Cropland abandonment is a human-induced land use change that is emerging globally in the last century. The rate of cropland abandonment is increasing in Nepal in the last decade, impacting soil functions and productivity. Restoration options have been actively sought by land managers and policymakers, and the potential of pasture establishment has been seen as a potential option to maintain soil quality while also providing a productive base for landholders. Following abandonment, changes in geomorphology and secondary vegetation succession can have a significant impact on soil organic carbon (SOC). Few studies have been conducted in Nepal to understand the impact and restoration of post-abandonment land. In this work, abandoned croplands, forest, pasture, and existing cropped land were examined in Gorkha and Kavre districts of Nepal with the aim of understanding the impacts of cropland abandonment by measuring the change in SOC across soil depths. A field experiment was undertaken in Gorkha district of Nepal where four pasture species were grown in a prior cropland abandoned for 2 years, in order to understand the potential of pasture species for carbon (C) sequestration. To explore the mechanism of SOC formation by pasture roots, a greenhouse experiment at University of New England, Australia was also conducted in which four pasture species were grown in two contrasting soil types (Ferrosol and Chromosol). Abandoned cropland in Nepal subject to secondary vegetation succession had accumulated significant amounts of SOC, particularly the labile fraction (particulate organic carbon), compared with existing cropland, especially after 10 years of abandonment and that SOC values were on the trajectory towards those of forest and pasture. Around 23% of SOC has been recovered with secondary succession in more than 10 years abandoned cropland compared to the currently cropped land. Pasture establishment on abandoned cropland increased the SOC in which the labile carbon increased in the top-soil (0-20 cm) and stable carbon in sub-soil (20-40 cm). Pasture roots contribute to form new carbon through dissolved organic carbon formation and root exudation that is mostly fixed on the stable SOC fractions (mineral-associated organic carbon) in both Chromosol and Ferrosol. It is concluded that vegetation cover in abandoned cropland increased labile carbon in the surface soil which had been preferentially lost in the early years of abandonment. With the pasture establishment on abandoned cropland, SOC can be stabilised in sub-soil with the higher fixation of new carbon in the mineral soil matrix. So, we recommend that land managers and policymakers integrate a pasture component in abandoned cropland restoration program that has ability to restore abandoned cropland through increased SOC.

The allocation of soil organic carbon (SOC) to its component fractions can indicate the vulnerability of organic carbon stocks to change. The impact of vetiver on the composition and distribution of SOC can provide a complete assessment of its potential to sequester carbon in soil.

Purpose: This study quantified the distribution and impact of SOC under vetiver and the allocation of SOC to particulate (POC), humus (HOC) and resistant (ROC) fractions differentiated based on particle size and chemical composition under vetiver grass compared with other plant types.

Methods: Carbon fractions were measured on soil samples collected from Australia and Ethiopia to a depth of 1.0 m under three plant communities (vetiver, coffee, and Australian native pastures). We used the MIR/PLSR spectra to estimate SOC fractions based on fractionated, and NMR measured values.

Results: The stocks of SOC fractions indicated significant differences in the proportion of labile POC to HOC across sites and vegetation types. The dominant carbon fraction was HOC (71%) for all vegetation types. The average carbon sequestration rate under vetiver for OC was −2.64 to +7.69 Mg C ha⁻¹ yr⁻¹ , while for the POC, HOC and ROC was 0.04 to +1.17, -3.36 to +4.64 and −0.35 to +1.51 Mg C ha⁻¹ yr⁻¹ , respectively.

Conclusion: Growing vetiver and undisturbed native pastures has on average a high accumulation rate of a more stable carbon (HOC) which is less vulnerable to change, and change was largely driven by the HOC fraction. We, therefore, recommend the use and promotion of perennial tropical grasses like vetiver and similar grasses and undisturbed native pastures as potential options to facilitate soil carbon sequestration.

Cropland abandonment in Nepal is an emerging land use change that has significant impacts on ecosystem functions, particularly soil organic carbon (SOC) pools and stocks. Information relating to SOC is limited for the mountainous terrain of Nepal that is subject to either SOC loss from erosion or SOC accumulation from secondary succession after cropland is abandoned. The rate of cropland abandonment is accelerating in Nepal but the impact of the abandoned duration on SOC, fractions, and SOC stock has not yet been investigated. We used a space-for-time substitution approach to assess the impacts of abandonment on SOC concentrations, stocks and associated pools of carbon across abandoned cropland (<5, 5–10, and > 10 years) compared with forest, pasture, and currently cropped land. Soil samples were collected at four depth increments (0–10, 10–20, 20–40, 40–60 cm). The change in SOC and stock was found to be significantly higher in the surface (0–20 cm) compared with sub-surface soil (20–60 cm) and consistently decreased with increasing soil depth under all land uses. Change in soil nitrogen concentration followed a similar pattern to that of SOC at all land uses and depths. Among the SOC fractions, particulate organic carbon increased significantly by a greater proportion compared to mineral-associated organic carbon and dissolved organic carbon after 10 years of cropland abandonment. Abandoned cropland >10 years had a higher SOC stock (64.19 Mg ha⁻¹) in the soil profile (0–60 cm) compared with cropped land (58.76 Mg ha⁻¹) although it remained lower than forest (66.56 Mg ha⁻¹) and pasture (66.60 Mg ha⁻¹) soils. Although there was a decrease in the first five years, there was a significant accumulation of SOC, associated fractions, and SOC stock after ten years of abandonment and colonisation by vegetation indicating the inception of ecosystem regeneration in abandoned croplands in the mountainous region of Nepal.

Cropland abandonment is an emerging human-induced land use and is a growing national problem in Nepal. It is a key, current concern for local governance bodies and restoration options for this land are being actively sought. We initiated a systematic review of existing studies to identify the key causes and impacts of cropland abandonment and consider a range of restoration options. Data on the drivers, impacts, and restoration options for abandoned cropland were extracted from current relevant literature retrieved through four databases (Web of Science, Google Scholar, Scopus, and NepJol). Most studies focused on the drivers of cropland abandonment with less focus on the impacts and restoration options. Socio-economic drivers included outmigration, limited availability of labour or high labour cost, easy access to daily supply needs and high education level of the farming family members. Bio-physical factors were also reported as key drivers of cropland abandonment including unsuitable slope and rugged land topography, low soil fertility and low crop productivity. Both positive and negative impacts of cropland abandonment were also reported, principally bio-physical such as secondary vegetation succession as a positive impact and geomorphic damage and low crop production as negative impacts with a consequent decrease in food security. Most current studies focused principally on specific sites and typically lacked restoration options coordinated at a national level. Those studies that did identify restoration options for abandoned land suggested the adoption of short crop cycle, less labour-intensive crops, agro-forestry and conversion to pasture as key options. Most, however, failed to establish linkages between food insecurity, rural livelihoods, and ecosystem services after cropland is abandoned. As prior studies on abandoned land lack a defined study framework, we recommend an integrated research approach for Nepal. The approach suggests the need to adopt an integrated way for studying abandoned cropland restoration through the empirical analysis of local and national land abandonment issues. The proposed approach could also be applied in other areas where cropland abandonment is an issue.