Effect of sewage slude and compost application on soil health and plant growth

Abstract

Application of sewage sludge and solid waste compost to crop land is considered as the best option for their safe disposal. However, presence of heavy metals at high levels in such amendments warrants their characterization in order to assess their impacts on soil and plant health. This is particularly relevant for developing countries like Pakistan, where limited information on quality and/or composition of such amendments is available. Therefore, studies in laboratory, greenhouse and filed conditions were planned. The six sewage sludge samples collected from various sources were, (1) Water And Sanitation Agency waste water treatment plant Faisalabad (WSS), (2) I-9 sector wastewater treatment plant Islamabad (ISS), (3) Kasur sewage sludge (KSS), (4) Coca-Cola sewage sludge (CSS), (5) National University of Science and Technology sewage sludge (NTS), (6) Nestle company sewage sludge (NSS) and seven compost samples collected were (1) Green Force Fertilizer (GCC), (2) Super Bloom company (SBC), (3) Lahore compost company (LCC), (4) University of Agriculture, Faisalabad (UAC), (5) Uni-Grow Bio fertilizer (BFC), (6) N.K Agro chemicals (NAC) and (7) Estate care management compost, University of Agriculture, Faisalabad (ECC). The collected samples were analyzed for macronutrients, micronutrients heavy metal/oids and some physicochemical properties. The sewage sludge and compost samples showed neutral to slightly alkaline pH. The sewage sludge samples showed higher salt content as compared to composts with the highest value of 12.06 dS m-1 in WSS. The compost samples contained more concentration of macro and micronutrients compared to sewage sludge samples with the highest value of SBC and UAC. The concentrations of micronutrients varied a lot, SBC and LCC showed the highest levels of boron (1.15 mg kg-1) and copper (5.08 mg kg-1). On an average, the sewage sludge samples contained higher contents of heavy metals as compared to composts except LCC which had more contents of cadmium (Cd) and lead (Pb). The highest concentration of arsenic (As), chromium (Cr) and Pb were found in CSS, KSS and ISS, respectively. On the basis of characterization study, another trial was undertaken in glass settings using pots without leaching provision. In this study, impact of selected composts and sewage sludges on growth and yield performance of rice and subsequent wheat crop and levels of various heavy metals in soil solution and plant tissues were evaluated. Four types each of compost and sewage sludge were selected from first study and soil applied at 0.5% (w/w) and 1.0% (w/w). Control pots for treatment comparison received recommended dose of fertilizer only (RDF, no sewage sludge/compost). All these treatments were tested in two types of soils which differed in their texture. The plant physiological attributes like stomatal conductance, transpiration rate, photosynthetic rate and chlorophyll contents were negatively affected by the concentration of heavy metals in plant parts (rice and wheat). Among composts, the LCC increased the concentration of heavy metals (Cd and Pb) in rice and wheat plant tissues grown on both soils at harvest. The Cd concentration in rice grains with LCC, ISS, KSS and CSS applied at 1.0% was above the Indian safe limit (1.5 mg kg-1) in both types of soils. The concentration of dissolved organic carbon (DOC) in soil solution decreased with time during both rice and wheat crops and it showed a linear relationship with heavy metals in plant tissues in both type of soils. Composts performed better in terms of growth and yield and metal accumulation in grains of both crops remained lower than sewage sludges except LCC in both soils which showed higher levels of Cd and Pb in plant parts compared to control. Among soils, the plants grown on clay loam soil (Soil-I) showed higher yield and less heavy metal accumulation as compared to plants grown on sandy clay loam (Soil-II). Irrespective of amendment type, amendment addition at 0.5 % produced higher grain yield and lower metals accumulation in both the crops and both type of soils. Similarly, irrespective of amendment type, their application at 1.0% improved grain yield of both the crops only in Soil I, whereas the grain yield of both the crops declined at this level with LCC, WSS, ISS, KSS in Soil-II. A field experiment was conducted to evaluate the best application level among selected sewage sludges and composts samples by growing rice and wheat as test crops. Maximum straw and grain yields of both the crops were recorded with SBC at 0.5% in both type of soils. The lowest rice and wheat grain yield in Soil-I recorded with LCC and control treatment, respectively. In case of Soil-II, the lowest rice and wheat grain yields were recorded with WSS at 0.25% and 0.5% level, respectively. Overall, the plant grown in Soil-I showed higher grain and straw yields at each level of treatments as compared to Soil-II. However, the application rate of 0.5% is not recommended to avoid the possible risk of metals accumulation in soil and plants. At this rate, however, yield is to low but the possible risks of metal uptake and accumulation in the soil-plant system is less. The SBC at both the levels (0.25% and 0.50%) of application remained safer for rice and wheat plants as well as for soil health.