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

Abstract

Evaluating the capacity of soil to supply N and the plant characteristics associated with N uptake are key to determining economical and environmentally sound fertilization practices. I reviewed the literature to assess the effects of management practices (e.g., tillage, crop rotations and N fertilization) on soil N supply and to examine the utility of different biochemical assays to predict N availability (Chapter 1). Published work shows that management affects N supply by influencing inorganic and organic N pools, by modifying the activity of soil decomposers that mineralize organic N sources, and by influencing soil processes involved in N losses. Also, management can affect plant root properties associated with N and water uptake (e.g., root depth, root density). A variety of different methods for assessing potential N supply were evaluated, with an emphasis on field-applied methods that could be practical for soil testing and fertilizer management. The highest correlation between N uptake by crops and N supply were for aerobic incubations and mild extracts. The lowest correlations involved methods using anaerobic incubation and intensive extractions. Although traditionally-used indices of soil N supply based on biological and/or chemical assays work well in some circumstances, none work consistently in all cases due to variability associated with sample collection, handling, and processing, as well as site-specific circumstances. Com (Zea mays L.) yields in Venezuela are highly variable, and a better understanding of the multiple factors affecting yields is needed. In a field study, I studied com response to N application during three successive years on a Mollisol soil of Venezuela (Chapter 2). I evaluated the effects of initial contents of soil mineral N and water, and different amounts and distribution of precipitation during the crop cycle on N uptake and soil N dynamics. Total N uptake by the crop was more closely associated with the initial soil mineral N content and soil water than was yield and dry matter production. This probably is because most of the N uptake occurred before silking, while a great proportion of the total dry matter production occurred later. Interannual variations in crop response to N application were associated with variations in soil capacity to supply N and variations in water availability during the crop cycle. Soil factors affecting the amount and timing of N supply to plants were evaluated for 15 soils in Venezuela (Chapter 3). I attempted to identify measurable soil N fractions associated with soil N supply capacity, along with other measures such as initial soil mineral N content. Because organic N sources associated with macroaggregate size fractions have been found to be easily mineralizable, I hypothesized that some of these fractions could be used as an index of soil N availability measured by N uptake by a perennial grass (Brachiaria brizanta) in the greenhouse. Although initial mineral N content was closely associated with the N uptake by Brachiarai, it represented only a small proportion of the total N uptake so that most of the N came from an easily decomposable organic N source. In soils having a high capacity to supply N, high correlations between macroaggregation and N uptake were found, suggesting that macroaggregate-associated N was a significant labile N source. In contrast, a poor association between N in macroaggregates and N uptake was observed in the soils with low capacity to supply N. However, there was no single soil property that could reliably separate soils of high capacity from low capacity to supply N other than the initial content of mineral N. Future research should be oriented to identify and to isolate the organic sources of N responsible for soil N availability in different soils. The plant root system is a primary determinant of plant growth, controlling the uptake of nutrients and water. Due to the diversity of factors that affect root growth, simulation modeling can be useful in evaluating root dynamics and crop responses to management. In Chapter 4, I describe a general root growth model that emphasizes root properties associated with water and nutrient uptake as a function of physical and chemical properties, C and N availability, plant phenology, and climatic variables. The model 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 (com and cotton) and different soil conditions. The simulated root length density and rooting depth were similar to the observed data, both in terms of the magnitude and the temporal patterns. The root model was sensitive to plant and soil parameters and it could be satisfactorily utilized to predict the root system dynamics in the soil profile for different crops.