Phosphorus dynamics and phytoavailability in a calcareous soil applied with composts of sewage sludge and biochar

Abstract

With the increasing demand for P fertilizers and a gradual decrease in P resources, recyclable P is the focus of researchers in recent years. Sewage sludge (SS) is a municipal waste that contains considerable amounts of P and probably other nutrients. Various experiments were conducted using SS, SS-biochar and/or SS-composts to investigate the P-availability to different crop plants. In the first experiment, SS were collected from different areas (city populated area and disposal) and their biochars (450 °C for 2h) were prepared. After physiochemical characterization, their effect on soil properties and P uptake by wheat (Triticum aestivum L.) with and without P fertilizer were investigated in a pot experiment. In the second experiment, various sewage sludge (SS)-based composts were prepared and their influence on P uptake, soil properties and rice yield were investigated under field conditions. The composts mixtures comprised of various combinations (V/V) of sewage sludge (SS), farm manure (FM) and rock phosphate (RP). The treatments were control (T1), nitrophos (T2), SS50:FM50 (T3), SS100:FM0 (T4), SS25:FM25:RP50 (T5), SS5O:FM25:RP25 (T6), SS75:FM0:RP25 (T7), SS50:FM0:RP50 (T8). The nitrophos were applied at the rate of 9 bags per hectare (50 kg per bag, 450kg), while rest of the treatments were applied equivalent to NP based on their total P content. Later, wheat crop was grown on the same rice field without disturbing experimental layout to investigate the residual effect of SS-composts. The agronomic parameters and P uptake by wheat plants were investigated in this study. In the third experiment, an incubation study was performed using SS-composts previously used in the field experiment. Soil-SS mixtures were incubated at 26 °C under controlled conditions of temperature and moisture. In the last study, P sorption-desorption isotherms were investigated using the soil samples collected from the field and incubation experiment. The Freundlich and Langmuir equation were calculated, and sorption parameters were investigated. Results showed a positive effect of sewage sludge on phosphorus availability to crop plants. In the first experiment, it was observed that SS application increased the wheat yield and P concentrations 104 in plants than control depending upon SS types, whereas biochar application decreased the P concentration in roots. Grain yield and P concentration in shoots were not significantly affected for the treatment with P fertilizers than without P. It was concluded that SS and biochars might be a potential source of P, but further research is needed to recommend the use of modified SS biochars as a source of available P for crops. In the second experiment, the treatments significantly influenced soil properties and rice yield parameters under field conditions. The maximum grain yield (1.63 Mg ha-1 ) was obtained in T5 followed by T6 (1.52 Mg ha-1 ). Lower yields were obtained in those treatments where FM was higher. The P concentration in rice grains were recorded in the trend as T8 (2.55 %) > T6 (2.24 %) > T4 (1.92 %) =T3 (1.88 %)>T7 (1.62 %). A similar trend was noted for P concentration in rice straw. FM and RP enhanced the effect of SS in terms of P bioavailability and yield parameters. In the third residual experiment, the results indicated that composts had positive effects on the wheat growth and P availability. The investigated parameters (biomass, grain and straw yield, plant height and elemental concentrations in plant parts) were significantly increased in all composts as compared to the control treatment. However, the composts and NP were statistically similar to each other for plant growth parameters and Olsen’s P concentration. The P uptake by the plants was higher in compost treatments as compared to the control and NP that shows the long-term residual effect of applied composts. The results suggest SS-composts can be a suitable alternative of P fertilizers for sustainable agriculture. In the fourth experiment, after 5d and 45d of the incubation period soil pH, soil EC, Olsen P and P adsorption isotherms were determined. Freundlich and Langmuir equation were calculated after equilibrating soil with various concentration of P followed by determination of adsorbed P. Results showed that after incubation of 5 and 45 days, treatments significantly affected and increased the P sorption capacity. Stronger conformity (R2) was observed for the 45d incubated soil samples compared to 5 d. Based on the findings of our experiments, it is concluded that SS must be processed either through pyrolysis or composting before application in soil. The composting of SS in combination with farm manure and rock phosphate increase the bioavailable P and could serve as a recycleable an economical fertilizer for farming communities across the world.