Soil/plant relationships and nutrient cycling | [Relacion planta - suelo y ciclo de nutrientes]

The main objectives of the section are: (1) to understand the mechanisms of adaptation of grasses and legumes to acid infertile soils; (ii) to determine the factors at the root-soil interface that affect nutrient uptake and competition for nutrients; (iii) to evaluate the role of nutrient cycling in grazed pastures; and (iv) to determine soil-plant interactions. that affect pasture-crop integrated systems. During 1990, the section has focused Its research work on three main areas: (1) plant adaptation to acid soils, (2) nutrient cycling in pastures, and (3) pasture establishment and rice production. Plant adaptation responses to soil acidity and nutrient supply include several morphological, physiological and biochemical changes in shoots and roots. Three glasshouse experiments were conducted to test the influence of soil clay content and nutrient supply on the performance of three different grass + legume associations, a wide range of grasses and legumes and somaclones of Stylosanthes guianensis. Plant adaptation responses were measured in terms of total biomass production; dry matter partitioning between roots and shoots; leaf area production; leaf chlorophyll, soluble protein and inorganic phosphate levels; net photosynthesis; acid phosphate activity in leaf and root extracts; nutrient uptake efficiency; nutrient use efficiency and nutrient transport from roots to shoots. The results obtained from these measurements indicate that in plants adapted to nutrient-poor acid soils, fixed carbon is preferentially partitioned towards root growth at the expense of leaf expansion and shoot growth. This is achieved by maintaining normal photosynthetic activity in the existing green leaves. These plants seem to adapt to a decrease in soil clay content and nutrient supply by increasing root density as well as root biomass. This strategy enables these adapted species to be productive on nutrient-poor acid soils by scavenging efficiently for nutrients. The dramatic increase in phosphorus uptake efficiency and root acid phosphatase activity observed in legumes when compared to grasses indicates that the mechanisms of phosphorus acquisition and transport in legumes may be different from those of grasses. Improved understanding of the physiological, biochemical and genetic basis of differences in shoot to root partitioning of fixed carbon in relation to nutrient supply could help to increase the persistence and productivity of the forage species. In order to evaluate the contribution of animal dung pats to the recycling of nutrients to grass-legume pastures under grazing, some preliminary measurements were carried out this year. The influence of stocking rate on spatial distribution of dung pats was determined in grass alone as well as grass + legume pastures. Distribution of dung pats was more even under higher stocking rate. The activity of beetles was higher in grass + legume pastures than grass alone pastures resulting in an increase in lateral and vertical distribution of dung pats.