Carbon and Nitrogen Content in Aggregate Size Classes Under Continuous Cropped Maize.

Although high nitrogen fertilization in continuous cropped maize under NT system can increase soil organic C and N concentration, the allocation of the elements in different aggregate size has not been reported for our region. The effects of long-term (13-yr) nitrogen fertilization of continuous cropped maize (Zea mays L.) on aggregate size distribution and, aggregate organic C and N mass were assessed on acuic Argiudolls at Parana, Argentina (31,5º S 60,3º W). The experiment was conduced under NT with four nitrogen fertilization rates (0, 69, 138 y 276 kg N ha^{- 1}). Samples were taken at three soil depths (0-5, 5-10, 10-20 cm) and wet sieved into three aggregate size classes (>2000, 250-2000, 53-250 µm). Proportion of aggregate in the sample, total C and N (%) and for each aggregate size class was quantified. The proportion of >2000 µm aggregate was higher in 0-5 cm than in other depths. Nitrogen fertilization with 138 kg N ha^{- 1} increased total N content (0-5 and 5-10 cm) and the C and N in >2000 µm aggregate size class (0-5 cm). Soil organic C (%) and C mass were unaffected by nitrogen fertilization in any depth. The percentage of aggregates was strongly associated in the class > 2000 µm (0-5 and 5-10 cm). Nitrogen after 13 year of continuous cropped maize not increased total C and N. In spite of the fractionating sample in different aggregate size classes, there was not a detectable effect of N on the C and N storage.