Abstract
Dumping of hazardous waste causes land degradation, air, and water pollution, deteriorates landscape and aesthetics, which can be controlled by reclaiming with grass-legume seeding. The study aimed to examine the effect of grass-legume revegetation between 1- and 5-years in a restored waste dump (hazardous waste of an integrated steel plant) reclaimed with coir-matting, topsoil blanketing followed by grass (Pennisetum pedicellatum ) and legume (Stylosanthes hamata ) seeding. We hypothesized that the synergistic effect of the grass-legume mixture would lead to an increase in productivity and soil fertility. To assess the effects, changes in root and shoot biomass, mulch accumulation, nitrogen (N) mineralization, and its effect on soil fertility were measured. Our results showed between 1- to 5-years legume and grass biomass increased by 44% and 37%, respectively. An increase in mulch density and thickness along with revegetation age potentially increased the soil moisture by 7.5% and lowered soil temperature by 9°C at 10 cm depth. Cumulative N-mineralization by legume was three-fold higher than the grass. Soil organic carbon (SOC), available N, total N, N-stock, and soil respiration was doubled after 5-years of revegetation. Dehydrogenase and urease activity increased by 44% and 56% respectively, indicating greater C and N accumulation at the dump surface. The study concluded that grass (P. pedicellatum ) and legume (S. hamata ) mixture can be used for reclamation of the waste dump that accelerates recovery of the fertility of disturbed topsoil by contributing mulch with increasing age of revegetation.
Keywords: Reclamation; topsoil; mulch; nitrogen; soil CO2 flux; waste dump
INTRODUCTION
Soils are being degraded worldwide due to industrial activities and the inevitable dumping of solid wastes. The solid waste generated from an integrated sponge iron plant consists of dolochar, slag, and fly ashes, dumped together causes deterioration of aesthetics, and acts as continuous sources of pollution. They caused severe air pollution problems during the summer, while in monsoon, the loose waste was easily carried away along with run-off to pollute the nearby water bodies. The waste is very homogenous, loose, and devoid of nutrients, hence stabilization of this type of waste poses a severe challenge. Additionally, the dumping of waste materials alters the surface soil horizon (native topsoil) at the disposal site. Therefore, proper handling and sustainable management of hazardous waste are essential to minimize land degradation and environmental pollution.
Ecological restoration is the only remedy that causes the degradation of solid wastes with time and regenerating the natural aesthetics of the land. The restoration method may involve the application of fertile topsoil, covering the exposed slope area with geotextile mat, different soil amendments, revegetation with grass-legume seed mixture, and selective plantation. Every step involved its associated primary benefit which aids in the process of restoration. Geotextile mats are beneficial for topsoil water retention and vegetation growth on abandoned wastelands (Shao et al., 2014). To develop quick vegetation cover, control erosion, and minimize pollution, seeding of grass-legume mixtures has now become a widely used technique to restore soil fertility of wastelands (Maiti, 2012; Shang et al., 2014). With time, grass and legume litter eventually dry and produces mulch which decomposes to form humus and regenerates the soil organic layer (Maiti & Maiti 2015).
A variety of plant species has been used to stabilize dump surface but the selection of plant species that can provide an adequate vegetation cover has an added advantage. Grasses and legumes quickly aid in developing thick ground cover and decomposition of biomass residues contribute organic matter (Maiti, 2012; Maiti & Maiti, 2015; Shang et al., 2014). For instance, proportional seeding of the grass-legume mixture showed improvement in the soil nutrients in degraded pasture soil of Cerrado, Brazil (Marques et al., 2016). Seeding of grass (Cynodon dactylon and Paspalum notatum, ) and legume (Medicago sativa, Indigofer tinctoria , Amorpha fruticose,and Lespedeza bicolor ) mixture attributed significant improvement in the initial vegetation cover of highway slopes along with recovery of soil physicochemical properties (Yang et al., 2016). Natural regeneration of exotic legume Leucaena leucocephala showed the potential to recover soil fertility compared to grasses predominant on mine waste dumps of varying ages in Brazil (Lima et al., 2018). The introduction of legume (M. sativa ) on abandoned farmlands increased biomass cover as well as soil N concentrations during 1 to 11 years of revegetation (Yuan et al. 2016). Addition of legume M. sativa also increased aboveground net primary productivity along with soil carbon (C), nitrogen (N) and phosphate (P) storage in a sandy grassland amended with sediment (Wu et al., 2019). Grass species, such as Chrysopogon zizanioides , Pennisetum macrourum ,Pennisetum polystachion, and Pennisetum purpureum were used to restore wastelands in Southwest Ethiopia (Talema et al., 2019). Under drought stress conditions, root length and root area of grasses are more than legumes at the 30-60 cm depth of soil (Wang et al., 2020). A greenhouse study reported that an increase in soil N concentrations and development of diverse microbial communities were observed when invasive legume (Lespedeza cuneate ) introduced with native grass (Schizachyriu mscoparium) in a mixture (Fill, Pearson, Knight, & Crandall, 2020).
Soil organic matter (SOM) and available N instantly declines in topsoil due to land degradation which usually takes time to recover (Maiti & Maiti, 2015). Plant-derived biomass residues in the form of mulch can accelerate the accretion of soil nutrients. The quantity and chemical composition (N concentration and C:N ratio) of the organic mulch residues are variables which determines decomposition rate, predict the amount of potentially mineralizable N (PMN) and nutrients release affecting soil quality (Halde & Entz, 2016; Ibrahim, Abaidoo, Fatondii, & Opoku, 2018; Radicetti et al., 2017). Mulching thickness shows a direct positive effect on soil moisture content and reduces soil surface temperature (Kader, Senge, Majid, & Ito, 2017; Pramanik et al., 2015; Wang et al., 2017). Indirectly, decomposition of litter/organic mulches by microbes also adds to the SOM that simultaneously augments the C and N pools and helps in carbon sequestration (Frouz, 2018; Guoju et al., 2020; Zhang et al., 2019). Organic mulching has been preferred for years because it is eco-friendly, saves labor cost and after decomposition, adds biomass nutrients to soils.
We investigated the effect of forage grass (Deenanath grass;Pennisetum pedicellatum Trin.) and legume (Caribbean stylo;Stylosanthes hamata (L.) Taub.) revegetation and gradual enrichment of mulch on soil fertility of the reclaimed waste dump. We hypothesized that the synergistic effect of grass-legume revegetation would lead to an increase in the restoration of productivity and soil fertility with time. The objectives of the present study were to (i) assess the changes in soil physicochemical and biological properties under grass-legume mixture at two developmental ages of revegetation (between 1- and 5-years), (ii) examine the effect of biomass residue (mulch) on soil fertility of the revegetated waste dump, and (iii) analyze the role of grass and legume biomass residue in N-mineralization.