The nitrogen cycle diagram has one stage left blank. In a soybean field, nitrogen gas taken from the air is converted inside the root nodules into ammonium that the plant can absorb. Which step belongs in the blank?

The nitrogen cycle diagram has one stage left blank. In warm, well-aerated soil, ammonium is converted first into nitrite and then into nitrate, the form most plants take up. Which step belongs in the blank?

The nitrogen cycle diagram has one stage left blank. In a flooded rice paddy where the soil holds almost no oxygen, nitrate is converted back into nitrogen gas that escapes into the atmosphere. Which step belongs in the blank?

The nitrogen cycle diagram has one stage left blank. On a forest floor, fallen leaves and animal waste are broken down and the nitrogen locked up in their proteins is released into the soil. Which step belongs in the blank?

The nitrogen cycle diagram has one stage left blank. A corn plant's roots take up nitrate dissolved in soil water, and the plant uses that nitrogen to build amino acids, proteins and DNA. Which step belongs in the blank?

A fertilizer factory combines nitrogen gas drawn from the air with hydrogen under high pressure and temperature to make ammonia. Which natural step of the nitrogen cycle is the factory carrying out industrially?

The nitrogen cycle diagram has one stage left blank. Nitrogen gas makes up most of the atmosphere, but plants cannot use nitrogen in that form. What makes atmospheric nitrogen available to plants in most soils?

The nitrogen cycle diagram has one stage left blank. In well-aerated soil, ammonium is steadily converted into nitrite and then into nitrate. Which organisms carry out this conversion?

The nitrogen cycle diagram has one stage left blank. A field stays waterlogged for two weeks and the soil runs out of oxygen. Which organisms then convert the soil's nitrate back into nitrogen gas?

In a forest, the nitrogen held in fallen leaves and animal waste is returned to the soil as ammonium. Which organisms do this?

The water cycle diagram has one stage left blank. On a warm day, liquid water in a leaf's air spaces leaves the leaf as vapor through the stomata on its underside. Which process belongs in the blank?

The water cycle diagram has one stage left blank. Rising water vapor cools as it climbs, and the vapor collects on tiny dust particles as liquid droplets that build up into a cloud. Which process belongs in the blank?

The water cycle diagram has one stage left blank. After rain stops, water sinks from the surface down through the spaces in the soil and porous rock until it reaches the saturated zone below. Which process belongs in the blank?

The water cycle diagram has one stage left blank. Sunlight warms the surface of a lake, and liquid water at the surface gains enough energy to become water vapor in the air above it. Which process belongs in the blank?

The soil of a hillside pasture is already saturated when a storm arrives, so the new rain flows across the ground surface and down into a stream at the bottom of the slope. What is this part of the water cycle called?

Heavy rain washes nitrogen fertilizer from farm fields into a shallow bay. Within weeks a thick algal bloom covers the water, and a month later many fish are dead. What killed the fish?

A farmer grows corn in the same field year after year, harvests the whole crop, and adds no fertilizer and no legume crop. Yields fall a little further every year. Why?

A field is flooded for two weeks and the soil runs out of oxygen. After the water drains away, a soil test shows far less nitrate than before the flood. Where did most of the missing nitrate go?

A town replaces its fields with roads, roofs and parking lots. After storms the creek now floods faster than it used to, and the town's wells draw less water. Which change to the water cycle explains both?

A large area of tropical forest is cleared for pasture. Over the following years the region receives less rainfall than it did before. Which change to the water cycle best explains this?