Marginal tropical soils, particularly Dystric Histosols, present severe constraints for sustainable forage production due to their low fertility, poor nutrient retention, and susceptibility to degradation. These soils are characterized by low cation exchange capacity, high acidity, and low organic matter content, which collectively limit their ability to sustain perennial grasses such as Pennisetum purpureum cv. Mott (Kurniawan et al., 2022). This forage species, widely cultivated in Southeast Asia, is a cornerstone of livestock feeding systems, providing high biomass yields under favorable conditions. However, in degraded landscapes dominated by Imperata cylindrica, productivity is severely compromised, threatening both forage availability and nutritional quality. Addressing these challenges requires soil management strategies that not only enhance biomass yield but also improve forage quality to support animal nutrition and overall livestock system resilience (Siddiqui, 2025).

One promising approach is the application of biochar‑enriched amendments, inspired by the ancient Amazonian Dark Earths (Terra Preta). Biochar, a carbon‑rich product derived from biomass pyrolysis, has attracted global attention for its multifunctional role in soil improvement (Hoque et al., 2025). Its porous structure and chemical stability enable it to act as a long‑term carbon sink while simultaneously enhancing soil nutrient retention, cation exchange capacity, and microbial activity (Ganesh et al., 2025). However, the magnitude and direction of these effects depend on biochar feedstock characteristics and production conditions, which influence its physicochemical properties. Biochar with favorable physicochemical characteristics, including alkaline pH, porous structure, and strong nutrient-retention capacity, is often considered particularly suitable for tropical soils, where nutrient leaching and poor structural stability are persistent problems. By improving soil physical and chemical properties, biochar can contribute to both immediate agronomic benefits and long‑term environmental sustainability (Siddiqui, 2025).

Recent studies emphasize that biochar integration into tropical agroecosystems can significantly improve soil fertility and plant productivity. For example, biochar has been shown to increase soil organic carbon stability, enhance phosphorus availability, and reduce nutrient losses through leaching (Babu et al., 2025). These improvements directly support sustained plant growth and nutrient use efficiency. Moreover, the role of biochar in enhancing water retention is particularly relevant for marginal lands, where conventional fertilizers often fail to deliver lasting improvements due to rapid nutrient depletion and poor soil structure (Eissa et al., 2025). In tropical forage systems, these benefits translate into higher biomass yields and improved resilience against climatic variability.

Building upon this foundation, the concept of pre‑Terra Preta formulations represents an innovative adaptation of traditional knowledge to modern agricultural contexts. Inspired by the processes that created Terra Preta soils in the Amazon, these formulations combine biochar with organic matter and microbial inoculants to generate synergistic effects (Evans, 2022; Lehmann & Joseph, 2024; Neina & Agyarko-Mintah, 2023). Ancient Terra Preta soils remain fertile for centuries due to their high carbon content and stable nutrient pools, offering a model for sustainable soil management. By mimicking these processes, pre‑Terra Preta formulations are immature forms of biochar-rich amendments that aim to create resilient soil systems that enhance both biomass yield and forage quality (Melo & Sánchez-Monedero, 2024; Murtaza et al., 2023; Ramezanzadeh et al., 2023).

Previous studies have examined the role of biochar in improving soil fertility and plant productivity, but they have not clearly identified the threshold proportion of biochar within pre-Terra Preta formulations required to significantly enhance forage biomass yield and quality in marginal soils such as Dystric Histosols (Nasution et al., 2025). This gap is critical because marginal tropical soils are characterized by severe nutrient limitations, low organic matter content, and poor structural stability, which collectively constrain sustainable forage production. Addressing these constraints requires integrated pre-Terra Preta formulations that combine biochar, organic matter, and microbial inoculants to stabilize soil fertility and improve forage quality. The novelty of this study lies in establishing the threshold effect of biochar within pre‑Terra Preta formulations for forage systems. By systematically testing different biochar proportions within pre-Terra Preta (0 – 60%), this research provides new empirical evidence that a specific application rate and formulation of pre-Terra Preta can rehabilitate marginal soil and strengthen tropical livestock systems.

This study, therefore, seeks to examine the role of pre‑Terra Preta formulations in enhancing biomass yield and forage quality of Pennisetum purpureum cv. Mott cultivated on marginal soils in South Kalimantan, Indonesia. By integrating soil chemistry, proximate composition, and fiber fraction analyses, the research provides empirical evidence for the effectiveness of biochar‑enriched amendments in tropical forage production. The study further employs correlation and PCA to elucidate the multivariate relationships linking soil fertility variables with forage quality traits, thereby offering a holistic perspective on soil–plant interactions. Ultimately, the findings contribute to the broader discourse on sustainable soil management and livestock system resilience in degraded tropical landscapes. By demonstrating that significant yield improvements occur only at ≥ 40% biochar, with 60% biochar offering the most pronounced benefits, the study highlights the importance of targeted amendment strategies. These insights not only advance scientific understanding but also provide practical guidance for rehabilitating marginal lands and strengthening tropical livestock systems.

The present study demonstrates that variations in the composition of pre-Terra Preta formulations play a decisive role in enhancing forage productivity and quality on Dystric Histosols. The results revealed a clear threshold response, where significant improvements in biomass yield were observed only at ≥40% biochar proportion within the formulation, with optimal performance achieved at 60%. This finding indicates that the effectiveness of pre-Terra Preta depends on the balance among its constituent components and their proportional configuration within the formulation matrix. The substantial increase in biomass yield, reaching up to fourfold compared with the non-pre-Terra Preta treatments, suggests that higher biochar proportions within the formulation enhanced the overall effectiveness of the amendment system. These responses should be interpreted as the outcome of integrated interactions among biochar, organic materials, microbial inoculants, and soil components rather than the isolated effect of any single constituent.

The observed biomass enhancement can be attributed to the synergistic modification of soil physicochemical properties induced by variations in the composition of pre-Terra Preta, particularly at higher biochar proportions within the integrated system. Within this formulation, biochar contributes significantly through its highly porous structure and large specific surface area, which enhances soil organic carbon (SOC) stabilization and increases cation exchange capacity (CEC), thereby improving nutrient retention (Babu et al., 2025; Baquy et al., 2022; Gaurav et al., 2025; Haghighatnia et al., 2024; Nasution et al., 2025; Neina & Agyarko-Mintah, 2023; Neina & Glaser, 2024). In this study, the increase in SOC and available phosphorus (P) under higher biochar treatments suggests that biochar functions as both a nutrient reservoir and a buffer against nutrient leaching, which is particularly critical in highly weathered and acidic soils such as Dystric Histosols.

The sharp biomass response observed at higher biochar proportions within the pre-Terra Preta system is consistent with these findings and suggests that a threshold level of the formulation is required to generate a stable soil matrix capable of supporting sustained forage productivity. Notably, the absence of a statistically significant biomass increase at 20% biochar indicates that lower proportions within the amendment system may be insufficient to effectively mitigate nutrient leaching and improve cation retention in highly degraded soils. Similar nonlinear biomass responses to increasing biochar rates have been reported in tropical forage systems, where positive yield effects became evident only beyond moderate application thresholds (DeLuca et al., 2024; Hoque et al., 2025; Lehmann & Joseph, 2024; Nair et al., 2023; Yang et al., 2024). Thus, the present results support the concept that pre-Terra Preta acts through cumulative structural and chemical effects rather than through short-term nutrient supply alone. Beyond these physicochemical modifications, the effectiveness of the system is further reinforced by biological processes occurring within the soil matrix. Biochar, as a key component within the pre-Terra Preta formulation, likely contributes to improved soil biological functioning. Recent studies have highlighted that biochar provides a favorable microhabitat for soil microorganisms by offering protective niches within its pore structure, thereby enhancing microbial colonization and activity (Ramezanzadeh et al., 2023; Zhang et al., 2022). This microbial stimulation is essential for nutrient cycling processes, including mineralization and immobilization, which regulate nutrient availability for plant uptake. The enhanced microbial activity may also promote root–microbe interactions in the rhizosphere, leading to improved nutrient acquisition efficiency and root development. Consequently, the observed increase in biomass yield can be interpreted as the outcome of a coordinated improvement in soil structure, nutrient dynamics, and biological activity.

These results are further supported by multivariate analyses, where correlation patterns and principal component analysis (PCA) collectively reveal that improvements in soil chemical properties under pre-Terra Preta application are strongly interrelated and play a central role in shaping forage productivity and quality. The positive associations among soil organic carbon (SOC), available phosphorus, exchangeable potassium, and CEC indicate a coordinated enhancement of soil nutrient status, consistent with the known behavior of biochar-based amendments in tropical environments. Recent high-impact studies have demonstrated that biochar-rich substrates can increase soil SOC stability while simultaneously improving nutrient availability by enhancing sorption sites and reducing nutrient losses through leaching (Lehmann & Joseph, 2024; Luo et al., 2023; Ramezanzadeh et al., 2023). The moderate to strong correlations between SOC and key macronutrients observed in this study support the interpretation that pre-Terra Preta promotes nutrient retention through organic–mineral interactions rather than through direct nutrient release alone. CEC emerged as a central variable in both correlation and PCA analyses, reinforcing its role as a key mediator of soil fertility in marginal soils. Elevated CEC enhances the soil’s capacity to retain ammonium, potassium, and calcium ions, thereby stabilizing nutrient supply to plants across growth stages. Similar findings have been reported in tropical pasture systems, where increases in CEC following biochar application were directly linked to improved forage yield and nutrient use efficiency (Antonangelo et al., 2024; Latawiec et al., 2023; Nasution et al., 2025).

One of the most robust patterns emerging from the heatmap and PCA is the strong internal coherence among fiber fractions, particularly neutral detergent fiber (NDF), acid detergent fiber (ADF), and cellulose. These components consistently loaded together along the primary PCA axis, reflecting their shared structural role in plant cell walls. The strong positive correlation between ADF and cellulose (r = 0.87) is consistent with established lignocellulosic frameworks, wherein cellulose constitutes the primary structural carbohydrate of the acid detergent fraction. Recent forage quality studies emphasize that such structural coherence is indicative of normal cell wall development rather than stress-induced lignification (Bhandari et al., 2023; Katoch, 2023; Main et al., 2025; Tedeschi et al., 2023). Importantly, acid detergent lignin (ADL) exhibited weaker and partially negative correlations with cellulose and hemicellulose, suggesting that lignin accumulation did not increase proportionally with polysaccharide fractions.

This pattern implies that increased biomass production under higher pre-Terra Preta composition did not result in excessive lignification, which would otherwise reduce forage digestibility. Similar decoupling between lignin and structural carbohydrates has been reported in biochar-amended forage systems, where improved nutrient availability promoted cell expansion rather than secondary wall thickening (Dixon et al., 2024). Consequently, the present findings suggest that pre-Terra Preta enhances biomass yield without compromising forage quality through excessive lignification. Although biomass production exhibited the strongest quantitative response to increasing pre-Terra Preta composition, forage quality should not be interpreted solely in terms of crude protein concentration. In forage systems, feeding value is determined by the combined effects of nutrient composition and structural fiber fractions, including NDF, ADF, cellulose, and lignin, which influence digestibility and nutrient utilization. The relatively low ADL values observed across treatments, together with the coordinated behavior of fiber fractions, indicate that increased biomass production was not accompanied by excessive structural deterioration. Therefore, the benefits of pre-Terra Preta formulations arise not only from greater forage availability and nutrient yield per unit area, but also from the preservation of favorable forage-quality characteristics that are important for livestock feeding systems. Proximate composition variables occupied intermediate positions in the PCA space, indicating that these traits respond to both soil fertility gradients and structural carbohydrate development. Crude protein showed weak to moderate correlations with soil nutrients and did not cluster strongly with any single soil chemical variable, reflecting the complex physiological controls governing nitrogen assimilation in forage crops. Recent studies highlight that crude protein content in perennial grasses is often more sensitive to plant developmental stage and nitrogen uptake dynamics than to total soil nitrogen alone (Larsen et al., 2024). The modest correlations observed here suggest that while pre-Terra Preta improved soil nitrogen availability, protein accumulation was regulated by multiple interacting factors, including growth rate, biomass dilution effects, and internal nitrogen partitioning.

Importantly, the proportion of variance explained by the first two PCA axes (49.6%) is consistent with expectations for complex agroecosystems and does not indicate analytical weakness. Rather, it reflects the inherent multidimensionality of soil–plant interactions, as emphasized in recent methodological syntheses on multivariate analysis in agronomy (Zhang et al., 2026). From a practical perspective, the results demonstrate that pre-Terra Preta formulations with higher biochar proportions can substantially enhance forage biomass while maintaining favorable forage quality characteristics on marginal tropical soils. This outcome has significant implications for sustainable livestock systems in regions where soil degradation and low fertility constrain forage availability. Recent global assessments emphasize that integrated soil amendments combining organic matter, biochar, and microbial inputs represent a promising pathway for restoring productivity on degraded lands while contributing to carbon sequestration goals (Lima & da Penha Simon, 2025). The present study provides empirical support for this approach under tropical conditions and highlights the importance of optimizing biochar proportions to achieve meaningful agronomic benefits.

Despite these promising findings, this study has limitations that should be considered. The experiment was conducted over a single growing season and at a single site, which may restrict the generalizability of results across different environments and climatic conditions. Long-term and multi-location studies are therefore required to validate the persistence and scalability of the observed threshold effects under varying tropical agroecosystems. In addition, future research integrating microbial community dynamics, greenhouse gas emissions, and animal performance is needed to fully elucidate the ecological and practical implications of pre-Terra Preta application. Economic feasibility assessments will also be critical to ensure that this approach can be implemented sustainably at larger scales.

This study showed that increasing the proportion of biochar within pre-Terra Preta formulations improved the biomass production of Pennisetum purpureum cv. Mott on marginal Dystric Histosol. A clear threshold response was observed, with significant increases occurring at biochar proportions of 40% and above, while the 60% composition gave the highest yield. The results also indicated that pre-Terra Preta application was associated with improved soil chemical properties and favorable forage quality characteristics. Overall, these findings suggest that optimized pre-Terra Preta formulations have potential as a soil amendment strategy for improving forage production on marginal tropical soils. Further studies under longer-term and broader field conditions are needed to confirm these results.