Nitrogen dynamics in sewage sludge and commercial fertilizer enriched soils

Abstract

This study indicated that sewage sludge could be used as an alternative to commercial inorganic fertilizers. It is common practice in South Africa to use agricultural land for disposal of sewage sludge. The disposal of sludge must however be done in a responsible manner to avoid environmental pollution such as nitrate (NO3-) leaching. In South Africa strict guidelines exist regarding sludge disposal, and a maximum of 8 ton ha-1 year-1 (dry mass basis) sludge may be applied. This value was based on possible NO3- leaching, but no equivalent legislation exists for commercial fertilizer that could result in the same harmful effects. In this study the possible pollution hazard in terms of NO3- leaching from sewage sludge was investigated and compared to commercial fertilizer. An incubation trial was done to determine the mineralization rate of sludge and fertilizer. The rate at which inorganic N, NO3- and NH4+ was produced from sewage sludge was measured during this experiment. Sludge was applied at three different loads: 5, 10 and 20 ton dry ha-1. Commercial fertilizer was also applied on three different levels and each level was equivalent to 30% of the N content of the corresponding sludge treatments. It was found that in the sludge treatments the NH4+ levels immediately increased possibly due to microbial activity. The NH4+ levels reached a maximum on day 7, with a production rate of 14, 26 and 60 mg kg-1 NH4+ for the 5, 10 and 20 tondry ha-1 treatment, respectively. After day 7, the production rate decreased while the NO<sub3- production started increasing at the same rate at which the NH4+ levels decreased, as a result of nitrification. The 5, 10 and 20 tondry ha-1 did not show any increases in NO3- production initially, but production started increasing on day 7 and stabilized after 28 days. In the fertilizer treatments the NH4+ levels increased immediately after application. The levels decreased again due to nitrification. Unlike the sludge treatments, and immediate increase in NO3- production was observed. This is because of the inorganic nature of the applied fertilizer. The NO3- content increased over time, and at the same time the NH4+ content decreased. All the inorganic N from the fertilizer is immediately available, while the N in sludge must first be mineralized before the inorganic fraction becomes available. NO3- production from sludge is steady, and after day 28 the total NO3- production from sludge exceeds the total production from fertilizer. Even though more NO3- (mg kg-1) was produced from sludge, the distribution was different and could be utilized more effectively by plants. The risk of NO3- leaching from commercial fertilizer is therefore possibly more than the risk of NO3- leaching from sewage sludge applications. To correlate the above laboratory mineralization values to that of field conditions, a field trail was done. The potential leaching and the effect of plant uptake were also investigated. Two trials were done: one for winter and one for summer conditions. Each trial continued for three months, and soil samples were collected every two weeks on depths of 00-30 cm, 30-60 cm and 60-90 cm. For the winter trial, application levels of 4, 8 and 16 tondry ha-1 sludge were applied. Low mineralization rates and subsequently low NO3- and NH4+ levels were measured that could be attributed to low microbial activity. No NO3- leaching was detected. To obtain better results the application rates were adjusted to 20tondry ha-1 sludge and corresponding fertilizer treatments during the summer trial. Maize was used as a crop to measure the effect of plant uptake on NO3- leaching. No significant differences were obtained between the open blocks and the maize blocks. Significant leaching occurred under fertilizer treatments, but none under sludge treatments. Mineralization was also much slower in the field trial than in the laboratory trial, and after 90 days, there was still NH4+ production in the top horizons. A part of the organic N in sludge was lost through denitrification. These losses could reduce the available inorganic N and subsequently reduce the risk of NO3- leaching. An incubation study was done to measure the gaseous losses of N through N2O and N2. Gas samples were taken and measured on a gas chromatograph. Similar application rates that were used in the previous incubation study were applied, as well as different moisture contents. N2O production from sludge increased immediately after sludge application, after which the production rate steadily decreased until day 14. No significant differences were obtain between 50% and 100% moisture content, and very little N2O production was found from the fertilizer treatments. N kinetics were done on the data obtained from the incubation studies. These values indicate the rate at which sludge could be mineralized and the subsequent rate at which inorganic N, such as NO3- were produced. N balances were also drawn to indicate the quantity of organic N that is mineralized, as well as the change between different inorganic N fractions over time. This study compared the potential N pollution from sewage sludge and commercial fertilizer in agricultural soils. By using all the results mentioned above, a better idea on the dynamics of sewages sludge compared to commercial fertilizer could be obtained. This knowledge could assist to apply sludge as a fertilizer to achieve the maximum benefit from the N content in the sewage sludge, without detrimental environment impact such as groundwater contamination.