From VOA Learning English, this is the Agriculture Report.

这里是美国之音慢速英语农业报道。

An area known as a dead zone develops every spring in the Gulf of Mexico near the mouth of the Mississippi River. It can spread as much as 13,600 square kilometers, extending all the way to the eastern Texas coast. Scientists know what causes the dead zone -- too much nitrogen, but the solution might be hard to accept.

墨西哥湾附近的密西西比河河口每年春天都会变为死水区。它能扩散到1.36万平方公里，一路延伸到德克萨斯州东部海岸。科学家知道导致死水区是因为含氮量太高，但解决方案可能难以被接受。

Bayani Cardenas is a professor of water studies at the University of Texas at Austin. He says the movement of rivers usually removes material like nitrates. So he wondered why that natural filtration process does not remove nitrates from the Mississippi River.

巴雅尼·卡德纳斯(Bayani Cardenas)是得克萨斯大学奥斯汀分校一位水资源研究的教授。他说，河流的流动通常会去除硝酸盐之类的物质。所以他想弄明白，为什么自然过滤过程未能去除密西西比河的硝酸盐。

Professor Cardenas says his recent study shows that more than 99 percent of the river's water does pass through the river's sediment which is on the shore or bottom of the river. But he says the study found that there is so much nitrogen in the river system that is simply can not filter all of it.

卡德纳斯教授表示，他最近的一项研究显示，99%的河流水确实会穿过河岸边或河底的沉积物。但他表示，这项研究发现，河流系统中的含氮量如此之高，根本无法被完全过滤掉。

The water that has nitrogen still in it supports the growth of algae. As the algae dies, it sinks to the bottom of the river where it breaks down or decomposes. It then takes oxygen from the water. This condition is called hypoxia and it is deadly to fish and shrimp.

仍然富含氮的水有利于藻类的生长。随着藻类死亡，它会下沉到河底并分解，然后会带走水中的氧气。这种情况被称为缺氧，它对鱼虾来说是致命的。

Where does all this nitrogen come from? The answer is farms. The Mississippi River system carries water from 33 American states and part of Canada to the gulf of Mexico. Along the way nitrogen and other chemicals used in farming enter the system. Farmers say these chemicals must be used to produce enough food for a growing world population.

但是氮从何来呢？答案就是农场。密西西比河水系将美国33个州以及加拿大部分地区的水带到墨西哥湾。一路上农场经营使用的氮和其它化学物质进入了该水系。农民们表示，必须使用这些化学品，以生产足够多的食物来满足不断增长的世界人口。

Aaron Packman is a professor of Civil and Environmental Engineering at Northwestern University. He says farmers may be able to better control the amount of nitrate fertilizer they put on fields.

亚伦·帕克曼（Aaron Packman）是美国西北大学土木与环境工程学教授。他说，农民们也许能够更好地控制他们投放到田地里的氮肥总量。

"How much fertilizer do you need to give you good yields and then how much is maybe a marginal gain from adding lots more fertilizer? There is really a question here: can you maybe [reduce the amount] and get close to the same level of yield without having such a negative impact?" said Packman.

帕克曼：“多少氮肥才能让你获得良好收益？增加更多肥料的边际收益可能有多少？问题就在于：是否可以在不产生如此负面影响的情况下减少施肥总量并获得近似收益？”

The water in the Mississippi River system should be able to clean itself naturally as it flows down stream. But flood controls and other human-made structures have hurt this filtering process.

密西西比河水系的水在流向下游的过程中本来可以很好地通过自然方式自我清理。但防洪和人造建筑伤害了这个过滤过程。

Storms in the next few month will mix the gulf water and the dead zone will disappear, but it will return next year. And scientists say it will grow larger in years to come, if something is not done to reduce the amount of nitrogen in the Mississippi River.

未来数月的暴风雨将会搅动墨西哥湾的水，死水区将会消失，但明年又会重现。科学家表示，如果不采取措施减少密西西比河的含氮量，未来几年死水区会愈发扩大。

And that's the VOA Learning English Agriculture Report. For more agriculture stories, go to our website 51voa.com. I'm Caty Weaver.