A kinetic approach to evaluate salinity effects on carbon mineralization in a plant residue-amended soil

A kinetic approach to evaluate salinity effects on carbon mineralization in a plant residue-amended soil first-author †E-mail:farshid@cc.iut.ac.ir †E-mail:farshid@cc.iut.ac.ir farshid@cc.iut.ac.ir Department of Soil Science, School of Agriculture, Isfahan University of Technology, Isfahan 84154, Iran Department of Soil Science, School of Agriculture, Isfahan University of Technology, Isfahan 84154, Iran The interaction of salinity stress and plant residue quality on C mineralization kinetics in soil is not well understood. A laboratory experiment was conducted to study the effects of salinity stress on C mineralization kinetics in a soil amended with alfalfa, wheat and corn residues. A factorial combination of two salinity levels (0.97 and 18.2 dS/m) and four levels of plant residues (control, alfalfa, wheat and corn) with three replications was performed. A first order kinetic model was used to describe the C mineralization and to calculate the potentially mineralizable C. The CO 2 -C evolved under non-saline condition, ranged from 814.6 to 4842.4 mg CO 2 -C/kg in control and alfalfa residue-amended soils, respectively. Salinization reduced the rates of CO 2 evolution by 18.7%, 6.2% and 5.2% in alfalfa, wheat and corn residue-amended soils, respectively. Potentially mineralizable C ( C 0 ) was reduced significantly in salinized alfalfa residue-treated soils whereas, no significant difference was observed for control treatments as well as wheat and corn residue-treated soils. We concluded that the response pattern of C mineralization to salinity stress depended on the plant residue quality and duration of incubation. Salinity stress Carbon mineralization First-order kinetics Plant residues Residue quality INTRODUCTION 1 Salt-affected soils are an important ecological entity in the landscape of semi-arid and arid regions of the world (Pathak and Rao, 1998 1998 ). The soils occupy more than 7% of the earth land surface and represent a major limiting factor in crop production (Feng et al., 2002 2002 ). Soil water potential is a critical factor when microbial activities are of concern. The osmotic component of water potential is related to the concentration of soluble salts present (Harris, 1981 1981 ). McCormick and Wolf ( 1980 1980 ) observed decreased CO 2 evolution, ammonification and nitrification in a sandy soil salinized with NaCl. Carbon dioxide evolution from Sesbania cannabina residue-amended soils decreased significantly as salinity increased beyond ECe (electrical conductivity of the saturation extract) of 16.0 dS/m (Pathak and Rao, 1998 1998 ). Dehydrogenase enzyme activity was shown to be severely inhibited by salinity whereas, the hydrolases showed a much lesser degree of inhibition (Frankenberger and Bingham, 1982 1982 ). Sustainable management of agricultural ecosystems requires incorporation of plant residue in soil. Plant residue input can influence soil organic matter as a result of their quantity and biodegradability (Dick and Gregorich, 2004 2004 ). The ameliorative influence of Sesbania cannabina residues on CO 2 evolution, microbial biomass, dehydrogenase and urease activities of salt-affected soils has been reported (Rao and Pathak, 1996 1996 ). Crop residues returned to soil may provide significant input of N and easily available C to the decomposer populations. The incorporation of crop residues therefore, has significant influence on the N turnover in soil (Christensen, 2004 2004 ). Similarly, phosphorus transformations in soil-plant system are controlled by a combination of biological, chemical and physical processes which can be affected by plant residue incorporation (Condron, 2004 2004 ). The release of inorganic forms of N, P and other organically-bound nutrients in soil is functionally associated with carbon mineralization (Mafongoya et al., 2000 2000 ) and hence, is affected by the salinity stress that inhibits the mineralization of plant materials in soil. The importance of soil organic C in maintaining soil functionality and as one of the major C pools interacting with atmospheric CO 2 has been acknowledged (Puget and Drinkwater, 2001 2001 ). A first-order kinetic model can describe the CO 2 evolution in soils amended with various plant materials (Ajwa and Tabatabai, 1994 1994 ). The effects of biochemical composition of organic amendments on their decomposition kinetics have been addressed (Thuriès et al., 2002 2002 ). We could not find information regarding the salinity effects on the kinetics of C mineralization in plant residue-amended soils. Besides, the interaction between salinity effects and plant residue initial quality is still obscure. We hypothesized that the biodegradation of plant residues of dissimilar composition would be differently influenced by salinity. The objective of this study was to investigate the effects of NaCl-derived salinity on the kinetic parameters of C mineralization in a soil amended with alfalfa, corn and wheat residues. MATERIALS AND METHODS 1 Soil and plant residue characteristics 2 The soil sample was collected from 0~15 cm depth, Shervedan Research Station (32°30′ N, 51°36′ E), Isfahan, Iran. The soil (Typic Haplocambid) was clayey (576 g/kg clay and 59 g/kg sand) with pH 8.1, containing 18.01 and 1.96 g/kg organic C and total N, respectively. The soil was calcareous (394 g/kg calcium carbonate equivalent), non-saline ( ECe =0.97 dS/m) and non-sodic (sodium adsorption ratio ( SAR )=1.42). The soil sample was passed through a 2-mm sieve and kept at 4 °C before the incubation experiment. Alfalfa ( Medicago sativa L.) shoots were sampled before flowering. Corn ( Zea mays L.) and wheat ( Triticum aestivum L.) shoots were sampled one week prior to harvest. Plant residues were washed with running tap water, rinsed three times with distilled water, then dried at 65 °C for 24 h, passed through a 1-mm sieve and kept in sealed glass containers until analysis and/or incubation experiments. Hemicellulose, cellulose and lignin content of the plant materials were measured sequentially in duplicate samples (van Soest et al., 1991 1991 ). Kjeldahl digestion and distillation were used to analyze the residues for total N (Bremner and Mulvaney, 1982 1982 ). Organic C was determined using wet digestion procedures (Nelson and Sommers, 1982 1982 ). The initial compositions of the plant residues are summarized in Table 1 1 . CE: Cellulose; HE: Hemicellulose; LG: Lignin Plant C:N C (g/kg) N (g/kg) CE (g/kg) HE (g/kg) LG (g/kg) Wheat 22.4 413 18.4 234.4 230.5 8.3 Corn 42.3 440 10.4 256.1 254.0 33.4 Alfalfa 12.7 380 29.8 108.9 69.8 28.1 Soil salinization 2 An artificially salinized sub-sample was prepared from non-saline soil. For this purpose, the non-saline soil was treated with NaCl at a rate which produced electrical conductivity of the saturation extract (ECe) of 18 dS/m. The ECe of 18 dS/m corresponds to 50% reduction in barley yield (Ayers and Westcut, 1985 1985 ), simulating a saline condition predominantly existing in salt-affected soils. The weight of salt applied was calculated on an oven-dried basis and mixed thoroughly with the soil sample. The actual ECe level achieved was 18.2 dS/m. Incubation experiment 2 To study the C mineralization kinetics, triplicates of 50 g soil samples (both non-saline and salinized samples) were mixed thoroughly with plant residues at the rate of 10 g residue C/kg soil. Moisture was adjusted at 50% water holding capacity. Control treatments without application of plant residues were also run for both salinity conditions. The soils were kept in sealed glass jars containing alkali solution vial (15 ml 1 mol/L NaOH) to trap CO 2 -C respired. The jars were kept at 25 °C. The trapping alkali solution was replaced periodically after 1, 2, 3, 4, 5, 6, 7, 9, 11, 13, 15, 18, 21, 29, 37 and 46 d of incubation. The CO 2 -C trapped was measured by titrating the aliquot with 0.25 mol/L HCl following precipitation of carbonates by BaCl 2 solution (Alef, 1995 1995 ). A first order kinetics equation was used to calculate the potentially mineralizable C ( C 0 ): C m = C 0 (1−e − kt ), where, C m is the organic C mineralized at any specific time t , and k is the first order rate constant. SYSTAT (8.1) was used to calculate C 0 and k (Wilkinson, 1988 1988 ). RESULTS AND DISCUSSION 1 In general, the amount of CO 2 -C released from plant residue-treated soils increased at a decreasing rate. This was seen as rapid increase during the initial stages of incubation, followed by a slower, relatively linear release (Fig. 1 1 ). In this study, the decomposition rate of the native soil organic C in the presence of organic material (priming effect) is assumed to be the same for each type of organic material. The same assumption was made earlier by other researchers (Ajwa and Tabatabai, 1994 1994 ; Trinsoutrot et al., 2000 2000 ). The different initial composition of the three residues applied to the soil resulted in different rates of CO 2 evolution (Table 2 2 ). The total amount of CO 2 -C released from alfalfa residue treated soils under non-saline condition ( ECe =0.97 dS/m) was about 6, 1.9 and 1.4 times that of control, corn residue- and wheat residue-treated soils, respectively (Table 2 2 ). Similarly, under salinized condition ( ECe =18.2 dS/m), the total amount of CO 2 -C released from alfalfa residue treated soils was 4.8, 1.6 and 1.2 times that of control, corn residue- and wheat residue-treated soils (Table 2 2 ). The greater amounts of CO 2 produced in alfalfa residue-treated soils can be attributed to the higher biodegradability of alfalfa residues induced by higher N content, lower C/N and lignin/N ratios of alfalfa residues (Table 1 1 ). In a similar experiment, Nourbakhsh and Dick ( 2005 2005 ) demonstrated that the rate of net N mineralization potential of a calcareous soil treated with a wide range of plant residues heavily depended on the N content of plant residues. The C/N and lignin/N ratios of the plant residues influenced the rate of N mineralization process to a lesser extent. Different letters in each column indicate significant difference (LSD, P < 0.05) 2 2 Non-saline 2 Salinized C m (mg C/kg) C p (%) C m (mg C/kg) C p (%) Control 814.6 a − 817.9 a − Corn 2620.1 b 18.1 a 2482.8 b 16.7 a Wheat 3500.0 c 26.9 b 3284.2 c 24.7 b Alfalfa 4842.4 d 40.3 c 3938.7 d 31.2 c Expressed as a percentage of organic C in plant residues, the cumulative amounts of C mineralized as CO 2 in 46 d under non-saline condition ranged from 18.1% for corn residue- to 40.3% for alfalfa residue-amended soil. This suggests that the readily decomposable fractions of organic C in alfalfa residues are greater than those in wheat and corn residues. There were considerable differences in chemical composition of the plant residues. Carbon to N ratio varied from 12.7 for alfalfa to 42.3 for corn residues. Since the C contents of the plant materials are not widely ranged, the wide range of the C:N ratios can be attributed to the N concentrations. The N concentration in alfalfa residues was about 3 times that of corn (Table 1 1 ). Cellulose and hemicellulose concentrations in the wheat and corn residues were similar, while lignin content in corn residues was about 3.4 times that of wheat. Lignin is known to be a natural recalcitrant and will be subjected to degradation more slowly in soil (Melillo et al., 1982 1982 ). The extent of salinity effects on the cumulative CO 2 -C emitted and the percentage of plant residue C released as CO 2 -C depended on the type of plant residues (Table 2 2 ). The non-saline and salinized alfalfa residue-treated soils showed significant differences with respect to the CO 2 -C evolved, from day 15 of incubation in alfalfa residue-treated soils. The difference increased as the incubation proceeded (Fig. 1a 1a ). Carbon dioxide evolution was not negatively influenced by the NaCl-derived salinity ( ECe =18.2 dS/m) in wheat (Fig. 1b 1b ) and corn (Fig. 1c 1c ) residue-treated soils as well as the control treatment. The percentage of plant C released in alfalfa residue-amended soils was decreased from 40.3% to 31.2% by salinity whereas, in wheat and corn residue amended soils, very small changes were observed due to salt amendment (Table 2 2 ). The results suggest that ECe (18.2 dS/m) can only decrease the decomposition rate of the plant residues containing high amounts of readily decomposable material. Pathak and Rao ( 1998 1998 ) reported that ECe 16 (dS/m) decreased the C mineralization rates of Sesbania cannabina treated soils. They suggested that the Sesbania residues are readily decomposable, N rich plant material containing 28 g/kg N and with C:N of 15 are commonly used as green manure (Pathak and Rao, 1998 1998 ). The response pattern of alfalfa residue-amended soil to salinity in the current study is similar to the pattern observed by Pathak and Rao ( 1998 1998 ) for Sesbania cannabina , however; the response pattern of Sesbania cannabina residue-treated soils has not been compared with those of other plant residues to address the interaction of salinity stress and initial quality of the plant residues on the C mineralization. The lack of response in the wheat residue- and corn residue-amended soils to the salinity stress (ECe of 18.2 dS/m) indicated that the decomposition rates of the cereal residues are presumably controlled by their inherently low biodegradability rather than the salinity stress. There are some evidences revealing that low N content of cereal residues is a major limiting factor for their biodegradation in soil (Recous et al., 1995 1995 ; Trinsoutrot et al., 2000 2000 ). It can be hypothesized that increasing ECe to levels higher than those examined in the current study may also influence C mineralization of less biodegradable plant residues. Decomposition kinetics 2 All results of CO 2 -C evolution from plant residue-treated soils conformed well to the exponential model C m = C 0 (1−e − kt ) described earlier. The model has been widely used in literature to describe the C (Ajwa and Tabatabai, 1994 1994 ; Martinez and Tabatabai, 1997 1997 ) and N mineralization kinetics (El Gharous et al., 1990 1990 ). The potentially mineralizable C was significantly decreased by salinity stress in alfalfa residue-treated soils (Fig. 2 2 ). In contrast, there were no significant differences between non-saline and salinized soil samples either when treated with wheat and corn residues or remained untreated (control treatments). Apparently, ECe as high as 18.2 dS/m can only reduce C mineralization potential in low C/N ratio residue-treated soils in which enough N for decomposer populations can be provided. Carbon mineralization in wheat and corn residue-treated soil seems to be controlled by N shortage rather than salinity stress. significant difference indicated by different letters, LSD, P < 0.05 It has been suggested that the product of C 0 and k ( kC 0 ) is more accurate than C 0 (Murwira et al., 1990 1990 ; Saviozzi et al., 1993 1993 ). Pascual et al.( 1998 1998 ) reported that the product kC 0 was sensitive to organic waste quality in an arid soil, amended with organic wastes of varying degrees of stability. We observed that kC 0 is significantly decreased by salinity in alfalfa- and wheat-residue treated soils while remaining unaffected in the control and corn residue-treated soils (Fig. 3 3 ). Heterotrophic soil microorganisms in the control soils are supplied by native soil organic C which is relatively resistant to biodegradation processes. The response pattern of kC 0 in corn-residue treated soils is similar to that of the control soils. Corn residues are shown to be resistant to biodegradation (Recous et al., 1995 1995 ) and hence, behave similarly as the control treatments. Lastly, this research was done under laboratory conditions where the salinity stress was created by artificial addition of NaCl. 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SYSTAT The authors wish to express special thanks to Dr. Susanne Klose, University of California (Davis) for reviewing the manuscript and her valuable comments. 10.1631/jzus.2006.B0788 Project (No. 1AGH811) supported by Isfahan University of Technology, Iran 2006-05-12 2006-07-10 2006-09-14 2022-01-05 Copyright © 2006, Journal of Zhejiang University Science 2006 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 10 07