Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma Biochars mediated degradation, leaching and bioavailability of pyrazosulfuron-ethyl in a sandy loam soil Suman Manna, Neera Singh ⁎ Division of Agricultural Chemicals, ICAR-Indian Agricultural Research Institute, New Delhi 110 012, India ARTICLE INFO Handling Editor: A.B. McBratney Keywords: Rice and wheat straw biochars Soil amendment Herbicide fate Mobility ABSTRACT Burning of crop residues after harvest is a quick, cheap and an easy way to manage the large quantities of agricultural biomass for timely preparation of the field for next crop. Conversion of the crop harvest residues into biochars and incorporating them back in the same field can address the issue of land clearing, waste utilization and nutrient conservation but, any amendment to the soil changes its physico-chemical characteristics and can affect the fate of soil applied herbicides. The present study reports the effect of low (400 °C) and high (600 °C) temperature wheat (WBC) and rice (RBC) straw biochar's amendment on leaching, degradation and bioavail- ability of pyrazosulfuron-ethyl (PYRAZO) in a sandy loam soil. The PYRAZO was poorly retained in the control soil column where 78% of the soil-applied herbicide leached out of the control soil column. Biochars addition (0.02 and 0.05 g kg -1 ) significantly reduced the PYRAZO leaching by affecting herbicide's breakthrough time and its maximum concentration in leachate. The biochars reduced PYRAZO degradation, both in the flooded and the nonflooded soils; but, effect was more pronounced in the nonflooded soils. The effect of biochars on PYRAZO's leaching and degradation was the function of the nature of feedstock, biochar production temperature and its dose. The high temperature biochars were more effective in reducing the leaching and degradation of PYRAZO than the low temperature biochars and the rice biochars were better than the wheat biochars. The bioavailability of PYRAZO was assayed by its effect on mustard seedlings and results suggested that low temperature biochars, even at 0.05 g kg -1 level, had no ne- gative effect on herbicide efficacy. However, high temperature biochars significantly reduced the herbicide bioavailability and higher concentration of PYRAZO was required for the desired effect. These findings are relevant in assaying the fate of PYRAZO in the biochar amended soils. 1. Introduction Pyrazosulfuron-ethyl (ethyl-5-(4,6-dimethoxypyrimidin-2-ylcarba- moylsulfamoyl)-1-methylpyrazole-4-carboxylate) (PYRAZO) belongs to the sulfonylurea group of herbicides. PYRAZO is recommended for the selective control of pre-emergent and early post-emergent grassy and broad leaved weeds in direct seeded and transplanted rice (Mathew et al., 2013). PYRAZO is weakly acidic in nature (pKa - 3.7) and has lower aqueous solubility (14.5 mg L -1 ) than its other counterparts metsulfuron-methyl (pKa - 3.3, K OW – 0.018; aqueous solubility – 2.79 g L -1 ) and sulfosulfuron (pKa - 3.5, K OW – 0.17; aqueous solubility – 1.63 g L -1 ), which are highly soluble in water, have low octanol water partition coefficient and are poorly sorbed, especially in alkaline soils with K f values ranging from 0.21–1.88 (metsulfuron-methyl) and 0.37–1.17 (sulfosulfuron) (Singh and Singh, 2012). These character- istics make them prone to leaching and runoff losses and they have been detected in the surface water as well as in the ground water (Sarmah et al., 2000; Sondhia, 2009; Singh et al., 2014). Even though PYRAZO has a high octanol-water partition coefficient (P ow - 3.16), has been shown to be poorly sorbed in the same sandy loam soil that was used in the present study with a Freundlich adsorption coefficient (K f ) of 0.22 μg (1–1/n) g -1 mL 1/n (Manna and Singh, 2015). Application of wheat and rice straw biochars, even at agronomically feasible rates (0.01–0.02 mg kg -1 ), significantly increased the herbicide sorption and the rice straw biochars were nearly 1.5 times better than the corre- sponding wheat straw biochars (Manna and Singh, 2015). No in- formation is available on PYRAZO's leaching behavior in soils. How- ever, Chu et al. (2002) reported half-life values of 16–27 days in soil and 9–16 days in water; while, Singh et al. (2012) reported half-life (t 1/ 2 ) of 5.4 and 0.9 days in soil and water, respectively. Ok et al. (2012) observed that temperature affected PYRAZO's persistence in the paddy water and the t 1/2 in spring and summer were 3.1 and 1.6 days, https://doi.org/10.1016/j.geoderma.2018.07.032 Received 16 April 2018; Received in revised form 3 July 2018; Accepted 21 July 2018 ⁎ Corresponding author. E-mail address: drneerasingh@yahoo.com (N. Singh). Geoderma 334 (2019) 63–71 0016-7061/ © 2018 Elsevier B.V. All rights reserved. T