Nitrogen transformation rates and nutrient availability under conventional plough and conservation tillage

Abstract

The effect of 14-years of plough and conservation tillage on macro- and micro-nutrient availability and on N transformation rates was evaluated in a temperate humid region soil (0–5 and 5–15cm) under an annual Italian ryegrass-maize forage rotation. Nutrients were extracted with an NH4Ac-DTPA solution. Gross N mineralization (m), nitrification (n) and immobilization (i) rates were calculated by 15N isotope dilution technique (experiment with 15NH4, 48-h incubation) with the FLUAZ model. Our results demonstrate that long-term conservation tillage increases organic C in the upper soil layer and the availability of most of studied nutrients (Co, Fe, Mg, Mn, Na, P and Zn) compared with plough tillage. On the contrary, conservation tillage resulted in lower contents of NO3−-N, and extractable K and Mn in the 5–15cm soil layer. Extractable Al, Ca and Cu were unaffected by tillage system or soil depth. No significant effect of tillage practices on gross and net N transformation rates was observed. Soil C contents was not related to gross and net N fluxes, while positive relationships were found between: (1) m and soil available NH4+-N; (2) n and soil available K and δ15N; (3) i and soil available K, Mn, Mg, total N and NO3−-N; and (4) net m and n with soil available NO3−-N and K. The negative relationships of n and δ15N with soil Co content suggested that Co availability could affect the nitrifying activity. Results suggest that tillage practices had a limited effect on N transformation rates in this soil and that NO3−-N leaching could decrease under conservation tillage. For N fluxes in these agricultural soils, K was a more important factor than the other nutrients, suggesting close relationships between N and K availability; however, some micro-nutrients could also play a relevant role on soil N cycle and, therefore, they must be considered in future studies on gross N fluxes.