Nitrogen distribution in soil density fractions and its relation to nitrogen mineralisation under different tillage systems

Cropping leads to a depletion of soil organic matter which is associated with a decrease in crop yields. In order to reduce land degradation, conservation tillage systems have been developed over the last few decades. We evaluated the effects of 12 years of no-tillage, chisel tillage, and plough tillage, in a Typic Argiudoll from the Argentine Pampa, on nitrogen distribution in the light (<1.6 g/mL), medium (1.6-2.0 g/mL), and heavy (>2.0 g/mL) soil density fractions and its mineralisation potential. Under no-tillage, nitrogen in light and heavy fractions, and mineralised nitrogen of the whole soil, diminished markedly with depth. Meanwhile, in ploughed soil these variables remained constant up to 20 cm depth. Under chisel tillage, an intermediate condition was observed. In the first 20 cm, no-tillage accumulated more nitrogen in light and medium fractions. A higher and positive correlation was observed between the percentage of organic nitrogen mineralised and the nitrogen in the soil medium fraction, as a percentage of the total nitrogen. Cumulative nitrogen production fitted significantly to the exponential (R-2 > 0.931), hyperbolic (R-2 > 0.932), and logistic (R-2 > 0.930) models, while the Gompertz equation described the data best, obtaining the highest determination coefficient (R-2 > 0.989). In vitro nitrogen mineralisation was highest under no-tillage. These results could be attributed to the accumulation of labile organic components associated with a lower mineralisation intensity in the field, a consequence of lower temperatures. This increase under no-tillage in fertility can represent a nitrogen reserve for future crops.