Our objective was to determine the effect of tillage system, crop rotation, and N fertilization on soil organic carbon (SOC) storage in the 0‐ to 90‐cm profile in a long‐term (20‐yr) experiment established in 1986 on a rainfed Mediterranean Vertisol in southern Spain. The treatments studied were: conventional tillage (CT) vs. no‐tillage (NT); five crop rotations: wheat (Triticum aestivum L.)–chickpea (Cicer arietinum L.) (WC), wheat–sunflower (Helianthus annuus L.) (WS), wheat–bare fallow (WF), wheat–faba‐bean (Vicia faba L.) (WFB), and continuous wheat (WW); and N fertilizer applied at four rates (0, 50, 100, and 150 kg N ha−1). The SOC content of soil samples was determined in 1995, 1997, 2000, 2003, and 2006 for four different soil layers (0–15, 15–30, 30–60, and 60–90 cm). The application of N fertilizer did not influence SOC sequestration. The other treatments showed a gradual increase of total SOC content over time, although there were no differences between some consecutive years. The SOC accumulation was higher for 30‐ to 60‐ and 60‐ to 90‐cm depths than other depths due to characteristic cracks of Vertisol. Over the 20 yr of the study, WW and WFB sequestered 21 and 15 Mg C ha−1 more under NT than under CT, respectively. The other crop rotations did not show any difference in C sequestration between NT and CT. Under CT, WS sequestered more SOC than other rotations, while under NT, WW and WFB sequestered more SOC. In general, the crop rotation intensification and NT had a positive effect over time on SOC sequestration in this rainfed Mediterranean Vertisol.

Little research has examined the influence of tillage system on root growth in wheat grown on rainfed Vertisols. A 3-year field study (2003, 2004 and 2005) was carried out on a typical Vertisol (southern Spain), to determine the effects of tillage system on root growth in spring wheat (Triticum aestivum L) grown in continuous rotation with faba bean (Vicia faba L), within the framework of the longterm "Malagón" experiment started in 1986. Tillage treatments were no-tillage (NT) and conventional tillage (CT), and the experiment was designed as a randomized complete block with three replications. The following parameters were measured: above-ground biomass, grain yield, root length density (RLD), root biomass (RB) and root N content. In the topmost 10 cm of soil, higher values were found under CT than under NT for RLD in the rainiest year (20.2 km m −3 vs. 9.6 km m −3 respectively) and for RB (512 kg ha −1 vs. 261 kg ha −1 respectively) in all study years. In deeper layers, no difference was recorded between the two tillage systems. Greater wheat root development in the upper soil layer under CT may reflect the greater soil penetration resistance found in the topmost 10 cm under NT. Root separation using a sieve with a 0.5 mm mesh screen led to a marked underestimation of RLD and RB, with values up to three times higher when using a 0.2 mm mesh screen. Mean wheat root N content in the topmost 30 cm of soil accounted for over 80% of total root N content. The highest grain yield was observed under NT, since this system provided greater water storage in the soil profile in the mostly dry study years.

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all rights reserved agricultural soils, having been depleted of much of their native c stocks, have a significant co 2 sink capacity. 1 for this reason, the quantification of c content in various types of soil, according to climate and soil management, is of great interest. Moreover, c sequestration in agricultural soils will also benefit productivity and sustainability, and trading of c credits or tax benefits could be a source of income for agricultural producers. 2 awarding these benefits would require the monitoring of soil organic c content. It has been estimated that a soil c monitoring system, including all uS farms, using existing analytical techniques, could cost more than $1 billion for the first four-year commitment period. 3 clearly, a reduction in per-sample cost of analysis