Pasture Species as an Option for Soil Organic Carbon Sequestration to Restore Abandoned Croplands in Nepal

Abstract

<p>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.</p>