Soil-plant dynamics related to nitrogen uptake and soil nitrogen availability

The effects of management practices on soil N supply and different biochemical assays to predict N availability were reviewed, with an emphasis on field-applied methods for soil testing and fertilizer management. The highest correlations with N uptake were for aerobic incubations and mild extracts and the lowest correlations involved methods using anaerobic incubation and intensive extractions. Although commonly used N supply indices work well in some circumstances, problems remain due to variability associated with sample collection, handling, and processing as well as site-specific circumstances.; Corn (Zea mays L.) yields in Venezuela are highly variable and a better understanding of the multiple factors affecting yields is needed. I studied corn response to N application during three years on a Mollisol soil. Total N uptake by the crop was more closely associated with initial soil mineral N content and soil water than was crop yield. This probably is because most of the N uptake occurred before silking, while a great proportion of the total dry matter production occurred later.; Soil factors affecting N supply to plants were evaluated for 15 soils of Venezuela. I hypothesized soil aggregate fractions could be used as an index of soil N availability, measured by N uptake by a perennial grass ( Brachiaria brizanta). In soils having a high capacity to supply N, strong correlations between macroaggregation and N uptake were found. In contrast, a poor association between N in macroaggregates and N uptake was observed in the soils with low capacity to supply N.; Simulation modeling can be useful in evaluating root dynamics and crop responses to management. I describe a general root growth model that simulates vertical root development and lateral root proliferation, root depth and density distribution and includes a full carbon budget. The model was parameterized for two different crops (corn and cotton) and different soil conditions. The simulated root length density and rooting depth were similar to the observed data and model analysis demonstrated that the model can be used to predict the root system dynamics in the soil profile for different crops.