The global challenge of meeting increased food demand and protecting environmental quality will be won or lost in cropping systems that produce maize, rice, and wheat. Achieving synchrony between N supply and crop demand without excess or deficiency is the key to optimizing trade-offs amongst yield, profit, and environmental protection in both large-scale systems in developed countries and small-scale systems in developing countries. Setting the research agenda and developing effective policies to meet this challenge requires quantitative understanding of current levels of N-use efficiency and losses in these systems, the biophysical controls on these factors, and the economic returns from adoption of improved management practices. Although advances in basic biology, ecology, and biogeochemistry can provide answers, the magnitude of the scientific challenge should not be underestimated because it becomes increasingly difficult to control the fate of N in cropping systems that must sustain yield i...

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Agroecosystems, Nitrogen-use Efficiency, and Nitrogen Management

Abstract The global challenge of meeting increased food demand and protecting environmental quality will be won or lost in cropping systems that produce maize, rice, and wheat. Achieving synchrony between N supply and crop demand without excess or deficiency is the key to optimizing trade-offs amongst yield, profit, and environmental protection in both large-scale systems in developed countries and small-scale systems in developing countries. Setting the research agenda and developing effective policies to meet this challenge requires quantitative understanding of current levels of N-use efficiency and losses in these systems, the biophysical controls on these factors, and the economic returns from adoption of improved management practices. Although advances in basic biology, ecology, and biogeochemistry can provide answers, the magnitude of the scientific challenge should not be underestimated because it becomes increasingly difficult to control the fate of N in cropping systems that must sustain yield increases on the world's limited supply of productive farm land.

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Agroecosystems, Nitrogen-use Effic iency, and Nitrogen Management

CropSyst, a cropping systems simulation model

STICS (Simulateur mulTJdiscplinaire pour les Cultures Standard) is a crop model constructed as a simula- tion tool capable of working under agricultural conditions. Outputs comprise the production (amount and quality) and the environment. Inputs take into account the climate, the soi1 and the cropping system. STICS is presented as a model exhibiting the following qualities: robustness, an easy access to inputs and an uncomplicated f~~ture evolution thanks to a modular (easy adaptation to various types of plant) nature and generic. However, STICS is not an entirely new model since most parts use classic formalisms or stem from existing models. The main simulated processes are the growth, the development of the crop and the water and nitrogenous balance of the soil-crop system. The seven modules of STICS - development, shoot growth, yield components, root growth, water balance, thermal environment and nitrogen balance - are presented in tum with a discussion about the theoretical choices in comparison to other models. These choices should render the model capable of exhibiting the announced qualities in classic environmental contexts. However, because some processes (e.g. ammoniac volatilization, clrought resistance, etc.) are not taken into account, the use of STICS is presently limited to several cropping systems. (O InraIElsevier, Paris.) crop modelling / wheat / corn / water balance / nitrogen balance

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STICS : a generic model for the simulation of crops and their water and nitrogen balances. I. Theory, and parameterization applied to wheat and corn

The rate of N uptake of crops is highly variable during crop development and between years and sites. However, under ample soil N availability, crop N accumulation is highly related to crop growth rate and to biomass accumulation. Critical N concentration has been defined as the minimum N concentration which allows maximum growth rate. Critical N concentration declines during crop growth. The relationship between critical N concentration and biomass accumulation over the growth period of a crop is broadly similar within major C(3) and C(4) cultivated species. Therefore, the critical N concentration concept is widely used in agronomy as the basis of the diagnosis of crop N status, and allows discrimination between situations of sub-optimal and supra-optimal N supply. The relationship between N and biomass accumulation in crops, relies on the interregulation of multiple crop physiological processes. Among these processes, N uptake, crop C assimilation and thus growth rate, and C and N allocation between organs and between plants, play a particular role. Under sub-optimal N supply, N uptake of the crop depends on soil mineral N availability and distribution, and on root distribution. Under ample N supply, N uptake largely depends on growth rate via internal plant regulation. Carbon assimilation of the crop is related to crop N through the distribution of N between mature leaves with consequences for leaf and canopy photosynthesis. However, although less commonly emphasized, carbon assimilation of the crop also depends on crop N through leaf area development. Therefore, crop growth rate fundamentally relies on the balance of N allocation between growing and mature leaves. Nitrogen uptake and distribution also depends on C allocation between organs and N composition of these organs. Within shoots, allocation of C to stems generally increases in relation to C allocation to the leaves over the crop growth period. Allocation of C and N between shoots and roots also changes to a large extent in relation to soil N and/or crop N. These alterations in C and N allocation between plant organs have implications, together with soil availability and carbon assimilation, on N uptake and distribution in crops. Therefore, N uptake and distribution in plants and crops involves many aspects of growth and development. Regulation of nitrogen assimilation needs to be considered in the context of these interregulatory processes.

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N uptake and distribution in crops: an agronomical and ecophysiological perspective

Decline in Percentage N of C3 and C4 Crops with Increasing Plant Mass

Growth Rate and % N of Field Grown Crops: Theory and Experiments

Quantitative relationships for growth rate and its dependence on plant nitrogen concentration are developed from the results of experiments on potatoes, cereals and vegetables. The relationships appear to be of general applicability and most coefficients in them are similar for widely different crops.

Quantitative relationships for the dependence of growth rate of arable crops on their nitrogen content, dry weight and aerial environment

A simulation model is described to interpret N fertilizer experiments on potatoes. It calculates the total growth of dry matter, the N uptake, the partition of dry matter and of N between tuber and foliage and the distribution of inorganic N down the profile for each day during the growing season. The validity of the model was tested against measurements of these parameters made at approximately fortnightly intervals on plots that received N fertilizer and those that received none in 4 experiments on a sand, 4 on a sandy loam and 3 on a clay soil. Simulated values were in reasonably good agreement with the measured values in all experiments. Overall the sums of squares of the differences between the simulated and measured values of the %N in the total plant, and the total amount of inorganic N in the top metre of soil and the logarithms of the total weight of dry matter, tuber dry weight, N uptake in the entire plant and N uptake in the tubers were each less than 25% of the sum of squares of the measured values about the mean. Only 9 inputs were required for the model. It was found essential to take account of differences in spring and summer leaching but not of inter-site differences in mineralization rate.

Response of potatoes to N fertilizer: dynamic model

Quantitative relationships for key processes influencing N response were derived from measurements of inorganic N in soil, the weights and N contents of foliage and tubers made at intervals during growth of maincrop potatoes in 11 N fertilizer experiments.

J. J. Neeteson

Papers

Decline in Percentage N of C3 and C4 Crops with Increasing Plant Mass

Growth Rate and % N of Field Grown Crops: Theory and Experiments

Quantitative relationships for the dependence of growth rate of arable crops on their nitrogen content, dry weight and aerial environment

Response of potatoes to N fertilizer: dynamic model

Response of potatoes to N fertilizer: quantitative relations for components of growth