Profile distribution and edaphic controls of soil organic carbon in dominant soil orders of Chitwan, Nepal

Terrestrial ecosystems represent a significant global carbon reservoir, with soils holding the largest fraction, influencing both agricultural productivity and climate feedback mechanisms. Comprehending how soil organic carbon (SOC) distributes across various soil depths and types, along with the factors governing its accumulation, remains essential for effective land management decisions. A recent study, published in Carbon Research, investigated the profile distribution of SOC in the predominant soil orders of Chitwan district, Nepal, addressing a critical gap in horizon-based analyses for the region. This work by researchers from Agriculture and Forestry University and the National Soil Science Research Center offers valuable insights into the intricate dynamics of soil carbon, especially pertinent to a landscape facing pressures from extensive agriculture and nutrient mining.

Unearthing Soil Secrets: A Horizon-Based Investigation

To characterize SOC dynamics comprehensively, the research team implemented a meticulous field and laboratory approach. They identified three dominant soil orders in Chitwan—Alfisols, Entisols, and Inceptisols—based on a soil map prepared by the National Land Use Planning Project (NLUPP). From each soil order, four replicated 1-cubic-meter soil pits were excavated, and samples were meticulously collected from master horizons down to 100 centimeters. This horizon-based sampling methodology allowed for a more nuanced understanding of SOC distribution than conventional fixed-depth approaches.

Advanced Analytical Framework Reveals Key Drivers

The collected soil samples underwent rigorous laboratory analysis at Agriculture and Forestry University, Chitwan, Nepal. Measurements included crucial soil properties such as pH, bulk density (BD), soil organic carbon (SOC), total nitrogen (N), and soil texture (sand, silt, and clay content). The Walkey and Black wet oxidation method was employed for organic matter determination, from which SOC was calculated, while total nitrogen was assessed using the Kjeldhal distillation method. Statistical analysis, including one-way Analysis of Variance (ANOVA), correlation, regression analysis, and Principal Component Analysis (PCA), elucidated the relationships between SOC and various edaphic factors. These techniques enabled the identification of primary controlling variables influencing carbon accrual within the different soil types.

Distinct Carbon Accumulation Across Soil Orders and Depths

The findings revealed significant variations in SOC concentration and stock among the studied soil orders. Alfisols, typically found in forested areas, exhibited the highest mean SOC concentration (10.1 ± 0.6 g kg−1) and cumulative SOC stock (200.01 ± 15.97 t ha−1) in the 0-100 cm profile. This was considerably higher than in Entisols (124.67 ± 12.20 t ha−1) and Inceptisols (113.27 ± 10.30 t ha−1), reflecting the influence of land use and pedogenic processes. Across all three soil orders, surface (A) horizons consistently demonstrated significantly higher SOC concentrations and stocks compared to sub-surface (B and C) horizons. This subsequent decline in SOC with depth is attributed to factors like continuous organic matter addition, enhanced biological activity, and reduced decomposition rates closer to the surface.

Edaphic Factors Shape Carbon Dynamics

Further investigation into the relationship between SOC and edaphic factors uncovered critical insights. Regression analysis showed a strong positive correlation between SOC and clay content (R2 = 0.45, p < 0.0001) and an even stronger positive correlation with total nitrogen (N) (R2 = 0.835, p < 0.001). These fine particles and nutrient interactions play a protective role, stabilizing organic matter against microbial degradation. Conversely, a significant negative correlation was observed with sand content (R2 = 0.19, p < 0.001) and bulk density (R2 = 0.04, p = 0.048). Principal Component Analysis further reinforced that the specific controlling edaphic factors vary depending on the soil type, emphasizing that soil pH, N, clay, and sand contents are primary drivers of SOC accrual across dominant Nepalese soil orders.

The study underscores that accurately estimating and predicting factors governing organic carbon content requires a holistic consideration of all soil properties—physical, chemical, and biological. While specific factors exert varying degrees of influence depending on the soil order and horizon, their cumulative impact dictates the soil's capacity for carbon storage. These findings have profound implications for developing tailored farm-level SOC management strategies in Nepal, which are vital for enhancing agricultural productivity and building climate change resilience within terrestrial ecosystems.

Corresponding Author Dinesh Panday reflected on the significance of these findings, stating, "Our research provides a foundational understanding of how soil organic carbon is distributed and regulated in Nepal's key agricultural and forest soils. This detailed, horizon-specific data is crucial for designing targeted, sustainable land management practices that can optimize soil health, improve agricultural yields, and bolster the ecosystem's ability to sequester carbon, contributing positively to Nepal's climate mitigation efforts."

Corresponding Author: Dinesh Panday

Original Source: https://doi.org/10.1007/s44246-024-00139-8

Contributions: All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Sunil Ghimire and Roshan Babu Ojha. The first draft of the manuscript was written by Sunil Ghimire and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.