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A better understanding of the P cycle is necessary to provide insights into management strategies that could enhance P bioavailability for crops. Our objectives were to evaluate the effects of continuous P fertilization on (i) the P budget, (ii) soil‐extractable P (Pe) dynamics, and (iii) the relationship between these variables. Two crop rotations during a 6‐yr period were analyzed in seven on‐farm experiments: corn (Zea mays L.)–double cropped wheat (Triticum aestivum)/soybean [Glycine max (L.) Merr.] (C‐W/S), and corn–soybean–double cropped wheat/soybean (C‐S‐W/S). Calculations of net P input considered only P removal (output) and fertilization (input). Initial soil‐test P was 13.4 mg P kg−1 (average for the four sites) for the C‐W/S rotation and 37.9 mg P kg−1 for the C‐S‐W/S rotation (average for the three sites). Treatment combinations were two annual rates: unfertilized and grain P removal estimation +10% (average 34 kg P ha−1 yr−1). Not applying P to soils testing 45 to 68 mg kg−1 of initial Pe resulted in a steep Pe decline (slope 0.2 mg kg−1 per unit of negative P budget). Nevertheless, soils with <25 mg kg−1 of initial Pe levels resulted in a very small and gradual decline (slope 0.02 mg kg−1). Use of current P rates applied by local farmers probably results in a small soil‐test P buildup with time. The P required to increase Pe by 1 mg P kg−1 was 11 and 8 kg P ha−1 yr−1 for the C‐W/S and C‐S‐W/S rotations, respectively.

In temperate cropping systems, a better understanding of soil C and P transformations is pertinent to evaluate crop management consequences in the medium term. Six-year cropping systems experiments consisting of corn {lea mays L.)-double-cropped wheat [TritUum aestivum L.)/soybean [Glycine max (L.) Merr.] (two sites) and corn-soybean-double-croppcd wheat/soybean (two sites), with (Fp) or without P added (Wp), were evaluated in the Pampean soils of Argentina. The objectives were (i) to quantify the effects of continuous P fertilization on the size and vertical distribution of C and P in total organic C (TOC) and particulate organic matter (POM) fractions and (ii) to evaluate the POM-P fraction as a potential indicator of crop P availability. Annually, an average P rate of 34 kg P ha"' was applied in Fp plots. The TOC, total P (TP), POM-C, and POM-P were measured in the 0-to 5-, 5-to 10-, and 10-to 20-cm soil depths at the end of the 6-yr period. Phosphorus fertilization increased TOC from 0.5 to 2.5 g Cleg"' and TP from 24.1 to 77.4 mg P kg"' in the 0-to 20-cm depth. Tlie POM-C and POM-P fractions were more sensitive than TOC or TP to P fertilization, increasing by 14 and 47%, respectively, in the 0-to 20-cm depth under continuous P addition. The greatest differences in POM-C and POM-P among treatments occurred in the surface soil layer. A significant relationship was found between POM-P and corn P uptake at anthesis.

work by Franzluebbers et al. (2000) obtained promising results with a short-term biological method. Franzlueb-Rapid estimates of mineralizable N in soil are important for management decisions and soil quality assessments. We adapted and evalu-bers et al. (2000) found that the CO 2-C evolved during ated a rapid method based on measuring the gas pressure generated the first 3 d of soil incubation at 25 ЊC was highly correwhen soil is treated with Ca(ClO) 2 in a closed vessel. An experiment lated (r 2 ϭ 0.67) with the amount of net N mineralized was conducted to determine the effects of reaction time, soil/reagent, in 24 d. and soil/water ratios on the gas pressure generated by the method. A promising, chemical method is the hot 2 M KCl, Based on this experiment, 5 g of soil, 5 mL of deionized water, 0.3 g which measures the NH ϩ 4 released from soil heated at Ca(ClO) 2 , and a reaction time of 25 min were selected as optimum conditions. The method was evaluated with 60 Cecil (fine, kaolin-100ЊC for 4 h (Gianello and Bremner, 1986). Several itic, thermic Typic Kanhapludults) sandy loam samples ranging in studies have shown that this method can be used to organic C from 4 to 16 g C kg Ϫ1. Nitrogen mineralized in 24 d and estimate mineralizable N in soil (Saint-Fort et al., 1993; soil microbial biomass C (SMBC) were measured and related to the