Sequential extraction of phosphorus in an Oxisol amended with biosolids in a long-term field experiment in Brazil

Abstract The continued growth of large cities is producing increasing volumes of urban sewage sludge. Disposing of this waste without damaging the environment requires careful management. The application of large quantities of biosolids (treated sewage sludge) to agricultural lands for many years may result in the excessive accumulation of nutrients like phosphorus (P) and thereby raise risks of eutrophication in nearby water bodies. We evaluated the fractionation of P in samples of an Oxisol collected as part of a field experiment in which biosolids were added at three rates to a maize ( Zea mays L.) plantation over four consecutive years. The biosolids treatments were equivalent to one, two and four times the recommended N rate for maize crops. In a fourth treatment, mineral fertilizer was applied at the rate recommended for maize. Inorganic P forms were extracted with ammonium chloride to remove soluble and loosely bound P; P bound to aluminum oxide (P–Al) was extracted with ammonium fluoride; P bound to iron oxide (P–Fe) was extracted with sodium hydroxide; and P bound to calcium (P–Ca) was extracted with sulfuric acid. Organic P was calculated as the difference between total P and inorganic P. The predominant fraction of P was P–Fe, followed by P–Al and P–Ca. P fractions were positively correlated to the amounts of P applied, except for P–Ca. The low values of P–Ca were due to the advanced weathering processes to which the Oxisol have been subjected, under which forms of P–Ca are converted to P–Fe and P–Al. The fertilization with P via biosolids increased P availability for maize plants even when a large portion of P was converted to more stable forms. Phosphorus content in maize leaves and grains was positively correlated with P fractions in soils. From these results it can be concluded that the application of biosolids in highly weathered tropical clayey soils for many years, even above the recommended rate based on N requirements for maize, tend to be less potentially hazardous to the environment than in less weathered sandy soils because the non-readily P fractions are predominant after the addition of biosolids.