Short communication

Effect of potassium salts and distillery effluent oncarbon mineralization in soil

Suresh Chandra, H.C. Joshi*, H. Pathak, M.C. Jain, N. Kalra

Division of Environmental Sciences, Nuclear Research Laboratory Building, Indian Agricultural Research Institute, New Delhi 110 012, India

Received 3 September 2001; received in revised form 4 November 2001; accepted 26 November 2001

Abstract

Distillery effluent, a rich source of potassium, is used for irrigation at many places in the world. A laboratory experiment was

conducted to study the influence of potassium salts present in post-methanation distillery effluent (PME) along with two other salts,

KCl and K2SO4, on mineralization of carbon in soil. PME oxidized with H2O2, raw PME, KCl and K2SO4 solutions containing K

equivalent to 10%, 20%, 40% and 100% of K present in PME were added to the soil separately, maintaining four replications for

each treatment and control. Addition of salts up to a certain concentration stimulated C mineralization but a decline was noticed at

higher concentrations. All the levels of salts caused higher CO2 evolution than the control suggesting that the presence of K salts

enhanced the microbial activity resulting in increased CO2 evolution. The influence of K2SO4 was significantly higher than KCl in

stimulating C mineralization in soil. Oxidized effluent had a higher stimulating effect than inorganic salts, showing the influence of

other salts accompanying K in the PME. Raw PME, which contained excess organic C, increased CO2 evolution even at the highest

salt level (100% PME) signifying the effect of added C on alleviating the salt stress on microbial activity. � 2002 Elsevier ScienceLtd. All rights reserved.

Keywords: Carbon mineralization; Microbial activity; Post-methanation distillery effluent; Potassium salts

1. Introduction

Distilleries are considered to be among the most pol-luting industries as their effluents, if discharged intowater bodies, defile the natural ecosystems. However, theeffluent is a rich source of potassium and can be suitablyapplied to agricultural fields (Pathak et al., 1999). Severalstudies have been conducted on manurial value of PME(Joshi et al., 1996) but the effect of its excess salt contenton carbon mineralization in soil has not been studied.Salts, which may occur or be applied to soils in variousoperations have significant influence on microbial ac-tivity. The literature suggests that the effects of salts onmicrobial activities are highly variable and are dependenton soil type (Laura, 1974, 1976, 1977; Pathak and Rao,1998). Although, at lower salt contents, increase in ac-tivity was noted in most of the studies where organiccarbon was less than 1%. An attempt was made to studythe effect of inorganic salts on C mineralization andcompare with the salts from distillery effluent with and

without its organic C component. Since distillery effluentis rich in potassium, KCl and K2SO4, which are also usedas K fertilizers, were chosen for the present study tocompare their impact with effluent application on Cmineralization in soil. The results may eventually help inevaluating the suitability of PME as a substitute forconventional potassium fertilizers.

2. Methods

2.1. Soil

Soil samples were collected from the Indian Agricul-tural Research Institute Farm, New Delhi, India fromthe 0 to 15 cm soil layer. The entire volume of soil wasthoroughly mixed and allowed to air dry. The soils weresieved through a 2 mm screen and a representativesample was drawn for analyses using standard methods(Page et al., 1982). The soil was sandy loam in textureand contained 0.34% organic carbon, 0.06% total N,water-soluble K 60 mg kg�1, ammonium acetate ex-tractable K 151 mg kg�1, electrical conductivity (EC,1:2 soil:water) 0:40 d S m�1, pH (1:2 soil:water) 8.6, andwater holding capacity 38.8%.

Bioresource Technology 83 (2002) 255–257

*Corresponding author. Tel.: +91-11-5781490; fax: +91-11-5811112.

E-mail address: him_ensc@iari.ernet.in (H.C. Joshi).

0960-8524/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.PII: S0960-8524 (01 )00230-9

2.2. Post-methanation distillery effluent

Post-methanation distillery effluent was obtainedfrom the biomethanation unit of the Shamli Distilleryand Chemical Works, Shamli, Uttar Pradesh, India. Thephysico-chemical composition of the effluent was ana-lyzed following the methods given by APHA (1980).The effluent had pH 8.8, EC 15:9 d S m�1, organic C15.0%, total N 0.02%, total K 3:76 g l�1, Ca 1:2 g l�1,Mg 0:62 g l�1, chloride 2:0 g l�1 and sulphate 0:6 g l�1.

2.3. Treatments

Four different sources of potassium: chemical gradeKCl, K2SO4, oxidized PME (oxidized with 30% H2O2 toremove organic carbon) and raw PME were used for thestudy. Four levels of potassium were obtained by adding10% (L1), 20% (L2), 40% (L3) and 100% (L4) of Kpresent in the raw PME through different sources. Themoisture level in the flasks was maintained at 1/3 ofwater holding capacity to simulate semi-arid conditions.

2.4. Carbon dioxide evolution

All the treatments were in quadruplicate and theflasks were incubated at 25� 1 �C. The rate of miner-alization of the organic carbon was determined in termsof CO2 evolved per 100 g of soil using the method ofPramer and Schmidt (1964) by absorbing the evolvedCO2 in NaOH solution and back titrating with HCl. Theexperiment was continued for 55 days.

3. Results and discussion

3.1. Effect of salt type on C mineralization

Addition of salts from all sources increased the evo-lution of CO2 as compared to control soils (Table 1).

This suggested that salt had a stimulating effect on Cmineralization. As the C mineralization is a microbialprocess, generally two effects of salts on microorganismsmay be expected. At low concentrations they stimulatemicrobial growth and activity, while at high concentra-tions they may become toxic to the organisms (Ryanand Sims, 1974; Rao and Pathak, 1996; Pathak andRao, 1998). Stimulation of C mineralization was higherwith K2SO4 than KCl (Table 1) indicating that the kindof salts affected mineralization. Aggarwal et al. (1971)and Heilman (1975) reported that chloride salts de-pressed nitrification, whereas lower concentrations ofsulphate promoted it. A similar process might be oc-curring for C mineralization, as both the processes(nitrification and C mineralization) are microbially me-diated. All the levels of oxidized effluent recorded higherC mineralization than did K2SO4 and KCl. This couldhave been due to the presence of calcium and magne-sium salts with potassium in the inorganic effluent sup-porting the microbial activity.

3.2. Effect of salt concentration

Addition of KCl at level L1 (10% of K in PME) in-creased the C mineralization over the control, which wasfurther increased at level L2 (20% of K in PME) and itwas highest at level L3 (40% of K in PME) of KCl(Table 1). Application of KCl at level L4 (100% of K inPME), however, decreased mineralization of C. K2SO4at level L2 recorded the highest C mineralization fol-lowed by level L1, but it was reduced significantlyat levels L3 and L4. At level L2 oxidized effluent re-corded highest total C mineralization, which decreasedat levels L3 and L4. The increased C mineralization atlow levels of inorganic salts was due to dissolutionof organic C in soil and making them more availableto the microbes (Pathak and Rao, 1998). The retarda-tion of microbial activities at high salt concentrations

Table 1

Carbon dioxide evolution from soil treated with different levels of potassium through different sources during 55 days of incubation

Treatments CO2 evolved ðmg kg�1 soilÞL1a L2 L3 L4 Mean

ECb CO2 EC CO2 EC CO2 EC CO2 EC CO2

KCl 1.3 740 2.6 753 5.1 781 11.4 753 5.1 757dc

K2SO4 1.3 794 2.6 805 4.6 786 10.0 759 4.6 786c

Ox. PMEd 1.3 819 2.5 871 4.9 826 10.6 839 4.8 839b

PME 2.0 820 3.7 855 7.9 926 15.9 1037 7.4 910a

Mean 1.5 793dc 2.9 821bc 5.6 830ab 12.0 847a

Control 0.4 701e

a L1, L2, L3 and L4 represent K levels equivalent to 10%, 20%, 40% and 100% of K present in raw PME.b Electrical conductivity ðd S m�1Þ of soil after addition of different salts, oxidized PME and raw PME.c In a column or row, means followed by the same letter are not significantly different at P < 0:05 by Duncan’s multiple range test.dOx. PME is PME oxidized with 30% H2O2.

256 S. Chandra et al. / Bioresource Technology 83 (2002) 255–257

could have been due to decrease in water availability,accumulation of ions to toxic levels in microbial tissuesand the hindrances in the use of the other essentialcations and anions by the microbes (Laura, 1974).

3.3. Effect of organic C added through PME on Cmineralization

Post-methanation distillery effluent evolved thehighest amount of CO2 as compared to the inorganicsalts due to organic C accompanying the inorganic saltsin PME. Unlike oxidized PME, mineralization of C in-creased significantly at each level (L1–L4) of raw PMEapplication. This indicated that lower C mineralizationwith inorganic salts was due to limited C availability,and salt stress at higher salt concentrations could bealleviated with the addition of organic C.The study showed that addition of inorganic salts at

low concentrations stimulated CO2 evolution. This is acause of concern because soils of tropical regions arealready low in organic C and increased salinity, which isaggravated in some areas due to application of poorquality water for irrigation, and fertilizer, will furtherreduce soil organic C due to enhanced C mineralization.At higher concentration, however, inorganic salts re-duced CO2 evolution due to suppression of microbialactivity by excess salt. Organic carbon present in PMEimproved the CO2 evolution by enhancing the microbialactivity and alleviating the salt stress. The results sug-gested that PME can be used as a substitute for con-ventional potassium fertilizers.

Acknowledgements

The authors are grateful to the Director, Indian Ag-ricultural Research Institute (IARI) and Dean, Post

Graduate School, IARI, New Delhi for funding thestudy.

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