Phosphorus (P) losses from sandy soils that are predominant in the 1.4 million ha of pastureland in Florida are a major cause of water pollution. We hypothesized that soil P loss would be lower from silvopastoral systems than from treeless pastures because soil P removal by a combined stand of trees and pasture would be more than that of treeless pasture. Four slash pine (Pinus elliottii Engelm.) + bahiagrass (Paspalum notatum Flüggé) silvopastoral systems located in Alachua, Suwannee, Manatee, and Osceola counties in Florida were selected for the study. The former two sites are on Ultisols, and the latter two on Spodosols. Soil samples were collected at different depth increments. Soil P storage capacity (SPSC), the maximum amount of P that can be safely applied to a soil before it becomes an environmental concern, was calculated. Water-soluble P concentrations in the 0-5 cm soil layer ranged from 4 to 11 mg kg −1 for the silvopasture sites and 10 to 23 mg kg −1 in the treeless pasture sites, with higher P concentrations in the treeless pasture at each location. Total SPSC in the upper 1 m depth ranged from 342 to 657 kg ha −1 in the silvopasture sites and −60 to 926 kg ha −1 in the treeless pasture sites (a negative value indicates that the soil is a P source). The results suggest that P buildup within the soil profile and therefore the chances for loss of P from soil to water bodies were less from silvopastures than from treeless pastures. Thus, silvopasture systems can be expected to provide greater environmental service in regard to water quality protection compared to treeless pastures under comparable ecological settings

We hypothesized that, because of the ability of trees to sequester carbon (C) in the deep soil profile and remove excess nutrients from soils, the silvopastoral agroforestry system could enhance the environmental quality of the agricultural lands. To test this hypothesis, two sets of experiments were conducted in two soil orders in Florida, Spodosols and Ultisols, with two major objectives: i) determining the soil C accumulation and tracing the plant sources of C in soil fractions, and ii) quantifying water soluble phosphorus (WSP) and estimating the Soil P Storage Capacity (SPSC). Total C in both soil orders was greater under silvopasture than in treeless pastures, particularly at lower depths. Stable-isotope signature analysis suggested that C 3 plants (in this case, slash pine, Pinus elliotii) contributed to a more stable C fraction than C 4 plants (in this case, bahiagrass, Paspalum notatum) at soil depths up to 1 m. WSP was consistently higher in treeless pastures, while the remaining SPSC was lower in this land-use system, suggesting the greater likelihood of P moving out of the soil under treeless pasture than in silvopasture. Thus, the presence of trees in pastures contributed to more stable C within the soil profiles, lower WSP, and greater SPSC, indicating more environmental benefits provided by silvopastoral systems as compared to treeless pastures under similar ecological settings. Key words: carbon sequestration, nutrients, soil P storage capacity, treeless pasture MELHORIA DA QUALIDADE AMBIENTAL DE TERRAS AGRICULTÁVEIS POR MEIO DA SILVOPASTAGEM NO SUDESTE DOS ESTADOS UNIDOSRESUMO: Nossa hipótese é de que devido à habilidade das árvores seqüestrarem carbono (C) no perfil profundo do solo e remover o excesso de nutrientes dos solos, o sistema de silvopastagem agroflorestal poderia melhorar a qualidade ambiental de terras agricultáveis. Para testar esta hipótese, dois grupos de experimentos foram conduzidos em duas ordens de solos na Florida, Espodossolos e Ultissolos, com dois objetivos principais: i) determinar a acumulação de C do solo e investigar as fontes de C para as plantas nas frações dos solos, e ii) quantificar o fósforo solúvel em água (FSA) e estimar a capacidade de armazenamento de fósforo no solo (CAFS). O C total em ambos os solos foi maior sob o sistema de silvopastagem do que sob pastagens com menos árvores, particularmente nas profundidades mais baixas. A análise por assinatura de isótopo estável sugeriu que as plantas C 3 (neste caso, slash pine, Pinis elliotti) contribuíram mais para a fração estável do carbono do que plantas C 4 (neste caso, bahiagrass, Paspalum notatum) nas profundidades dos solos acima de 1 m. O FSA foi consistentemente maior em pastagens com poucas árvores, enquanto que a CAFS foi mais baixa neste sistema, sugerindo a grande probabilidade do fósforo ser mais facilmente movido do solo sob pastagens com poucas árvores do que nos sob silvopastagem. Deste modo, a presença de árvores em pastagens contribuiu para C mais estável nos perfis dos solos e o mai...