J. R. Freney

Transformation of the Nitrogen Cycle: Recent Trends, Questions, and Potential Solutions

Humans continue to transform the global nitrogen cycle at a record pace, reflecting an increased combustion of fossil fuels, growing demand for nitrogen in agriculture and industry, and pervasive inefficiencies in its use. Much anthropogenic nitrogen is lost to air, water, and land to cause a cascade of environmental and human health problems. Simultaneously, food production in some parts of the world is nitrogen-deficient, highlighting inequities in the distribution of nitrogen-containing fertilizers. Optimizing the need for a key human resource while minimizing its negative consequences requires an integrated interdisciplinary approach and the development of strategies to decrease nitrogen-containing waste.

Regional nitrogen budgets and riverine N & P fluxes for the drainages to the North Atlantic Ocean: Natural and human influences

We present estimates of total nitrogen and total phosphorus fluxes in rivers to the North Atlantic Ocean from 14 regions in North America, South America, Europe, and Africa which collectively comprise the drainage basins to the North Atlantic. The Amazon basin dominates the overall phosphorus flux and has the highest phosphorus flux per area. The total nitrogen flux from the Amazon is also large, contributing 3.3 Tg yr-1 out of a total for the entire North Atlantic region of 13.1 Tg yr-1. On a per area basis, however, the largest nitrogen fluxes are found in the highly disturbed watersheds around the North Sea, in northwestern Europe, and in the northeastern U.S., all of which have riverine nitrogen fluxes greater than 1,000 kg N km-2 yr-1.

Global estimates of potential mitigation of greenhouse gas emissions by agriculture

Technologies to reduce net emissions of carbon dioxide, methane and nitrous oxide within the agriculture sector were reviewed to estimate the global potential for mitigation of these radiatively active greenhouse gases. Our estimates of the potential reduction of radiative forcing by the agricultural sector range from 1.15-3.3 Gt C equivalents per year. Of the total potential reduction, approximately 32% could result from reduction in CO2 emissions, 42% of carbon offsets by biofuel production on 15% of existing croplands, 16% from reduced CH4 emissions and 10% from reduced emissions of N2O. Agriculture encompasses large regional differences in management practices and rates of potential adoption of mitigation practices. Acceptability of mitigation options will depend on the extent to which sustainable production will be achieved or maintained and benefits will accrue to farmers. Technologies such as no-till farming and strategic fertilizer placement and timing are now being adopted for reasons other than concern for climate change issues.

Assessing and Mitigating N2O Emissions from Agricultural Soils

Agricultural cropping and animal production systems are important sources of atmospheric nitrous oxide (N2O). The assessment of the importance of N fertilization from synthetic fertilizer, animal wastes used as fertilizers and from N incorporated into the soil through biological N fixation, to global N2O emissions presented in this paper suggests that this source has been underestimated. We estimate that agricultural systems produce about one fourth of global N2O emissions. Methods of mitigating these emissions are presented which, if adopted globally could decrease annual N2O emissions from cropped soils by about 20%.

Prospects of improving efficiency of fertiliser nitrogen in Australian agriculture: a review of enhanced efficiency fertilisers

Fertiliser nitrogen use in Australia has increased from 35 Gg N in 1961 to 972 Gg N in 2002, and most of the nitrogen is used for growing cereals. However, the nitrogen is not used efficiently, and wheat plants, for example, assimilated only 41% of the nitrogen applied. This review confirms that the efficiency of fertiliser nitrogen can be improved through management practices which increase the crops ability to compete with loss processes. However, the results of the review suggest that management practices alone will not prevent all losses (e.g. by denitrification), and it may be necessary to use enhanced efficiency fertilisers, such as controlled release products, and urease and nitrification inhibitors, to obtain a marked improvement in efficiency. Some of these products (e.g. nitrification inhibitors) when used in Australian agriculture have increased yield or reduced nitrogen loss in irrigated wheat, maize and cotton, and flooded rice, but most of the information concerning the use of enhanced efficiency fertilisers to reduce nitrogen loss to the environment has come from other countries. The potential role of enhanced efficiency fertilisers to increase yield in the various agricultural industries and prevent contamination of the environment in Australia is discussed.

Ammonia flux into the atmosphere from a grazed pasture.

A micrometeorological technique has been used to measure the flux of ammonia and related gaseous nitrogen compounds into the atmosphere from a pasture grazed by sheep. During 3 weeks in late summer, the average daily flux density of nitrogen in these forms was 0.26 kilogram per hectare. This is a substantial part of the nitrogen turnover in grazed pastures.

A sampler for measuring atmospheric ammonia flux

Abstract This paper describes the design, construction and testing of a simple sampling device for determining ammonia fluxes in the atmosphere. Sampler performance was predicted theoretically and tested in a wind tunnel, the laboratory and in the field. The results showed that air flowed through the sampler at a rate linearly proportional to the external wind speed and that the ammonia it contained was absorbed quantitatively by the sampler. The mass of ammonia, M , collected by the instrument during a sampling period, t , is thus proportional to the mean convective flux density of ammonia, ( uϱ N ) since M = ( uϱ N )t , where u is the wind velocity, ϱ N the ammonia density, A is the effective cross-sectional area of the sampler, and the overbar represents a time-mean.

Soil organic matter fractions as sources of plant-available sulphur

Abstract Pot culture experiments are described which attempt to identify the organic fractions of soil sulphur that decompose during the growing season to provide plant-available sulphur. Soil organic matter was labelled with 35S by incubating soil with labelled sulphate and several organic sulphur fractions were determined before and after the growth of Sorghum vulgare and following a fallow treatment. The effect of moisture stress on the uptake of sulphur by plants was also investigated; this treatment had little effect. Changes occurred in both hydriodic acid-reducible and carbon-bonded sulphur fractions, 60 per cent of the sulphur taken up by the plants being derived from the latter. The changes in carbon-bonded sulphur involved changes in both Raney nickel-reducible and non-reducible sulphur fractions. All of the sulphur fractions investigated thus contributed available sulphur for plant uptake, and none of them are likely to be of any value for predicting the sulphur requirements of plants.

Policy implications of human-accelerated nitrogen cycling

The human induced input of reactive N into the globalbiosphere has increased to approximately 150 Tg N eachyear and is expected to continue to increase for theforeseeable future. The need to feed (∼125 Tg N) andto provide energy (∼25 Tg N) for the growing worldpopulation drives this trend. This increase inreactive N comes at, in some instances, significantcosts to society through increased emissions of NOx,NH3, N2O and NO3− and deposition of NOy and NHx.In the atmosphere, increases in tropospheric ozone andacid deposition (NOy and NHx) have led toacidification of aquatic and soil systems and toreductions in forest and crop system production. Changes in aquatic systems as a result of nitrateleaching have led to decreased drinking water quality,eutrophication, hypoxia and decreases in aquatic plantdiversity, for example. On the other hand, increaseddeposition of biologically available N may haveincreased forest biomass production and may havecontributed to increased storage of atmospheric CO2 inplant and soils. Most importantly, syntheticproduction of fertilizer N has contributed greatly tothe remarkable increase in food production that hastaken place during the past 50 years.The development of policy to control unwanted reactiveN release is difficult because much of the reactive Nrelease is related to food and energy production andreactive N species can be transported great distancesin the atmosphere and in aquatic systems. There aremany possibilities for limiting reactive N emissionsfrom fuel combustion, and in fact, great strides havebeen made during the past decades. Reducing theintroduction of new reactive N and in curtailing themovement of this N in food production is even moredifficult. The particular problem comes from the factthat most of the N that is introduced into the globalfood production system is not converted into usableproduct, but rather reenters the biosphere as asurplus. Global policy on N in agriculture isdifficult because many countries need to increase foodproduction to raise nutritional levels or to keep upwith population growth, which may require increaseduse of N fertilizers. Although N cycling occurs atregional and global scales, policies are implementedand enforced at the national or provincial/statelevels. Multinational efforts to control N loss tothe environment are surely needed, but these effortswill require commitments from individual countries andthe policy-makers within those countries.

Emission of nitrous oxide from soils used for agriculture

Nitrous oxide is emitted into the atmosphere as a result of biomass burning, and biological processes in soils. Biomass burning is not only an instantaneous source of nitrous oxide, but it results in a longer term enhancement of the biogenic production of this gas. Measurements of nitrous oxide emissions from soils before and after a controlled burn showed that significantly more nitrous oxide was exhaled after the burn. The current belief is that 90% of the emissions come from soils. Nitrous oxide is formed in soils during the microbiological processes nitrification and denitrification. Because nitrous oxide is a gas it can escape from soil during these transformations. Nitrous oxide production is controlled by temperature, pH, water holding capacity of the soil, irrigation practices, fertilizer rate, tillage practice, soil type, oxygen concentration, availability of carbon, vegetation, land use practices and use of chemicals. Nitrous oxide emissions from agricultural soils are increased by the addition of fertilizer nitrogen and by the growth of legumes to fix atmospheric nitrogen. A recent analysis suggests that emissions of nitrous oxide from fertilized soils are not related to the type of fertilizer nitrogen applied and emissions can be calculated from the amount of nitrogen applied. Legumes also contribute to nitrous oxide emission in a number of ways, viz. atmospheric nitrogen fixed by legumes can be nitrified and denitrified in the same way as fertilizer nitrogen, thus providing a source of nitrous oxide, and symbiotically living Rhizobia in root nodules are able to denitrify and produce nitrous oxide. Conversion of tropical forests to crop production and pasture has a significant effect on the emission of nitrous oxide. Emissions of nitrous oxide increased by about a factor of two when a forest in central Brazil was clear cut, and pasture soils in the same area produced three times as much nitrous oxide as adjacent forest soils. Studies on temperate and tropical rice fields show that less than 0.1% of the applied nitrogen is emitted as nitrous oxide if the soils are flooded for a number of days before fertilizer application. However, if mineral nitrogen is present in the soil before flooding it will serve as a source of nitrous oxide during wetting and drying cycles before permanent flooding. Thus dry seeded rice can be a source of considerable nitrous oxide. There are also indirect contributions to nitrous oxide emission through volatilization of ammonia and emission of nitric oxides into the atmosphere, and their redistribution over the landscape through wet and dry deposition. In general nitrous oxide emissions can be decreased by management practices which optimize the crops natural ability to compete with processes whereby plant available nitrogen is lost from the soil-plant system. If these options were implemented they would also result in increased productivity and reduced inputs.