Over fertilizing the world

Three faces of global over fertilization from agriculture in China and the USA, and its complex effects on food webs.

1) Chinese farmers are acidifying there soil by over applying fertilizer.  Acidic soils impede crop growth and amplify the leaching of toxins.  Since the early 1980s, pH has declined from 0.2 to 0.8 across China, mostly due to overuse of fertilizer.  This is shown in a new Science paper, Significant Acidification in Major Chinese Croplands (DOI: 10.1126/science.1182570) by JH Guo and others.

Topsoil pH changes from 154 paired data over 35 sites in seven Chinese provinces between the 1980s and the 2000s. The line and square within the box represent the median and mean values of all data; the bottom and top edges of the box represent 25 and 75 percentiles of all data, respectively; and the bottom and top bars represent 5 and 95 percentiles, respectively. (From Guo et al)

Reporting on the paper Mara Hvistendahl writes, “Beginning in the 1970s, Chinese farmers applied ever-increasing amounts of fertilizer with the hope that it would lead to bigger harvests. Instead of high yield, however, they got water and air pollution. Today, agricultural experts estimate that in many parts of China fertilizer use can be slashed by up to 60%.”  In another issue of Science she also reports on current Chinese efforts to reduce fertilizer use.  In the Wall Street Journal, Geeta Annad reports on overfertilization in India “Pritam Singh, who farms 30 acres in Punjab, says the more desperate farmers become, the more urea they use. Overuse is stunting yields.”

2) The Washington Post reports on how in the US large feed lots are causing water quality problems in Manure becomes pollutant as its volume grows unmanageable

Animal manure, a byproduct as old as agriculture, has become an unlikely modern pollution problem, scientists and environmentalists say. The country simply has more dung than it can handle: Crowded together at a new breed of megafarms, livestock produce three times as much waste as people, more than can be recycled as fertilizer for nearby fields.

… Despite its impact, manure has not been as strictly regulated as more familiar pollution problems, like human sewage, acid rain or industrial waste. The Obama administration has made moves to change that but already has found itself facing off with farm interests, entangled in the contentious politics of poop.

3) Fertilization of ecosystems can have complex ecological consequences. In a paper in PNAS, John Davis and others show that in a Long-term nutrient enrichment decouples predator and prey production DOI: 10.1073/pnas.0908497107.

Relationship between primary consumer and predator secondary production for the reference stream (gray circles), the treatment stream (black circles), and previously published data (open circles). The arrows represent the temporal trajectory of the treatment stream starting with the 2 years of pretreatment (P1 and P2) and ending with the fifth year of enrichment (E5). The data labels correspond to the sampling year for the reference and treatment streams. The previously published data include 5 years of production data from the reference stream (C53) and a similar Coweeta stream (C55) that had experimentally reduced terrestrial leaf inputs during 4 of those years (21). It also includes previously published data from an unmanipulated year that compared our current reference (C53) and treatment (C54) streams (22). AFDM is ash-free dry mass.

Their research showed that there were differences in how predators and prey responded to fertilization, but these only emerged over time.  Increases N and P entering a stream increased populations of both predators and prey, however later on prey populations continued to increase but predator populations declined,because fertilzation shifted the streams prey to larger, predator resistant species, which reduced the efficiency with which energy flowed through the food web.

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