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　　Perennial Solution

　　Annual grains feed the world, but they create problems. Perennials are thrifty. Their long roots hold on to soil, water, and fertilizer, which means less pollution.

　　By Robert Kunzig

　　Photograph by Rebecca Hale, NGM Staff

　　Humans made an unwitting but fateful choice 10,000 years ago as we started cultivating wild plants: We chose annuals. All the grains that feed billions of people today—wheat, rice, corn, and so on—come from annual plants, which sprout from seeds, produce new seeds, and die every year. "The whole world is mostly perennials," says USDA geneticist Edward Buckler, who studies corn at Cornell University. "So why did we domesticate annuals?" Not because annuals were better, he says, but because Neolithic farmers rapidly made them better—enlarging their seeds, for instance, by replanting the ones from thriving plants, year after year. Perennials didn't benefit from that kind of selective breeding, because they don't need to be replanted. Their natural advantage became a handicap. They became the road not taken.

　　Today an enthusiastic band of scientists has gone back to that fork in the road: They're trying to breed perennial wheat, rice, and other grains. Wes Jackson, co-founder and president of the Land Institute in Salina, Kansas, has promoted the idea for decades. It has never had much money behind it. But plant breeders in Salina and elsewhere are now crossing modern grains with wild perennial relatives; they're also trying to domesticate the wild plants directly. Either way the goal is crops that would tap the main advantage of perennials—the deep, dense root systems that fuel the plants' rebirth each spring and that make them so resilient and resource efficient—without sacrificing too much of the grain yield that millennia of selection have bred into annuals.

　　We pay a steep price for our reliance on high yields and shallow roots, says soil scientist—and National Geographic emerging explorer—Jerry Glover of the Land Institute. Because annual root crops mostly tap into only the top foot or so of soil, that layer gets depleted, forcing farmers to rely on large amounts of fertilizers to maintain high yields. Often less than half the fertilizer in the Midwest gets taken up by crops; much of it washes into the Gulf of Mexico, where it fertilizes algae blooms that cause a vast dead zone around the mouth of the Mississippi. Annuals also promote heavy use of pesticides or tillage because they leave the ground bare much of the year. That allows weeds to invade.

　　Above all, leaving the ground bare after harvest and plowing it in planting season erodes the soil. No-till farming and other conservation practices have reduced the rate of soil loss in the U.S. by more than 40 percent since the 1980s, but it's still around 1.7 billion tons a year. Worldwide, one estimate put the rate of soil erosion from plowed fields at ten to a hundred times the rate of soil production. "Unless this disease is checked, the human race will wilt like any other crop," Jackson wrote 30 years ago. As growing populations force farmers in poor countries onto steeper, erodible slopes, the "disease" threatens to get worse.

　　Perennial grains would help with all these problems. They would keep the ground covered, reducing erosion and the need for pesticides, and their deep roots would stabilize the soil and make the grains more suitable for marginal lands. "Perennials capture water and nutrients 10 or 12 feet down in the soil, 11 months of the year," Glover says. The deep roots and ground cover would also hold on to fertilizer—reducing the cost to the farmer as well as to the environment.

　　The perennial wheat-wheatgrass hybrid now growing at the Land Institute can already be made into flour. Yields are too low to compete with annual wheat in Kansas—but maybe not in Nepal, which has steeper slopes and a harsher climate, and where a researcher is now testing perennial hybrids in small plots. Amber waves of perennial grain may be decades away, but the emergence of cheap DNA sequencing is allowing plant breeders to work much faster than they used to. Buckler thinks that for a tiny fraction of the billions spent annually on corn research, one could create field-testable perennial corn in as little as ten years. "I think we should take a shot at revolutionizing agriculture," he says.

　　第五篇科学类文章，人们常年以来一直用的是单年生的农作物来种庄稼，现在科学家想要发展多年生植物。这个话题在去年也出现过很多次，关于粮食安全和农业的话题，也是现在科学类文章当中会经常出现的话题。

　　阅读题目分析：

　　阅读的题目部分，加强了对主旨题的考查，尤其是全文的主旨题，几乎每一篇都出现了这篇文章的 primary purpose、main focus等题型，所以文章和段落的主旨仍然是一个重点。

　　非联动题的寻证题也出现了，也就是给好了结论让寻找证据，这就意味着文章当中多出了没有行数范围的细节题，所以一些细节题和推理题需要没有行数提示的情况下自己寻找，对文章的结构要求比较高。

　　词汇题非常典型，没有出现很难的单词，基本都是简单但是有多个含义的单词，要求找出单词在原文当中的意思。考到的词汇有：style(动词) 、general、steep、bulk、natural。

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