Long term nitrogen fertilization: Soil property changes in an Argentinean Pampas soil under no tillage

The main objective of nitrogen (N) fertilization is to achieve high yields and/or to increase grain quality. However, nutrient application may affect soil processes and cycles. These could involve increases in crop residues return to soil, changes in soil organic matter dynamic, NO3−-N content and pH decrease. The aim of this study was to determine the effect of N application on: crop residue input, soil organic carbon (SOC) and N (SON), their particulate (POC and PON) and mineral associated (AOC and AON) fractions, mineralizable N (anaerobic incubations, AN), and pH on a Molisoll of the southern Buenos Aires Province under no tillage (NT). A long-term crop rotation experiment has been conducted between 2001 and 2008 on a complex of Typic Argiudoll and Petrocalcic Paleudoll soils at Balcarce, Argentina (37°45′S, 58°18′W). Three N rates (N0, N1 and N2) were evaluated, with an average N input of 0, 57 and 105 kg ha−1 year−1, respectively. Crop sequence was integrated by maize (Zea mays L.), soybean (Glycine max (L.) Merr.) and wheat (Triticum aestivum L.)/soybean double crop. Soil sampling was done in 2008, previous to maize planting.Nitrogen fertilization increased carbon (C) return to soil during 2001–2008 (11.1 and 18.7% for N1 and N2 respect to N0) but no differences in SOC, SON, AOC, and AON were observed among N rates in 0–5 and 0–20 cm depth. It was only found more PON in N1 and a slight tendency to increased POC (3% and 13% for N1 and N2 respect to N0) in 0–5 cm depth. At the same time, NO3−-N content in 0–60 cm depth was similar among N rates (89.6 ± 8.4, 88.6 ± 6.4, and 81.6 ± 10.3 kg N ha−1 for N0, N1, and N2, respectively). By contrast, it was determined soil acidification (5.8 ± 0.3, 5.5 ± 0.2, and 5.3 ± 0.2 for N0, N1, and N2, respectively) and AN reductions in 0–5 cm depth as N rate increased, (76.1 ± 3.2; 74.9 ± 6.3 and 57.9 ± 3.5 for N0, N1 and N2 respectively). The high frequency of soybean in the rotation could have prevented higher increases in C return to soil and, as a consequence, mitigated the changes in related soil properties. In addition, the absence of N application to soybean also could have prevented enhances in soil acidification and AN depletion.

► Nitrogen fertilization increased carbon return to soil during the period 2001–2008.
► There were no differences in soil organic carbon and nitrogen among N rates.
► Nitrate content was similar among N rates.
► Increases in N rate caused a slight soil acidification and reductions on anaerobically mineralized nitrogen in 0–5 cm depth.