Short and long-term distribution with depth of soil organic carbon and nutrients under traditional and conservation tillage in a Mediterranean environment (southwest Spain)

The impacts of tillage and cropping sequences on soil organic matter and nutrients have been frequently reported to affect the uppermost soil layers, but there is little published information concerning effects at greater depth. This article reports results on the distribution of soil organic carbon (SOC), active carbon (AC), N, Olsen-P and extractable K within 100 cm in short (4 yr) and long (16 yr) term experiments under different tillage systems. Short (TT4) and long (TT16) traditional tillage are compared with conservation tillage, reduced (RT16) and non-tillage (NT4). The results show more accumulation of SOC in the near-surface under RT16 and NT4 in both experiments compared with traditional tillage. Moreover, greater C content occurs to 40 cm depth in the long-term experiment. The results demonstrate the importance of time on C accumulation, not only in near-surface layers but also at greater depths. Active C is an indicator of the increase in soil quality in the long-term experiment. This trend is only apparent for the first 10 cm in the short-term experiment. Patterns in N, Olsen-P and extractable K are similar to that of SOC. However, only extractable K is significantly greater in soil under conservation tillage (RT16 and NT4) after short and long periods. Potassium availability is a good indicator of the changes caused by tillage. Our results indicate that studies of soils at depth could be very useful in long-term experiments to demonstrate the effect of conservation tillage on C and nutrient distribution.