Soil Carbon and Phosphorous Fractions in Field Crop Rotations of the Argentine Pampas.

In temperate cropping systems, a better understanding of soil carbon and phosphorus pools transformations in a medium-term is required. Two cropping systems, corn-double cropped wheat/soybeans (C-W/S; two sites) and corn-full season soybeans-double cropped wheat/soybeans (C-S-W/S; two sites), with or without P added during six years were evaluated. The objectives were to: (i) quantify the effects of continuous P fertilization on the size of total C and P (TOC and TP) and associated with particulate organic matter (POM) fractions, (ii) establish a relationship between P in particulate organic matter (POM-P) with corn plant nutrition and (iii) evaluate these fractions as soil quality indicators. The farm fertilization experiments were established in the year 2000 and the average P rate was 34 kg P ha^-1, accumulating a total of 204 kg P ha^-1. All parameters were measured at 0- to 5 cm, 5- to 10 cm and 10- to 20 cm soil depths. In all P fertilized treatments a significant increase was observed for TOC, TP, POM-C and POM-P concentrations, reaching an average value of 1.1 g kg^-1, 51.8 mg kg^-1, 0.7 g kg^-1 and 3.8 mg kg^-1, respectively, at 0- to 20 cm soil depth. For all evaluated parameters the greatest differences appeared at 0- to 5 cm soil depth. POM-P was a variable that showed a high response (47%) to P fertilization. The POM-C/POM-P ratio presented the lowest values in the fertilized treatment. The POM-C/TOC ratio was in average 22.9%. In average, for each ton ha^-1 of POM we found 1 kg P ha^-1. The POM-P/TP ratio was in average 1.7%, greater in the fertilized P treatment than in the check. At corn silking time, a quadratic model was established between accumulated P in maize plants and POM-P soil fraction, which implies a close relationship between these two P fractions.