Hydrolysis and availability to plants of polyphosphates added to soils

Abstract

<p>The hydrolysis by plant roots and in soils of seven linear oligomers (P(,2), P(,3), P(,5), P(,15), P(,25), P(,35), and P(,65)) and one cyclic polyphosphate, trimetaphosphate (TMP), was investigated. Nonsterile intact corn roots showed greater rates of hydrolysis than sterile corn roots, with hydrolysis rates in the order: P(,2) > P(,3) > P(,5) > TMP > P(,15) > P(,25) > P(,35) > P(,65). An experiment with sterile intact corn roots suggested that pyrophosphatase was induced in corn roots in the presence of its substrate. The amount of polyphosphate-P hydrolyzed in four Iowa surface soils at 25(DEGREES)C with incubation times ranging from 1 to 14 days decreased with increasing polyphosphate chain length, with 37 to 74% hydrolysis in 14 days. The relationship between the nonhydrolyzed polyphosphate and time of incubation showed that polyphosphate hydrolysis in soils was controlled by two first-order reactions. The initial faster rate (k(,1)) changed to a slower rate (k(,2)) at incubation times ranging from 2 to 7 days, depending on the polyphosphate, soil type, and soil moisture status. The k(,1) and k(,2) values for soils incubated aerobically ranged from 1.7 x 10('-4) to 3.3 x 10('-5)/min and from 4.5 x 10('-5) to 7.4 x 10('-6)/min, respectively. The corresponding values for soils incubated under waterlogging conditions ranged from 2.4 x 10('-4) to 2.6 x 10('-5)/min and from 5.2 x 10('-5) to 1.0 x 10('-5)/min. Hydrolysis of polyphosphates in 29 Iowa soils incubated for one week at 25(DEGREES)C showed that from 27 to 47% of the orthophosphate produced was due to chemical hydrolysis; the rest was due to biochemical hydrolysis. Multiple regression analysis showed that polyphosphate hydrolysis was significantly correlated with pH, water-soluble Ca('2+), and nonbuffered pyrophosphatase activity, and negatively correlated with clay content, water-soluble Mg('2+) (except for P35 and P(,65)), and dithionite-extractable Al('3+) (except for P(,3) and TMP);The uptake of P from four Iowa surface soils amended with four linear oligomers (P(,2), P(,3), P(,15), P(,45)), one cyclic polyphosphate (TMP), and orthophosphate in a greenhouse study using ryegrass and corn as indicator plants showed that all of these sources of P are equally available to plants. Tests indicated that the residual P levels were similar among the P compounds after a short cropping period with corn (35 days) and a long cropping period with ryegrass (120 days).</p>