Biochars effects potentially toxic elements and antioxidant enzymes in Lactuca sativa L. grown in multi-metals contaminated soil

Highlights

• Rice husk, corn cobs and peanut shells biochars were amended at 5%.

• Bioaccumulation of metals reduced highly with amendment of peanut shells biochar.

• Stimulation and suppression of antioxidant enzymes were biochars dependent.

Abstract

Geogenic and anthropogenic activities can leads to agriculture soil pollution and land degradation. Many cost-effective and environment friendly strategies are applied to improve soil fertility, reduce soil pollution and human health risks caused by consumption of metals contaminated vegetables. In this study we evaluate the effects of rice husk biochar (RHB), biochar from corn cob (CCB) and biochar from peanut shells (PNB) on the bioavailability of potentially toxic elements (PTEs) in soil, its bioaccumulation and antioxidant enzymes activities in Lactuca sativa L. plants.

RHB, CCB and PNB amendments significantly (P$\le$0.05) increased Lactuca sativa L. biomass production (39%, 65% and 100%) as well as soil fertility. Amendments of PNB, RHB and CCB significantly (P$\le$0.05) increased soil available phosphorous (P), cation exchange capacity (CEC), pH, total nitrogen (TN), total carbon (TC) and dissolved organic carbon (DOC) concentration, but markedly reduced bioavailable concentrations of cadmium (Cd) (31%, 20% and 22%) arsenic (As) (33%, 22% and 27%), and lead (Pb) (46%, 24% and 32%). In addition, CCB and PNB amendments significantly (P$\le$0.01) decreased the shoot accumulation of Pb, Cd and As, while RHB amendment increased the shoot accumulations of nickel (Ni) and chromium (Cr). The reduction in PTEs accumulation may be linked with increased sorption of PTEs by biochars. Furthermore, amendments of CCB and PNB significantly (P$\le$0.05) suppressed the activities of SOD (53% and 69%), POD (22%, 31%) but stimulated (38% and 31%) with amendment of RHB. However, RHB, CCB and PNB amendments significantly (P$\le$0.05) suppressed the activity of CAT (21%, 41% and 48%) in Lactuca sativa L. plants.

PNB was the most effective soil amendment as compared with RHB and CCB. However, to fully elucidate the effects of the tested biochars, long-term field trails are needed.

Introduction

The pollution of agricultural soils by potentially toxic elements (PTEs) represents major risks to the environment and to human health. Because they are not biodegradable (Habiba et al., 2015, Bandara et al., 2017), and restrict the opportunities for future land use. Geogenic and anthropogenic activities are the major sources of PTEs particularly As, Cd, Pb, Cr and Ni globally (Pratas et al., 2013, Galuszka et al., 2016). In China, rapid economic development and industrialization in most coastal areas has led to elevated concentrations of PTEs (particularly Ni, Cr, As, Cd and Pb) in arable fields (Khan et al., 2014, Ibrahim et al., 2017). It is therefore important that methods of remediating PTEs affected soils are developed and applied quickly for China to meet its pressing needs to provide sufficient safe food. Elements such as Cd, Pb, Cr and As are essential in small amounts for normal plants growth and development (Noctor et al., 2007). However, in excess they become potentially toxic, inducing oxidative stress, toxicity and surplus accumulation of reactive oxygen species (ROS) in plants (Monteiro et al., 2009, Kim et al., 2015, Quartacci et al., 2015). This overproduction of ROS can result in DNA and RNA damage, enzyme inhibition and protein oxidation in plant cells (Chao and Seo, 2005, Lin et al., 2007). Plants have developed limited protective mechanisms, including the production of stress response proteins and synthesis of antioxidant enzymes (includes SOD, POD and CAT).

PTEs alter the normal ecosystem functioning and induce toxicity in vegetation (Singh et al., 2013, Gill et al., 2015). Green vegetables grown in PTEs contaminated soils are the main exposure route of PTEs to humans (Khan et al., 2008, Niu et al., 2013) and food contamination with PTEs is more prevalent in urban areas of China than some other countries such as United Kingdom and United States of America (Yuan et al., 2017). The dietary intake of excessive Pb and Cd can cause lung cancer, abdominal pain, kidney failure and stomach trouble (Patrick, 2003, Meharg et al., 2013). Elevated As intake can cause cardiovascular diseases, neurological disorders and infertility (Smoke and Smoking, 2004). Many biological, physical and chemical-based remediation technologies were developed to minimize PTEs availability in metals polluted soil (Kumpiene et al., 2008, Bolan et al., 2014). In-organic minerals, compost, agricultural residues and sewage sludge were used to reduce PTEs mobility and bioavailability in metals contaminated soil by ion exchange, co-precipitation, adsorption and surface complexion (Tsang and Hartley, 2014, Zhang et al., 2015). In-addition biodegradable cationic salts, organic acids and chelating agents have shown good effects on soil quality (Makino et al., 2008, Kim and Baek, 2015). Increased crop production and Pb precipitation was achieved with soil amendment with phosphogypsum (Anikwe et al., 2016, Yan et al., 2016) and phosphate minerals (Cao et al., 2009).

In the last few decades, biochar had been recognized as a significant element for soil fertility, crop growth and long term carbon sequestration (Lehmann, 2007, Laird et al., 2010, Luo et al., 2014, Zhao et al., 2014, Lima et al., 2018). It is emerging that soil amendment with biochar has the potential to restore metals polluted soils due to its porous structure, feedstock type, temperature, heat transfer rate, surface area, pH and cation exchange capacity (Jiang et al., 2012, Beesley et al., 2013, Ibrahim et al., 2016, Prapagdee and Tawinteung, 2017). Biochar is usually produced in oxygen-limited conditions at different pyrolysis temperatures and is commonly used for soil fertility and sorption of in-organic and organic contaminants (Melo et al., 2015, Xu et al., 2016, Hagemann et al., 2018). Biochar amendment affects the physio-chemical properties of the soil, notably bulk density, pH, carbon concentration, water holding capacity (WHC) and CEC (Rajapaksha et al., 2016, Beiyuan et al., 2017). There is research gap about the effects of plants based alkaline biochars on the remediation of multi-metals contaminated soil and its effects on antioxidant enzymes in vegetable plants.

Here we explore the effects of different plant based biochars amendments on multi-metals contaminated soil and the responses of the most widely consumed vegetable in China, lettuce (Lactuca sativa L.). In 2013, lettuce production reached 24.9 million tons globally, 13.5 million tons from China (FAOSTAT, 2013). Our approach was to a glasshouse study in which antioxidant enzymes were used as biomarkers for oxidative damage in plants (Sun et al., 2010, Wei et al., 2013), following previous studies (Li et al., 2013, Wu et al., 2013) that showed that PTEs accumulation can activate oxidative stress, thus promoting changes in the normal activities of antioxidant enzymes. Glasshouse pot trails were conducted from early November to late December 2016 to determine the effects of RHB, CCB and PNB on Lactuca sativa L. (1) biomass production, (2) PTEs bioaccumulation and (3) antioxidant enzymes, including SOD, POD and CAT.