Abstract: A calcareous saline-sodic soil (pHs= 8.26, ECe = 9.07 dS m‾1, SAR = 37.90, CaCO3 = 8.58%, texture = sandy clay loam) packed in concrete lysimeters was tested for phytoremediation. The treatments were: No-Sesbania (SO), Sesbania-harvested (SH) and Sesbania-incorporated (SI) before flowering. Rice-wheat crop rotation was followed after Sesbania. Each crop was irrigated with high RSC water (3.1 mmolc L‾1) according to its water requirement and 20% additional water was allowed to infiltrate to carry down salts. Thirteen leachates were collected during the course of study. At termination of the experiment, pHs of soil increased while ECe and SAR decreased with all the treatments, change was more at 15-30 cm soil depth compared to that of 0-15 cm. The ECe: SAR ratio decreased with all the treatments showing an increase in Na+ hazard. It suggests that the use of water having high RSC could not be useful for reclamation without the use of a chemical amendment (Ca2+ source) and phytoremediation seems of little usefulness to improve soil under such conditions.
Introduction
Effectiveness of crops for phytoremediation is a matter of controversy. Crops could ameliorate salt-affected soils through promoting their hydraulic conductivity upon introducing root channels (Lal et al., 1979) as well as releasing CO2 during respiration (Robbins, 1986) and exudates (Amrhein et al., 1985). Deep-rooted crops are preferred over shallow rooted ones. Leguminous crops could be useful as they decrease pH of the rhizosphere through protonation (Hinsinger, 1998). Sesbania could be preferred in this case (Ahmad et al., 1989 and Qadir et al., 1996).
A decrease in pHs, ECe and SAR of a saline-sodic soil have been observed by Akhtar et al. (1988) while growing Leptochloa fusca for four years. Similarly, Nye (1981) and Kumar and Abrol (1984) observed change in pH of the rooting medium. Rhoades et al. (1973) found that salt concentrations in drainage water and lime dissolution were the greatest when alfalfa was rapidly growing. Contrarily, Wienhold and Trooien (1995) observed an increase in ECe and SAR in soils with fine-textured subsoil during alfalfa (Medicago sativa L) cropping using two water qualities (EC 1 dS m-1, SAR 4 and EC 0.34 dS m-1, SAR 16). Ghafoor et al. (1987) also observed similar results during a three-year wheat-rice cropping experiment.
Crops could cause significant alterations in soil physical properties. Several researchers have reported improvements in soil structure under pasture grasses and some other crops (Reid and Goss, 1981 and Yadav, 1975). Deterioration of structure also occurs under arable crops such as wheat, barley and soybeans (Low, 1972 and Ojeniyi, 1978). Presence of a crop can cause significant increase in bulk density of soil compared with the un-cropped soils due to soil compaction immediately around the roots (Bauder and Brock, 1992).
It is devised that proper crop rotations (depending on specific salt tolerance and ameliorative role) are essential for achieving continued improvements of saline and sodic soils besides crop varieties (Yadav, 1975). Saraswat et al. (1972) suggested that alternating crops such as barley and rice would accelerate the reclamation process although little work is found regarding their effect on ECe: SAR ratio of soil. Keeping these facts in view a lysimeter study was conducted to investigate effect of Sesbania especially and crop rotations in general on chemical properties of calcareous saline-sodic soil.
Materials and Methods
A calcareous saline-sodic soil (Table 1) was packed in concrete lysimeters to prepare 35 cm soil columns for the experimental purpose. The treatments included were no Sesbania (SO), Sesbania harvested (SH) and Sesbania incorporated (SI) before flowering. Rice-wheat crop rotation was followed after Sesbania. Wheat and Sesbania seeds (10 per pot) were sown, whereas thirty days old five seedlings per pot of salt-tolerant rice variety KS-282 were transplanted. Sesbania was harvested thrice in treatment SH as fodder, while it was chopped and incorporated into the soil after growing for 45 days in SI. Then SI pots were irrigated to facilitate decomposition of Sesbania and after 15 days of irrigation rice seedlings were transplanted. After harvesting rice, wheat cultivar SARC-1, a salt-tolerant variety, was sown.
Each crop was irrigated with water (Table 2) according to its water requirements and 20% extra water (1.08 L irrigation-1) was passed to leach down the salts. Three leachates were collected during Sesbania, five during rice and four during wheat crop. One leachate was collected at pre-planting irrigation of rice. Sesbania was grown up to flowering whereas rice and wheat were grown up to maturity. Biomass produced was recorded for all the crops at harvesting stage. Soil and water chemical analyses were carried out according to the methods described by US Salinity Lab. Staff (Richards, 1954) whereas data were analysed by using MSTATC computer package.
Results
The pHs of soil increased with all the treatments at termination of the experiment (Table 3) over that of the initial soil level (Table 1). On an average, increase in pHs of lower soil layer (15-30 cm) was more compared to that of the upper layer (0-15 cm), although the increase was only 5.0-6.7 % over the initial soil pHs. The highest pHs was observed for SO (8.81) at 15-30 cm soil depth while the lowest for SH (8.67) at the same depth, which were higher than the critical level of 8.5 for saline-sodic/sodic soils.
Maximum ECe was in SI followed by SH and SO (Table 3). The ECe of 0-15 cm soil layer remained higher than that of 15-30 cm depth. The ECe of Sesbania incorporated treatment increased by 1.2% over initial one at 0-15 cm depth while in other treatments it decreased by 3.5 to 5.1%. At lower depth, this decrease was significant and ranged from 39.7 to 56.6 %.
The SAR of soil with all the treatments although remained statistically non-significant yet it decreased over that of the initial level; decrease being greater at lower depth as compared to the upper one (Table 3). It ranged from 28.0 to 30.5 at 0-15 cm and 22.3 to 27.5 at 15-30 cm soil depth, respectively. Percent decrease in SAR ranged from 8.9 to 41.1. Moreover, EC: SAR ratio was low in 15-30 cm soil depth compared to that at 0-15 cm. It was 0.32 in upper depth of SI followed by SH (0.31) and SO (0.28). At 15-30 cm depth it was the highest in SI (0.22) followed by SH (0.19) and SO (0.18).
Discussion
The pHs measures alkalinity of soils, which in turn affects dissolution/precipitation of compounds like CaCO3 (Bresler et al., 1982). The equilibrium between lime precipitation/dissolution exists at pHs 8.4. The pH of the system above this point tends to induce precipitation of CaCO3 while below this point help its dissolution.
| Table 1: | Physical and chemical characteristics of experimental soil |
| *By difference = TSS – (CO32- + HCO3- + Cl-), ions expressed as mmolc L-1 | |
| Table 2: | Characteristics of water used for irrigation |
Therefore, a change in pHs has sometimes been associated with the degree of soil reclamation, especially with a change in soil sodication process (Ghafoor, 1984 and Kumar and Abrol, 1984). Although pHs is not considered a valid criteria to characterize saline-sodic or sodic soils, since its values depend upon ECe to SAR ratios (Bohn et al., 1985).
Increase in pHs at termination of the experiment (Table 3) depicts precipitation of Ca2+ rather than its dissolution. It could be due to the use of high RSC irrigation water (Table 2) that counterbalanced the effect of CO2 produced during root respiration and caused precipitation of Ca2+ in the system. This enhanced the Ca2+–Na+ exchange that resulted in a gradual increase in the sodicity hazard in terms of pH. The elevated activity of CO32- in the soil system also showed a precipitation tendency of Ca2+ (Table 4). However, only a slight increase in pHs could be due to precipitation of CO32+ as CaCO3 that acted as buffer and resisted any remarkable change in soil pHs in alkaline range (Deveral and Fujii, 1990).
High ECe at 0-15 cm depth (Table 3) could be due to high root density in this depth, which caused most of the water to be used in transpiration coupled with evaporation from the surface. Only a part of applied water (Table 4) passed through upper into the lower depth to carry down the salts. Rhoades et al. (1973) reported that an increase in salt concentration occurred in the root zone because of continuous uptake and transpiration of water by plants leading to concentrate the soil solution leaving more salts in this zone. Moreover, soil samples were collected after soil had attained working condition and a part of salts must have trans-located from lower depth to upper depth through capillary action under fallow conditions during hot and dry month of May.
Decrease in SAR of post-experiment soil (Table 3) might be due to valence dilution as demonstrated by Reeve and Bover (1960) for reclaiming sodic soils.
| Table 3: | Effect of treatments on soil chemical properties after experiment |
Note: Data in parenthesis show percent increase (+) or decrease (-) over the initial soil level D1 = 0-15 cm, D2 = 15-30 cm | |
| Table 4: | Water leached (% of added) during each crop in different treatments |
The reverse is true when the soil solution is concentrated due to evapotranspiration. That is why SAR in the surface soil remained high in the study. Ghafoor et al. (1987) observed similar results that growing rice and wheat crops with tube well water (EC 1.6–2.2 dS m-1, RSC 3.6–9.2 mmolc L-1, SAR 6.6–12.2) made a saline sodic soil highly sodic without the addition of any amendment.
While considering the reclamation process ECe: SAR ratio of soil is important. Studies show that soil dispersion occurred at a very low SAR if EC: SAR ratio is also low (McNeal and Coleman, 1966a and Shainberg et al., 1981). Relatively larger and rapid decrease in ECe compared to SAR (Table 3) in the lower depth could cause dispersion of soil colloids. Soil gleying was observed at 15-30 cm of the Sesbania treatments where EC: SAR ratio was low (0.19 and 0.18, respectively). Although the 0-15 cm soil layer of these treatments seems dry yet the lower depth remained continuously saturated to affect gleying. McNeal and Coleman (1966b) observed that HC of montmorillonite soils started decreasing from normal at EC: SAR ratio of < 0.24 when SAR was greater than 20 and pronounced decrease occurred at SAR > 25. Moreover, sandy loam soils are prone to more dispersion and illuviation of clay particles (Keren and Singer, 1988); which could block pores in the deeper layers. This caused a decrease in hydraulic conductivity and an increase in evapotranspiration from the soil surface leading to salt accumulation in the surface soil. The exact levels of SAR at which HC is appreciably reduced also vary with mineralogy, clay content and bulk density of a soil (Frenkel et al., 1978). The results obtained in present study are not in line with the finding of Qadir et al. (1996) who reported a significant decease in ECe and SAR of soil by growing Sesbania and Elkins (1985) who considered it by improved soil permeability due to root action perhaps they used non RSC waters and different soil series in their studies.
Post-experiment soil pHs increased while ECe and SAR decreased with all the treatments, decrease being more at lower soil depth (15-30 cm) compared to upper depth (0-15 cm). The ECe: SAR ratio decreased with all the applied treatments that could cause clay illuviation. It suggests that the use of water having high RSC could not be useful for reclamation with studied crops without the use of a chemical amendment (Ca2+ source) and phytoremediation seems of little usefulness to improve soils under such situations.