Soil δ¹³C in Mixed Pasture with Forage Peanut and N-Fertilized Monoculture

The soil as a sink of Carbon (C) is a promising alternative to remove carbon dioxide (CO₂) from the atmosphere. The study assessed the natural abundance of δ¹³C ‰ among pastures system in monoculture fertilized with nitrogen (N) or grass-legume mixed pastures systems. The study was carried out at the Experimental Station of CEPLAC placed in Itabela – BA. The climate of the region is in transition between tropical rainforest climate (Af) and tropical monsoon climate (Am). The experiment followed a randomized block design with four blocks, arranged in a 2 x 8 factorial scheme (α = 5%). Factor one was composed of two pasture types: 1. Mixed pasture of Urochloa Brizantha cv. Marandu and Arachis pintoi cv. Belmonte, and; 2. Continuous pasture consisted of Marandu palisadegrass fertilized with 120 kg N ha^-1 year^-1. Factor two consisted of different soil depths (0-5, 5-10, 10-20, 20-30, 30-40, 40-60, 60-80 and 80-100 cm). Following sixteen years of mixed pastures establishment, the values of δ¹³C ‰ isotopic abundance, at 30 cm soil depth, were more negative than monoculture (p value > 0.05). On average, the δ¹³C values at the 0-5, 5-10, 10-20 and 20-30 cm depth intervals were, respectively, -18.15, -19.14, -19.84 and -22.19 ‰ for mixed pastures and -15.98, -16.18, -17.94 and -20.66 ‰ for fertilized pastures. There were no significant differences in deeper layers, suggesting that the time of pasture establishment was not enough to modify the soil isotopic signature in depth. Therefore, in mixed pastures, at 0-20 cm layer, the contribution of C₃ species was 45.78 % greater than fertilized pastures in monoculture, confirming the potential of legume to enhance C stocks. In general, a gradual reduction of ¹³C isotope was observed as the increase of soil depth, ranging between -17.06 % at 0-5 cm interval and -24.24 % at 80-100 cm interval. Since the average of δ¹³C values in the Native Forest at the 0-5, 5-10, 10-20 and 20-30 cm depth intervals were, -26.55, -26.55, -26.32 and -25.99 ‰, respectively, these data suggest that, with increasing depth, most of the C originates from natural vegetation, which was deforested twenty-four years ago. The deposition of plant residues from both shoots and roots increases soil organic matter turnover, modifying the isotopic composition, mainly in superficial layers. Hence, to compare C stocks between management systems, it’s essential to collect soil samples to a depth of at least 30 cm.