An Investigation of the Role of Organic Amendments on the Availability of Phosphorus in an Acidic Soil

© 2020 MD. Shahinur RahmanThe recovery of P from organic waste streams (known as organic amendments, OA) is becoming increasingly popular. Besides a source of P, OA have also been reported to increase soil P availability and reduce P adsorption in acidic soil. The main mechanism involves the reduction in the soil P adsorption sites (especially Al and Fe) by the organic anion producing compounds of the OA. Organic anions form organometallic complexes in the soil to reduce P adsorption. The potential representative organic anion producing compounds of the OA are phenolic and carbonyl compounds. The phenolic and carbonyl compounds usually represent the aromatic (such as benzoic, hydroxybenzoic, protocatechuic acids etc) and aliphatic (such as oxalic, malic, citric etc) organic acids, respectively. This study aimed to investigate if these signature phenolics and carbonyl compounds of the OA can be used to predict their role in soil P availability for future prescriptive applications. An acidic soil (Red Chromosol) from the Rutherglen, northeast of Victoria, Australia was collected and waste from two different streams namely food waste and biosolid and their three different processed forms (untreated or raw, biochar and compost) were utilized as sources of OA in this study. Nuclear magnetic resonance (NMR) spectroscopy was utilized to identify forms of C (solid-state 13C NMR) and P compounds (solution-state 31P NMR) in the OA. The effect of OA on soil P availability was studied in a 110-day laboratory incubation study. Further, some selected representative compounds of phenolic and carbonyl C of OA (aromatic and aliphatic organic acids) were tested against soil P availability in another 110-day laboratory incubation experiment. Finally, a plant bioassay experiment with Triticale (genus Triticosecale) was conducted to understand the effect of both OA and organic acid on soil P availability and plant P uptake with or without the addition of external P. Results from incubation of OA with soil showed an increase in soil P availability (Olsen P) and a decrease in soil P buffering capacity, (PBC). An additional increase in soil Olsen P was observed above the total amount of Olsen P from OA that was added. It might imply that this additive increase of the soil Olsen P was due to the substitution of previously fixed P by the action of organic anions (coming from the dissociation of phenolic and carbonyl compounds) and formation of the stable organometallic complexes. This was further supported by a strong and positive correlation between the delta Olsen P (difference between observed and expected Olsen P) and phenolic and carbonyl C content of the OA. The phenolic and carbonyl C content of the OA also showed a significant correlation with the decrease of soil PBC (delta PBC, the difference between treatment and control). In both correlation studies, the ratio of phenolic to carbonyl C compounds of OA showed a strong correlation with soil P availability compared to their total abundance. The contrasting nature of phenolic and carbonyl compounds was speculated as to the reason why there was a high correlation between soil P availability and the ratio of phenolic to carbonyl C content of OA. The OA with a nearly equal abundance of phenolic to carbonyl C content showed a maximum increase in soil P availability. Therefore, it was concluded that the ratio of phenolic and carbonyl C content of OA could be used to predict P availability in acidic soil. In the next study, some naturally occurring aromatic organic acids (representative of the phenolic compounds) and aliphatic organic acids (representative of the carbonyl compounds) were added either individually or in combination (at 1:1 ratio of aliphatic and aromatic organic acids). The experimental results suggested that aliphatic organic acids were more effective in increasing soil P availability and decreasing soil PBC compared to aromatic organic acids. However, the effect of aliphatic organic acids on increasing soil P availability was prolonged when applied in combination with aromatic organic acids at 1:1 ratio. Therefore, this study concluded that the effect of an OA can be predicted based on their phenolic and carbonyl C content. Finally, the results from the plant bioassay experiment showed that the addition of OA increased the soil P availability irrespective of addition of external P to the soil. As hypothesized in the previous study, this increase in soil P availability after OA addition was expected to show a strong correlation with the ratio of phenolic to carbonyl C content of the OA. The correlation study between the soil P availability and the ratio of phenolic to carbonyl C content of the OA showed a moderate to weak correlation. The differences in those studies (laboratory incubation vs pot) were speculated as to the reason for obtaining this moderate to week correlation between the soil P availability and ratio of phenolic to carbonyl C content of the OA. In the plant bioassay experiment, OA and P were added simultaneously to the soil. Therefore, a competition between organic anions (which was coming from dissociation of organic acid compounds of OA) and P and was expected to increase P availability in the soil by reducing P adsorption. However, the preferential sorption of P by Al and Fe compared to organic anions was speculated as the main reason for observed moderate correlation. The phenolic and carbonyl C content of OA would correlate strongly with the soil P availability if OA were added first (which might reduce P sorption by forming organometallic complexes) and then P to the soil as also observed in previous study. Therefore, it was concluded that the phenolic and carbonyl C content of the OA would predict soil P availability better if OA was first added (which would form organometallic complexes) and then P added to the soil rather than adding both simultaneously. The addition of OA significantly increased the plant P uptake and showed a positive correlation with soil Olsen P. There it was concluded that soil Olsen P was a good indicator of plant P availability. In the plant bioassay experiment, organic acid treatments showed no significant effect on soil P availability and plant P uptake compared to the control. This might be due to the addition of organic acids at 10 times lower concentration in this experiment compared to the previous incubation experiment. In conclusion, this thesis showed that the ratio of the phenolic to carbonyl C content of the organic amendments could be used to predict the role of organic amendments on increasing soil P availability. The availability of fertilizer P could also be predicted by phenolic and carbonyl C content, if OA added first and then P added to the soil compared to their simultaneous addition. The OA with a nearly equal abundance of phenolic and carbonyl C content would have a maximum effect on increasing soil P availability. This knowledge would be beneficial for farmers, agricultural landholders and especially OA producers to assess the impact of their organic waste products on soil P availability in the quickest possible time based on the abundance of the signature C compounds (phenolic and carbonyl)