Agroecosystems, Nitrogen-use Efficiency, and Nitrogen Management

Abstract

INTRODUCTIONThe focus of this paper is on nitrogen-use efficiency (NUE) in cereal production systems because maize (Zea mays L.), rice (Oryza sativa L.), and wheat (Triticum aestivum L.) provide more than 60% of human dietary calories either as cereals for direct human consumption or embodied in livestock products produced from animals fed with feed grains and their by-products (http:/apps.fao.org/, agricultural production). It is likely that these same cereal crops will continue to account for the bulk of the future human food supply because they produce greater yields of human-edible food, are easily grown, stored, and transported, and require less fuel and labor for processing and cooking than other food crops. Our analysis will examine the NUE of these primary cereals in the world's major cropping systems, which also account for the majority of global N fertilizer use. We define the NUE of a cropping system as the proportion of all N inputs that are removed in harvested crop biomass, contained in recycled crop residues, and incorporated into soil organic matter and inorganic N pools. Nitrogen not recovered in these N sinks is lost from the cropping system and thus contributes to the reactive N (Nr) (1) load that cascades through environments external to the agroecosystem.Our evaluation will focus on NUE in on-farm settings because estimates of NUE from experimental plots do not accurately represent the efficiencies achieved in farmers' fields. This lack of agreement results from differences in the scale of farming operations and differences in N-management practices-some of which are only feasible in small research plots. The effect of scale not only influences N fertilizer application, but all other management operations such as tillage, seeding, weed and pest management, irrigation, and harvest, which also affect efficiency. As a result, N-fertilizer efficiency in well-managed research experiments is generally greater than the efficiency of the same practices applied by farmers in production fields. For example, the average N-fertilizer uptake efficiency (defined as the percent-

Attribution of yield change for rice-wheat rotation system in China to climate change, cultivars and agronomic management in the past three decades

“…but fertilization use efficiency decreased with application rate increasing (Peng et al 2006). Consequently, the key to optimize trade-offs among yield, profit, and environmental protection is to achieve synchrony between fertilizer supply and crop demand without surplus or deficiency (Cassman et al 2002). In addition, other improvements in agronomic management practices such as pest, disease and weed management, soil melioration and no-tillage contributed to yield increase too, which however were not accounted in this study.…”

Food and fuel for all: realistic or foolish?

“…Th e resulting high grain prices may motivate farmers to achieve larger yields by using greater amounts of nitrogen fertilizer with current, relatively ineffi cient technologies, 21 and a reversion to conventional tillage from conservation tillage systems without regard for environmental consequences. While such gains may give a short-term spike in yields, they would not represent a new yield trajectory because they require practices that are not sustainable over the long term due to degradation of soil and water quality.…”

Modeling shows that alternative soil management can decrease greenhouse gases

“…However, this is an indication that in many crop systems the conventional amount of fertilizer applied is above what is actually taken up by the crop. Overfertilization is a common practice in California, due to farmers' desires to minimize the risk of yield reductions due to nitrogen limitation and the low price of nitrogen fertilizer (Cassman et al 2002). Fertilizer rates are selected so that the least productive parts of a field still receive sufficient nitrogen.…”