Soil organic C and N fractions under continuous cropping with contrasting tillage systems on mollisols of the southeastern Pampas

Cropping alters soil quality, and tillage intensity and residue input affects soil organic carbon (SOC) and nitrogen (SON). The aim of this work was to evaluate the changes in SOC and SON, of their particulate fractions (POC and PON), mineralizable N (anaerobic incubations, AN), and corn yield, in response to tillage systems and N fertilization under continuous cropping. Such variables were evaluated in 2001 and 2004 in a long-term experiment started in 1997 on a non-degraded complex of Typic Argiudoll and Petrocalcic Paleudoll soils at Balcarce, Argentina (37°45'S, 58°18'W). Tillage systems were conventional (CT) and no-tillage (NT) combined with continuous N fertilization and no N fertilization. Treatment factors did not differentially affect bulk density (BD) although a trend towards a higher BD could be seen under NT and in 2004. Neither tillage system nor N fertilization affected differentially SOC, SON, POC, nor PON contents with the exception of the 0-5cm layer where higher contents and a trend to higher contents were observed under NT and with N, respectively. However, despite little changes in SOC and SON contents respect to the beginning of the experiment occurred along it, significant decreases were observed between 1997 and 2001 and great increases arose between 2001 and 2004. These changes were related to the level of annual residue input in each period associated to weather conditions. No changes were observed in AN, which means that soil capacity to supply N to crops was not affected. Corn yield was highly affected by water availability in each of the studied growing seasons. However, when no N was applied, grain yield was greater under CT. Although potentially mineralizable N was not affected by treatment factors, the capacity of providing N to corn was expressed only when mineralization was promoted by tillage. Hence, rationally tilling this non-degraded mollisol with low risk of erosion would allow to take advantage of its potential of N supply and to a safe N fertilization reducing environmental risks due to excess of fertilizer. As long as it were assured a high average annual residue input, no significant changes in soil organic fractions would be expected during a relatively long period of use.