SHORT-TERM AND LONG-TERM PHOSPHORUS DYNAMICS IN A FERTILIZED ULTISOL UNDER SUGARCANE

In Brazilian sugarcane production systems, P fertilizer represents a s ubstantial input cost, yet it is used with low efficiency. Furthermore , extractants currently used to estimate available P in highly weather ed soils are not satisfactory. Understanding soil P transformations is a prerequisite to improving estimates of available P and fertilizer u se efficiency. We used a sequential P fractionation procedure to exami ne short-term dynamics of P from fertilizer, residue returns, and soil , and long-term changes in forms and quantities of soil P as a result of fertilization. Mulch and burn systems of residue management were im posed as treatments in field-grown sugarcane (Saccharum officinarum L. ) in northeast Brazil. Both treatments had similar inputs of P from re sidues (8.7 kg ha-1) but, despite the higher ''available'' P content o f the ashes, more of the added P was mobilized and taken up by the pla nt in the mulch system, probably due to the presence of roots within t he decomposing litter and more intense root exploration of the topsoil as a result of an improved moisture regime. This improved moisture an d P supply was reflected in a 45% higher ratoon yield in the mulch tre atment. Fertilizer P applied in a furrow 20 cm deep with the plant cro p was concentrated in the more labile fractions, and had a residual ef fect on the first ratoon of about 30%. Over the long term (10 yr) ther e was accumulation (about 144 kg ha-1) of fertilizer P in the top 30 c m (in both labile and stable fractions). Nearly one-half of the increa se was in the first 7.5 cm of soil, a reflection of limited mixing of broadcast applications and uptake from the 7.5- to 15-cm layer.