Stocks and fractions of organic carbon in an Oxisol under different management systems

O objetivo deste trabalho foi determinar os efeitos de sistemas de manejo do solo sobre os estoques e as frações de carbono orgânico de um Latossolo Amarelo, no cerrado do Piauí. Foram avaliados os sistemas: plantio convencional por três anos; plantio direto por três e cinco anos, com uso de milheto como cultivo de cobertura; e plantio direto por nove anos, dos quais sete com uso de milheto e dois com forrageira (Urochloa brizantha). Utilizou-se área de cerrado nativo como referência. Amostras de solo foram coletadas nos períodos chuvoso e seco, das camadas de 0,00–0,05, 0,05–0,10, 0,10–0,20 e 0,20–0,40 m, para determinação de: estoques e totais de carbono orgânico e nitrogênio; teores de substâncias húmicas; e índice de humificação. Na camada de 0,00–0,05 m, em ambos os períodos, foram observados os maiores teores de carbono no solo sob cerrado. No entanto, nos dois períodos e nas quatro profundidades, verificaram-se os maiores estoques de carbono e nitrogênio bem como os maiores teores de substâncias húmicas nos solos sob plantio direto por cinco e nove anos. Os maiores índices de humificação também foram observados nesses solos, no período seco. O plantio direto, associado ao uso de cultivos de cobertura, diminui a mineralização da matéria orgânica do solo, estoca carbono no solo em profundidade e favorece o sequestro de carbono em áreas do cerrado do Piauí.The objective of this work was to determine the effects of soil management systems on the stocks and fractions of organic carbon of an Oxisol (Xantic Ferralsol), in the cerrado of the state of Piauí, Brazil. The following systems were evaluated: a three‑year‑old conventional tillage; a three‑ and a five‑year‑old no‑tillage system, using millet as cover crop; and a nine‑year‑old no‑tillage, using millet as cover crop for seven and forage (Urochloa brizantha) for two years. A native cerrado area was used as a reference. Soil samples were collected in the wet and dry seasons from 0.00–0.05, 0.05–0.10, 0.10–0.20, and 0.20–0.40 m depths for determining: total organic carbon and nitrogen contents and stocks; humic substances content; and humification index. The highest carbon contents were observed in the soil under cerrado, at 0.00–0.05 m depth, in both seasons. However, the highest stocks of carbon and nitrogen as well as the highest contents of humic substances were verified in the soil with no‑tillage system for five and nine years, in both seasons, at all evaluated depths. The highest humification index was also observed in these soils, in the dry season. No‑tillage associated with cover crops decreases soil organic matter mineralization, increases soil carbon stocks in depth, and favors carbon sequestration in cerrado areas of Piauí state

<b>Repeatability, number of harvests, and phenotypic stability of dry matter yield and quality traits of Panicum maximum jacq.

Selection of superior forage genotypes is based on agronomic traits assayed in repeated measures. The questions are how repeatable the performance of individual genotypes is and how many harvests are needed to select the best genotypes. The objectives were to estimate repeatability coefficients of dry matter yield (DMY) and forage quality, their phenotypic stability and the number of harvests needed for an accurate selection. Two randomized complete block design experiments data with 24 genotypes each, undergoing 12 and 16 harvests, over a period of 2 and 3 years, respectively, were used. The DMY repeatability estimates ranged from 0.42 to 0.55, suggesting a low heritability. The mean numbers of repeated measures were 5 and 7 harvests for 0.80 and 0.85 accuracy, respectively. The inclusion of the first two harvests negatively affects the estimates. Repeatability for quality traits ranged from 0.30 to 0.69, indicating low to moderate heritability.