Nitrogen Use Efficiency by Annual and Perennial Crops

Abstract

The amount of nitrogen fertilizer applied to plants is usually large. Only part of this fertilizer, of about 6–50%, is taken up by plants, depending on fertilizer, plant and soil type, climatic conditions, and agricultural practices. The unaccounted nitrogen can be emitted from the ecosystem as trace gas and ammonia volatilization, or lost by leaching and runoff in the nitrate or ammonium form. The goal of reducing mineral nitrogen usage will be to this twenty-first century what the goal of reducing pesticides was to the last century. In the present study we reviewed the different concepts for nitrogen use efficiency by annual and woody plants. The major points were (i) understanding the terminology and the context in which each concept for nitrogen use efficiency has been used for annual and perennial woody plants, (ii) identifying the critical steps of controlling plant nitrogen use efficiency, and (iii) addressing new approaches to improve the efficiency for annual and perennial woody plants.

Some factors have been extensively studied for arable crops, but not for woody plants, and included the source, timing and rate of fertilizer nitrogen, plant growth curves, environmental factors, and cultural practices. Precision nitrogen management, and intercropping and crop rotation including legumes are recommended techniques to improve nutrient efficiency. Maximum crop yield has been achieved using high-yielding bred plants, with an optimization of photosynthetic capacity, but without maximizing the nitrogen use efficiency. At low nitrogen availability, C₃ plants have greater nitrogen use efficiency than C₄ plants, whereas at high nitrogen the opposite is true. Consequently, identifying the regulatory elements controlling the balance between nitrogen allocation to maintain photosynthesis and the reallocation of the remobilized nitrogen to sink organs in C₃ and C₄ species is vital for improving fertilizer use efficiency and reducing excessive input of fertilizer, while maintaining an acceptable yield.

In the present review we reinforced the importance of selecting plant genotypes from the ancient and modern germoplasm in order to improve nitrogen use efficiency. Efforts should include plant selection under low nitrogen, which has not been a priority for plant breeders. Plant breeders may need to find newer, more appropriate plant cultivars which can maximize the role of mycorrhizal in agriculture. The contribution of mycorrhizal in annual and perennial crops, particularly with ancient plant germoplasm, and the influence of rootstocks and plant N reserves in woody plants, and ammonia emissions from annual and perennial crops (1–4% of fertilizer nitrogen) should be considered for improving fertilizer nitrogen use efficiency. Beneficial traits can probably include the ability to maintain the plant photosynthetic capacity and nitrogen uptake under reduced (or high) nitrogen level perhaps through beneficial mycorrhizal associations and old (or modern) crop genotypes. Nitrogen absorption depends on fungi strain and plant cultivar. Rootstocks influence roots number and class, the nutrient uptake and translocation, and the bud break in woody plants which mechanism is not fully understood. This factor is probably crucial for attaining optimum nitrogen use efficiency and careful attention should be given for the choice of appropriate rootstock. Finally, we found that most studies for crop nitrogen recovery are based on recovery efficiency at harvest or fruit ripening which may underestimate the potential for fertilizer N utilization. Greater efficiency data arise when the efficiency is calculated on basis of mean nitrogen during the major growth period.