1 . If the grass looks greener after the skies have cleared, your eyes aren’t cheating you. “There are several reasons rain helps lawns green up,” said Jennifer Knoepp, a soil research scientist with the USDA Forest Service in Otto, North Carolina.
After it rains, there’s typically more water available in the soil for plants. When plants take up that water, they are also taking up nitrogen (氮) from the organic matter that’s in the soil. “As plants grow, their small roots die and new roots grow,” Knoepp said. When that happens, soil microbes (微生物) cause the dead roots to rot. This process is similar to adding fertilizers to your lawn, but only this action takes place underground and naturally, without your awareness. As rain runs into the soil, it activates the microbes to give off more nitrogen. The grass benefits from the freshly fallen rain because the water allows the roots to take up this “new” nitrogen as well as the nitrogen that the microbes have previously produced. At the same time, “the grass is very active with photosynthesis (光合作用)” when the sun returns, Knoepp explained.
Even though rain increases the nitrogen that’s available to your lawn in several ways, you can’t count on nitrogen from rain to meet all the fertilizer needs of your grass or your vegetable garden. Commercial fertilizers are still needed for a balanced fertilizer program, but Knoepp urges caution in applying them. While nitrogen is vital for good plant growth, be sure to follow package directions. Too much of a good thing can be harmful not only to plants but to nearby ponds, lakes, streams and rivers. After all, it’s not green streams but green lawns that homeowners want to see when the clouds depart and the sun returns.
1. What makes the grass look greener after the rain?| A．The cleanup of the rain. | B．Slower photosynthesis. |
| C．The appearance of new roots. | D．Increased absorption of nitrogen. |
| A．The production of nitrogen. | B．The process of plants’ growth. |
| C．The effects of photosynthesis. | D．The method of natural fertilizing. |
| A．Be careful not to overuse fertilizers. | B．Keep a commercial fertilizer program. |
| C．Collect as much rainwater as possible. | D．Take advantage of nitrogen from rain only. |
| A．Skeptical and critical. | B．Cautious and confident. |
| C．Objective and humorous. | D．Scientific and conservative. |
2 . Plenty of harvests of corn and other major crops rely on a mysterious phenomenon known as hybrid vigor (杂交活力). When highly naturally-born varieties are crossed, their next generations are taller, hardier, and bear more grain, Researchers report that this vigor is somehow influenced by microbes (微生物) in the soil, perhaps through a plant's immune system .
Charles Darwin was one of the first researchers to describe hybrid vigor. In the early 20th century, biologists began to apply this effect to agriculture by creating naturally-born parent plants that produced hybrid seeds. By the 1940s, almost every farmer in the United States was planting hybrid corn, and the harvests multiplied.
Biologists have proposed several theories about the cause of hybrid vigor, but no definitive explanation has emerged.
Maggie Wagner, plant biologist at the University of Kansas, and her colleagues wondered whether microbes might be it involved. Last year, Wagner and her colleagues found an interesting clue in a field study. They discovered that the leaves and roots of hybrid corn had microbial communities that are different from those living on naturally-born varieties of corn. “Something about being a hybrid makes a plant interact differently with microbes,” Wagner says. It could be that the naturally-born corn's immune systems react more actively to beneficial microbes, compromising their growth. Alternatively, hybrid plants may be better able to defend against weak pathogens (病原体) in the soil.
Wagner says the finding highlights the need for plant growers to match the hybrid crops to the microbial communities with which they live. The findings help scientists realize the importance of understanding the role of soil microbes in making agriculture more productive and sustainable, “This holds great promise.”
1. How did people use the effect of hybrid vigor in agriculture?| A．By producing naturally-born parent plants that bore hybrid seeds. |
| B．By creating naturally-born parent lines that produced hybrid vigor. |
| C．By providing more microbes communities that could produce more seeds. |
| D．By looking for hybrid parent plants in the field that produced hybrid seeds. |
| A．Hybrid plants react worse to weak pathogens in the soil. |
| B．Wagner connected hybrid vigor with microbial communities. |
| C．Being naturally-born makes a plant interact differently with microbes. |
| D．Hybrid corn and naturally-born corn have similar microbial communities. |
| A．Making farmers know what to grow to resist certain diseases |
| B．Helping scientist realize the importance of spreading soil microbes. |
| C．Showing the significance of associating soil microbes with specific crops. |
| D．Highlighting the need for plant growers to find more microbial communities. |
| A．Hybrid corn: a double-edged sword |
| B．How can farmers grow hybrid crops? |
| C．Hybrid vigor: a mysterious phenomenon |
| D．What role do soil microbes play in the harvests of corn? |
3 . Take a road trip from Wisconsin up through Northern Minnesota. Look out your window and watch as maple-oak forests gradually give way to plant species like aspens, spruce and birch. This changing view occurs because Northern Minnesota sits between temperate(温带的) forests in the southern US and boreal forests(北方针叶林) to the north. It makes the region an ideal place to track the impact of a warming climate.
Scientists have guessed that, as global temperatures rise, plant species of both temperate and boreal forests will move northwards to escape warmer conditions. To test this guess, researchers have been artificially heating 18 small forest plots in the transition zone of Northern Minnesota ever since 2012. Using covers and heaters, the team increased air and soil temperatures. The long-term experiment is now offering valuable insights into how our changing climate will affect the structure and ecosystems of boreal and temperate forests.
The team found that boreal species, such as balsam fir and white spruce, are less able to emerge and survive under warmer conditions. They measured the boreal forests species’ amount of photosynthesis(光合作用) taking place. Their results show that, as soils dry up in warmer conditions, boreal species reduce their photosynthetic rate, reducing the amount of organic matter available for growth. Overall, the seedling species richness in the temperate-boreal forest transition was reduced by warming. The researchers suggest that, unless the temperate species can offset the loss of the boreal species, the overall biodiversity of these forests is likely to decline.
This research is important because temperate and boreal forests contain46 percent of all trees on earth. Of the global carbon stock stored by forests, boreal forests contain 32 percent, temperate forests 14 percent. In the coming years, boreal forests will experience extreme increases in air and soil temperatures. An increase of 1.5°C has already been measured, with a rise of between 4°C and 6°C projected in the next century.
1. What made researchers choose Northern Minnesota to perform their experiment?| A．Its rich plant species. | B．Its easy accessibility. |
| C．Its special location. | D．Its warm climate. |
| A．The causes of decreasing forest areas. |
| B．The influence of human activities on climate change. |
| C．The changes of forests driven by rising temperatures. |
| D．The decline in the biodiversity of temperate and boreal forests. |
| A．Lead to. | B．Slow down. |
| C．Add up to. | D．Make up for. |
| A．absorb less carbon dioxide per square meter |
| B．face a high risk of being hit harder in the future |
| C．play an important role in fighting climate change |
| D．have drawn public attention to environmental protection |
