Thus, the amount of mined P that is included as an input in our budget is Because global weathering estimates vary widely due to differences in methodology or difficulties scaling up from local studies to arrive at a global estimate, we use a range of values in our budget Gardner , Newman For the minimal estimate of current weathering rates, we average the published values for global weathering noted above for an estimate of 15 Tg of P released per year.
Graham and Duce found that 3. Fluvial P flux is calculated as the total of dissolved and particulate riverine P flux. An extensive literature review led Howarth et al. Inputs of P to terrestrial soils and freshwater ecosystems in the current budget are between Approximately 23 Tg of P is output annually.
Total accumulation of P in surface soil and freshwaters in the modern budget was between A little over one-quarter of the P input is stored in upland soils and freshwaters, according to this budget. Mining input was negligible. As a maximal estimate, we assume that the preindustrial and current weathering rates are identical and we average the published values for global weathering for an estimate of 15 Tg P released per year.
In the preindustrial budget, P outputs were net atmospheric output and riverine flux to the oceans. The lower estimate for suspended sediment P concentration compared to current estimates is reasonable because it is likely that agricultural fertilization has increased the P concentration of eroding material Avnimelech and McHenry Preindustrial accumulation was probably variable in time and space, depending on factors such as glaciation and age of the soil. The excess P accumulation in the modern budget compared to the preindustrial budget is 4.
Our budgets address both terrestrial soils and freshwater ecosystems. While most of the excess P is probably accumulating in upland soils, some of the excess P may be accumulating in freshwater sediments.
We calculate the amount of P accumulating in freshwater sediments to have been between 1 and 1. Based on global freshwater area of Behrendt estimated that for the global average hydrologic output 0. This suggests P retention of 1. Phosphorus that is accumulating in freshwaters can be resuspended or mobilized to contribute to downstream eutrophication. Therefore, this P is included in our estimates of P accumulation in terrestrial and freshwater sediments globally.
We also calculated a global agricultural P budget to determine the amount of P accumulation that occurs in agricultural areas.
This budget included only agricultural inputs fertilizer and manure and outputs agricultural products such as meat and eggs, and runoff. Fertilizer inputs were calculated based on global estimates of fertilizer use and P content of fertilizer FAO — Manure inputs were calculated as in the current global budget. Outputs were calculated based on agricultural production data FAO — and the percentage of P of these products Pierrou The results presented in Figure 3 are budgets calculated for — at 5-year intervals.
This figure lies within the range of excess P accumulation in the modern budget as compared with the preindustrial one.
P accretion occurs in both developed and developing nations, but these areas show different patterns of P accumulation over time Figure 4.
For developing nations, P removal in crop yield was greater than P input and there was a slight depletion of P in soils in ; by , however, inputs greatly exceeded outputs. Of the 8 Tg of P accumulating in agricultural soils worldwide, approximately 5 Tg are accumulating in the agricultural lands of developing countries. Clearly, P is accumulating in Earth's surface soils, primarily in agricultural areas and at a faster rate than before large-scale mining for P began.
There is also greater throughput of P in the current budget than in the preindustrial estimate. Moreover, P accumulation caused by excess fertilizer may be qualitatively different from an increase in P stock due to weathering: Fertilizer P input changes both the mass and the concentration of P in soil, whereas an increase in weathering changes only the total P mass because it adds both P and other soil constituents.
The impact on aquatic ecosystems is therefore likely to be different as well, because the higher P concentration from fertilizers increases the flow of P per mass of soil transported to freshwater. Phosphorus accumulation is no longer a problem just in developed countries; it appears to be of increasing importance in developing nations as well. Human-caused changes in the global P budget have caused P to accumulate in upland soils, and greater global accretion of P in soil may lead to the heightened severity and prevalence of culturally eutrophic waters.
P is lost from soil in particulate and dissolved forms. Particulate losses, which are the dominant form of loss, occur during erosion and runoff events Sharpley et al. Dissolved losses can be significant in some soils, especially if the iron Fe , aluminum Al , and calcium Ca absorption capacity of the soil is saturated, allowing P to move more readily through the soil toward aquatic ecosystems Sims et al.
Although the long-term fate of P that accumulates in soils is uncertain Cassell et al. Of particular concern is that large amounts of soil P can be mobilized by exceptional precipitation and erosion events or by changes in land management practices, such as the conversion of agricultural land to residential development Daniel et al. Because it originates from dispersed sources and varies widely with environmental conditions, nonpoint source pollution is difficult to measure and regulate.
Policies and regulations have tended to approach P runoff to aquatic ecosystems and eutrophication as a problem of the particular lake, river reach, or estuary in question, rather than as part of a larger pattern.
Understanding the global ecological patterns behind eutrophication can affect local decisions and stimulate discussion of large-scale approaches to management. There are two basic approaches to decreasing the impact of soil P accretion on aquatic ecosystems. We can attempt to bring the P budget into balance by reducing P inputs to soil controlling sources , and we can try to reduce the transport of P from soils to aquatic ecosystems increasing sinks.
At the same time, it will be important to reduce P concentrations in soils already overenriched with P because of past budget imbalances. Drawdown of soil P could take decades or longer in many areas McCollum , Stigliani et al. Because of the increase in soil P concentrations, the risk of eutrophication will be elevated for a long while Cassell et al. Over this time period, changes in farm practices, urban expansion, or climate change could accelerate erosion, thus increasing the rate of transport of P from the soil into aquatic ecosystems.
By the time water resources are noticeably impaired, P accretion in terrestrial soils, upstream sediments, rivers, or lakes may already be great enough to maintain high loading to lowland aquatic systems for extended periods of time. Although there are few data on the long-term fate of P that accumulates in fertilized soils, the slow response times of Fe—P, Al—P, and Ca—P pools may reduce options for later management. When dealing with slowly changing variables such as soil P concentrations, mitigation takes a long time and aggregate costs can be large Pizer By controlling soil P accretion now, we may be able to avoid the costs of eutrophication in the future and create flexibility for coping with freshwater problems that could arise.
Delivery of P to receiving waters can be reduced not only by reducing P inputs to soils but also by increasing P sinks in watersheds. Among such sinks are riparian buffers and wetland areas, detention basins, and conservation agriculture practices NRC , Novotny and Olem , Soranno et al. However, riparian and wetland buffers have a limited capacity to retain P Richardson and Qian Some areas are at higher risk for increased sediment delivery rates and severity of eutrophication, and these will demand particular management attention.
Urban and suburban development—indeed, construction projects in general—expedite erosion of P-enriched soil into aquatic ecosystems Novotny and Olem Thus, water quality in areas where human population growth is rapid is likely to decline because of eutrophication.
Growing human populations make heavy demands on water supply and freshwater resources, yet diminish these services by increasing eutrophication. Thus, in rapidly urbanizing or suburbanizing areas, particular attention may need to be directed to reducing sediment delivery, drawing down soil P, and balancing the P budgets of surrounding agricultural areas.
Bringing the P budget of agricultural areas into balance by reducing fertilizer use would reduce P accumulation in agricultural soils, but doing so may diminish agricultural output. Global demand for food is predicted to increase as the human population continues to grow Daily et al.
Increases in food production will most likely derive from increased yields from more efficient water use on land already in production; increasing production may also require triple the amount of nitrogen and P now in use Daily et al. Some modifications in agricultural practices may allow a reduction in P use without sacrificing food production Frink et al. For example, manure and sewage P might be recycled more efficiently, fertilizer might be targeted to meet plant needs at specific times in the crop cycle, and changes in animal production systems might be made Matson et al.
Experience in developed countries suggests that the rate of P accumulation can be decreased even as crop yields increase Frink et al.
Methods of agricultural production have developed in response to society's demand for inexpensive, plentiful food Lanyon and Thompson Pressured to meet society's need for cheap food without going out of business, farmers have had to make decisions that have led to specialization and intensification of agricultural production systems. At the same time, society has taken for granted a continual supply of cheap, clean water.
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Secondary Production. Food Web: Concept and Applications. Terrestrial Primary Production: Fuel for Life. Citation: Chislock, M. Nature Education Knowledge 4 4 Eutrophication is a leading cause of impairment of many freshwater and coastal marine ecosystems in the world. Why should we worry about eutrophication and how is this problem managed?
Aa Aa Aa. The rise in intensive fertilizer use has serious implications for coastal habitats because greater application results in greater runoff, and the fraction of fertilizer lost from fields will increase with intensity of application. Increased global production of nitrogenous fertilizers have largely been linked to concerns over the relationship between water quality and eutrophication.
Nutrient removal in sewage treatment plants and promotion of phosphorus-free detergents are vital to minimize the impact of nitrogen and phosphorus pollution in Europe's waters [3].
The emissions of N and P to the coastal ocean have doubled during the past century. More than half of these emissions are related to agriculture [4]. About one quarter of this production finds its way mainly through river runoff and atmospheric deposition to the coastal ocean [5] , which receives an annual load of Tg N [4]. The annual global anthropogenic production of mineral phosphorus P in is estimated at about 20 Tg P [8] , almost entirely based on mining of phosphate rock.
When the use of livestock slurry and manure is included, the total phosphorus application in agriculture amounts to 25—29 Tg P per year [6]. It is estimated that about 10 Tg P is flushed to the coastal ocean [9]. Part of this emission is the gradual release of the historical phosphate enrichment of soil and sediments from human activities over more than a century [4]. Because phosphorus is hardly transported by air only as aerosol , anthropogenic P is more strongly concentrated near the land-sea interface than anthropogenic N.
Public awareness and political priority are overarching requirements for tackling the eutrophication problem [11]. Eutrophication is largely related to agriculture; point sources of nutrients play a minor role and can more easily be tackled [4] [5]. Dedicated fertilization strategies taking into account the N:P balance and agricultural practices such as crop rotation, drip irrigation, etc. Nutrient leaching from agriculture can be reduced by recycling animal manure to cropland within watersheds.
Individual behavior with regard to nutrition plays an important role; in large parts of the world, there is overconsumption of protein-rich food. Of the Tg N used for crop and grass production, about Tg is consumed by livestock while only 20 Tg is available for direct human consumption [13].
Preservation and restoration of landscapes, especially at the land-water interfaces, promotes denitrification processes and contributes to reducing the nutrient discharge to coastal waters [5].
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