth in PP and higher nitrogen input at the surface in LP.

Separating soil organic matter into particulate organic matter (POM) and mineral-associated organic matter (MAOM) is funda mental for understanding SOM cycling (Villarino et al., 2023). The labile soil organic matter fractions, such as particulate organic carbon and nitrogen, are characterized by their rapid turnover and are considered sensitive indicators of the effects of management practices (Yang et al., 2018). The associated organic matter fraction (in this case, MAOC and MAON) is considered a stable fraction. It is mainly comprised by labile compounds (i.e., low molecular weight) that are linked to mineral soil particles (clay and silt) and protected against decomposition (Cotrufo et al., 2013). So, management practices that stimulate their exposition to microbial activity, can also promote their quickly decomposition (Santos et al., 2020).

Our results show that the distribution of these fractions across the systems was differential. The higher surface POC values (0-20 cm) in PP are consistent with previous studies reporting elevated levels of this labile fraction in grasslands suggests that the accumulation is further promoted by conservation management practices that minimize soil disturbance (Semenov et al., 2019; Franzluebbers, 2024a). In a similar vein, several authors highlight that grassland management involving perennial forage cover qualifies as a conservation land use due to its positive effects on enhancing POC, mainly related to greater biomass input and SOM is not yet completely stabilized (Rodruigues de Oliveira et al., 2022). Conversely, grasslands subjected to poor management practices —characterized by reduced income due to lower aboveground productivity, lower quality, or both— may pose risks to environmental quality, including the degradation of the labile fraction of organic matter, as observed in LP (Franzluebbers, 2024a).

Subsoil carbon fractions have received little attention, as levels are often assumed to stabilize below 30 cm (Rodrigues Oliveira et al., 2022; Schmidt et al., 2011). At deepest layer, POC and MAOC content can be altered by tillage and subsoiling, root distribution, fertilization and burning that induce priming, and erosion exposing deeper layers (Rodrigues Oliveira et al., 2022). In this study, vegetation type, specifically the inclusion of a shrub–tree species such as Leucaena in LP treatment with a deeper root system, may have accounted for the observed differences at 50-100 cm of soil depth. These results highlight that the incorporation of deep-rooted legumes enhances SOC by simultaneously increasing both fractions, mainly through root biomass and exudates, which are now recognized as efficient contributors to SOC (Villarino et al., 2021).

Although N-POM content in the different soil layers was not particularly effective in distinguishing among the evaluated systems, the distribution of this fraction in the surface layer revealed a particular behavior between treatments. In LP, N-POM represented 69% of total nitrogen (TN), whereas in PP it accounted for only 56%. These results indicate that the relative contribution of N-POM to TN in the topsoil is closely linked to the type of management system. In particular, the incorporation of a leguminous species enhanced the buildup of organic nitrogen, mainly through its allocation into the more labile and readily available fraction. This is also observed in the C/N ratio of this fraction, where the lowest values in the upper soil layers were found in LP, showing a marked N enrichment in the labile fraction of this system (table 3).

In contrast, N in the stable fraction was higher in PP than in PL. Although residue composition (C:N ratio) was not analyzed in this study, Manzoni and Cotrufo (2024) indicate that residues relatively poor in N exhibit strong immobilization and accumulation of N in MAON, due to the preferential retention and stabilization of this N during the decomposition of residues with a high C:N ratio

For the same soil mass (2640 Mg/ha), PP stored higher SOC than LP (36.43 Mg SOC/ha vs. 32 Mg SOC/ha, respectively). In comparison to the previous soil survey, done in the same pastures 5 years after leucaena establishment (2014) (Banegas et al., 2019), SOC stocks increased by 1.77 Mg/ha under PP, reaching an annual increment rate of 0.35 Mg/ha/year, whereas the level of SOC in PL remained stable. The annual increment rate in PP is consistent with other results observed by Conant et al. (2017), who reported an average SOC increment of 0.47 Mg/ha/year in grazed grass pastures. In contrast to SOC, the increment of TN stock during the same period was higher in LP than in PP (0.10 Mg/ha vs. 0.07 Mg/ha), leading to an increment rate of 0.020 and 0.014 Mg/ha/year for LP and PP, respectively. The rate of TN increase under LP is consistent with the values reported by Radrizzani et al. (2011) in Australia, in a 20, 31 and 38-year-old leucaena-grass pastures (0.017, 0.011 and 0.014 Mg/ha/year, respectively). This focuses that while leucaena integration promotes TN accumulation, it may not lead to SOC sequestration without tailored grazing strategies.

CONCLUSIONS

This study evaluated the vertical distribution of soil organic car bon (SOC) and total nitrogen (TN), as well as their particulate and mineral-associated fractions, in soils under a 10-year-old Leucaena leucocephala–grass silvopastoral system and an adjacent grass-only pasture in the Argentinean Chaco region. The results revealed distinct patterns of C and N allocation between systems and across soil depths.

While the grass-only pasture (PP) accumulated higher SOC in the surface layers, the leucaena-based pasture (LP) showed higher TN contents, particularly within the labile fraction. These findings suggest that the integration of Leucaena enhances nitrogen enrichment through biological fixation and residue input but does not necessarily increase surface SOC without proper grazing management. Conversely, greater SOC contents at deeper soil layers under LP indicate that deep-rooted legumes contribute to subsoil carbon stabilization.

This study highlights the importance of integrating adaptive management strategies in silvopastoral systems to optimize both carbon sequestration and nitrogen cycling, reinforcing their potential as sustainable land-use alternatives in the Chaco region.

Acknowledgments

ACKNOWLEDGEMENTS

This work was supported by Instituto de Investigación Animal de la Región Chaco Semiárido (IIACS), operated by the Instituto Nacional de Tecnología Agropecuaria (INTA).

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