This paper contrasts the natural and anthropogenic controls on the conversion of unreactive N2 to more reactive forms of nitrogen (Nr). A variety of data sets are used to construct global N budgets for 1860 and the early 1990s and to make projections for the global N budget in 2050. Regional N budgets for Asia, North America, and other major regions for the early 1990s, as well as the marine N budget, are presented to highlight the dominant fluxes of nitrogen in each region. Important findings are that human activities increasingly dominate the N budget at the global and at most regional scales, the terrestrial and open ocean N budgets are essentially dis- connected, and the fixed forms of N are accumulating in most environmental reservoirs. The largest uncertainties in our understanding of the N budget at most scales are the rates of natural biological nitrogen fixation, the amount of Nr storage in most environmental reservoirs, and the production rates of N2 by denitrification.

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Nitrogen cycles: past, present, and future

1. Volatilization of Ammonia.- 2. Biological denitrification.- 3. Chemodenitrification.- 4. The measurement of denitrification.- 5. Micrometeorological methods for measuring gaseous losses of nitrogen in the field.- 6. Gaseous nitrogen losses from plants.- 7. Nitrogen loss from sewage sludges and manures applied to agricultural lands.- 8. Ammonia loss from fertilizer applied to tropical pastures.- 9. Gaseous nitrogen exchanges in grazed pastures.- 10. Fate of fertilizer nitrogen applied to wetland rice.- 11. The fate of nitrogen compounds in the atmosphere.- 12. Agronomic technological approaches to minimizing gaseous nitrogen losses from croplands.

Gaseous loss of nitrogen from plant-soil systems

Publisher Summary There are instances where the absolute amount of nitrogen in the tops of crops appears to have decreased before harvest, the maximum amount of nitrogen in the tops being reached nearer to anthesis. To the extent that they represent losses from the soil-plant system, they are important in assessing the results of nitrogen balance studies. It is not inconceivable that losses from the system, directly from plant tops, occur continuously over the full period of plant growth, only occasionally becoming apparent in records of aboveground nitrogen, when the rates of loss exceed the rates of uptake from the roots. Even if decreases in aboveground nitrogen merely reflect a reallocation of potentially harvestable amino acids, they are sources of inefficiency for those engaged in primary production. Careful attention must be paid to the fate of nitrogen in the tops of annuals between anthesis and maturity. The relation between plant nitrogen content and the capacity of the inflorescences to absorb nitrogen should be investigated.

Nitrogen losses from tops of plants.

Soil Nitrogen Budgets

Publisher Summary This chapter presents the estimates of nitrogen fixation by legume crops and discusses the methods of estimating fixation by crops. The pulses are sources of good-quality protein. The forages have a long-documented history of supporting livestock on poor soil. Both these factors are attributed in part to the ability of legumes, in symbiosis with rhizobia, to obtain nitrogen from the air. Legumes require more phosphate fertilizer than cereals, and many are more demanding of water. There is not a single legume crop for which valid estimates of the nitrogen fixed in agriculture is available. There are good estimates for soybean grown in representative locations in experimental plots. Dry beans are the important legume crop for human consumption in Latin America. There is no good evidence that any legume crop satisfies all its nitrogen requirements by fixation. The highest estimates are typical of low-fertility soil or soils artificially made N-poor by admixture or carbon amendment. The simplest estimate of nitrogen fixation is by total nitrogen accumulation of the crop. This is based on the intuitive assumption that the crop derives all its nitrogen via symbiotic fixation.

How Much Nitrogen do Legumes Fix

The stable isotope 15N was used to study the fate of nitrogen fertilizers applied to a black earth growing wheat. In a glasshouse experiment using soil packed at 56 % moisture (pF 2) into pots, added nitrate was almost completely recovered (mean, 98.8 ± 2.3 %) by soil and plant analysis. This experiment was repeated using 15N-labelled ammonium as well as nitrate; mean recoveries at 15 weeks were 96.8 and 97% respectively (± 2.2% at P < 0.05). A series of pots with soil wetter than pF 2 (at 63% moisture) was also included in an attempt to favour denitrification. Again, virtually complete recovery of added 15N was measured, 97 and 96.8% at 56 and 63% moisture respectively. Fallow and planted systems were then studied in a gas lysimeter. During experiments lasting up to 14 weeks, gaseous losses as 15N-labelled denitrification products were less than the equivalent of 0.2 µg nitrogen/g soil (the lower limit of detection with a mass spectrometer). Although analysis of plant and soil from the lysimeter in two experiments showed virtually quantitative recovery (99.7 and 97.05%), small (0.16%) losses of labelled ammonia into the lysimeter atmosphere were detected. The significance of these results is discussed in relation to the common finding of large deficits in nitrogen balance studies.

AE Martin

Papers

Isotopic studies of the nitrogen balance in a cracking clay. I. Recovery of added nitrogen from soil and wheat in the glasshouse and gas lysimeter