Effects of biosolids application on N mineralization

Sewage sludge is the solid, semi-solid or liquid residue generated during the treatment of domestic sewage. Biosolids are the treated form of sewage sludge. The use of biosolids as soil amendments is widely seen as a way to reduce the accumulation of wastes and at the same time to enhance soil fertility for crop production. Studies have shown that the use of biosolids as soil amendment is an effective means of recovering plant nutrients and improving the physical and microbiological properties of soils.

However, there are problems associated with the use of biosolids such as heavy metal contamination and nitrate pollution. Biosolids containing excessive levels of heavy metals should not be used as soil amendments. As for nitrate pollution due to excessive mineralization, Hseu and Huang (2005) proposed that this maybe avoided by regulating the annual rate of application of biosolids to soil based on crop N requirement and the anticipated net amount of organic N mineralized in the soil treated with biosolids.

In an interesting study aimed to characterize the influence of the application of biosolids on the soil potential for N mineralization (N0) and also to elucidate the kinetics of N mineralization in tropical soils treated with different biosolids, Hseu and Huang (2005) used anaerobic biosolids and aerobic biosolids obtained from the wastewater treatment plants in Kaohsiung and Taipei, Taiwan. The biosolids were applied at rates of 10, 50 and 100 Mg ha−1 to three tropical soils and incubated for 48 weeks.

Findings of the study revealed that addition of both kinds of biosolids to the soil increased N mineralization potential to an extent related directly to the application rate and the N content of the biosolids. However, the cumulative amounts of N mineralized for soil treated with aerobic biosolids greatly exceeded those for the soil treated with anaerobic biosolids. Sandy soil treated with biosolids exhibited a relatively low potential for mineralizing N.

The contamination of the biosolids with relatively high levels of heavy metals such as Cu and Zn did not prevent an increase in N mineralization resulting from the application of biosolids to the soils. Approximately 3–34% of the total N content in the biosolids-treated soils was mineralized for 48-week incubation. Based on a demand of 150 kg N ha−1 for vegetable production in Taiwanese soils, the rate of biosolids application in the three soils are safe and will not cause nitrate accumulation.

Reference

Hseu ZY and CC Huang. 2005. Nitrogen mineralization potentials in three tropical soils treated with biosolids. Chemosphere 59: 447-454.

Nitrate and phosphate leaching from Lake Danao soil (Leyte, Philippines)

Nitrogen and phosphorus are the most important nutrients that also function as environmental pollutants (Logan, 2000). The natural levels of these nutrients in soils are not high enough to cause environmental pollution. But the heavy and long-term use of chemical and organic fertilizers can lead to leaching of nitrate (NO₃^-) and phosphate (PO₄^3-) from the soil and thereby result in the contamination of the groundwater as well as of nearby surface waters such as rivers and lakes (e.g. Scheffer and Schactschabel, 1992; Logan, 2000). High nitrate level in surface waters contributes to fish kills and makes the water unsafe for animal and human consumption. Increased phosphate concentration in surface waters leads to eutrophication since phytoplankton in these waters respond to increased P level since it is a major limiting nutrient in fresh water ecosystems (Logan, 2000; Toor et al., 2003). The resulting accelerated growth of water plants and general degradation of water quality, limit the use of the affected surface waters for fisheries, recreation, industry and drinking (Lal and Stewart, 1994). According to WHO (1993) drinking water contamination with nitrate is presently the environmental issue of greatest concern in N management.

Lake Danao (also called Imelda Lake in former times) is a natural lake with an area of 1.9 km² in the form of guitar located at 700 m above sea level (ASL) in the central highlands of Leyte, Philippines (see photo). Surrounded by mountains with young loamy volcanic soils (Andisols), the lake provides water for home consumption and industrial uses, and opportunities for livelihood of people living nearby. It is a national reserve and popular tourist attraction due to its beautiful scenery and generally cooler climate than the lowlands of Leyte. In a recent Lake Danao watershed management study which was part of a VSU-Cornell University collaborative research and funded by USAID-ALO, it was revealed that there has been increasing signs of ecological degradation of the lake ecosystem in the last decade (Garcia et al., 2005). For example, many areas on the mountain slopes around the lake have been converted into agricultural farms which often use fertilizers and pesticides. Detergents used in households in the community on the bank of the lake can contribute substantial amount of P to the lake through leaching.

Until now there is lack of published data on the capacity of Philippine soils to filter pollutants as well as on the environmental impacts of the application of chemical and organic fertilizers to soils bordering surface water bodies like lakes and rivers. According to Sharpley et al. (2003) leaching of P is generally low except in sandy, acid organic, or peaty soils with low P fixation capacity and in soils where the preferential flow of water can occur rapidly through macropores and earthworm holes. Toor et al. (2003) observed that P leaching occurs in grassland soil largely in the form of organic P. Considering that the Lake Danao soil has very high porosity and organic matter content, it was thought that addition of fertilizers may enhance not only nitrate leaching but phosphate leaching as well despite of it being an Andisol.

In view of the above a laboratory study was conducted to find out if Lake Danao soil allowed leaching of nitrate and phosphate after addition of chemical fertilizers and manure. The leaching experimental set-up was designed and constructed using PVC cylinders containing the Lake Danao soil and amended with various amounts of poultry manure and chemical fertilizers like urea and solophos. The treatments were based on the application rate employed by the farmers around the lake. Results revealed that high amount of nitrate was leached from the soil amended with urea but only small amount in soil added with poultry manure. Findings also showed low amount of phosphate that was leached from the soil amended with either poultry manure or chemical fertilizer. Although field verification of the results maybe necessary, the study implies that the practice of using urea by the farmers can lead to eutrophication of the nearby lake. The use of poultry manure as fertilizer will minimize the said environmental effects.

Source:

Magahud JC and VB Asio. 2009. Nitrate and phosphate leaching from Lake Danao Andisol treated with manure and chemical fertilizer. Paper presented during the National Scientific Conference of the Philippine Society of Soil Science and Technology (PSSST), 20-23 May 2009, Davao City, Philippines.

References

Garcia P.P., E.A. Saz, V.B. Asio, T.A. Patindol and Z.M. de la Rosa. 2005. Multisectoral watershed planning in Lake Danao Natural Park through participatory approaches. Final Project Report, LSU-Cornell University ALO Project, 61pp.

Logan, T.J. 2000. Soils and environmental quality. In: Handbook of Soil Science (M.E. Sumner, ed.). CRC Press, Boca Raton, pp: G155-G169.

Scheffer F. and P. Schachtschabel. 1992. Lehrbuch der Bodenkunde. (13th ed.). Ferdinand Enke Verlag, Stuttgart, 491pp.

Sharpley AN, T Daniel, T Sims, J. Lemunyon, R Stevens, R Parry. 2003. Agricultural phosphorus and eutrophication. 2nd ed. USDA-ADS, ARS-149, 44pp.

Toor G.S., L.M. Condron, H.J. Di, K.C. Cameron and B.J. Cade-Menun. 2003. Characterization of organic phosphorus in leachate from a grassland soil. Soil Biol. Biochem.5: 1317-1323.

World Health Organization. 1993. Guidelines for Drinking Water Quality, Volume 1. Recommendations. Second Edition. WHO Geneva. 110 pp.

Fate of nitrate in the capillary fringe and shallow groundwater

Nitrate pollution of groundwater systems is a serious problem in many countries. Application of nitrogen-containing fertilizers to irrigated crops is widely known as the major cause of nitrate pollution in groundwater systems. Nitrate is assumed to move downward through the vadose zone (unsaturated zone) and then move horizontally in the groundwater. But a recent study revealed that this may not be the case. Abit et al. (2008) evaluated the fate of nitrate in the capillary fringe (i.e. the subsurface layer at the boundary between the vadose zone and the zone of saturation) and shallow groundwater for a sandy soil with shallow water table. They found that nitrate entered the capillary fringe from the unsaturated zone then moved horizontally in the capillary fringe until it was partially carried into the groundwater by the fluctuating water table following rain events.

Reference

Abit SM, Amoozegar A, Vepraskas MJ, Niewoehner CP. 2008. Fate of nitrate in the capillary fringe and shallow groundwater in a drained sandy soil. Geoderma 146: 209-215.