FACULTY NOTES

The suggested student active approach for Figure Set 4 is "Paired Think Aloud." This approach is designed to allow students that are shy to present their opinion to another student, without having to talk immediately in front of the whole class. Students work on the questions in pairs. We suggest that the older student answers the question while the younger student records the answers. The younger student may then present the answer to the larger group. You may choose another strategy for assigning the students to be talkers or recorders, such as using the last digit of a phone number.

Familiarize yourself with each of the six agroecosystems in Figure 4. Photos from the agroecosystems can be found at www.lter.kbs.msu.edu/photo_gallery/overview.php. Robertson et al. (2000) found that early successional plots dominated by herbaceous perennial plants (grasses, forbs, legumes) exhibited significant increases in soil organic carbon from 1989 to 1999, and sequestered the most carbon (>200 g CO₂ m-2 year-1) when established on previously cultivated soils from (Tables 1 and 2 in Robertson et al. 2000). Alfalfa and poplar trees, both perennial crops, also sequestered substantial amounts of carbon in soil during this time period. Several factors contributed to carbon sequestration by perennial cropping systems, including year round plant cover and root growth / turnover, high root productivity, and no soil tillage, which enhances decomposition.

Among annually cropped fields, no-till strategies were the most effective at sequestering soil carbon. Much of the plant residue from these fields was left on the soil surface after harvest, and decomposed slower than the residue in the tilled plots (conventional and organic). Cover crops were grown during the winter in organic plots, thus continuing plant growth across the entire season and building soil carbon whereas conventional plots did not utilize cover crops. Nitrogen fertilizer was also applied in conventional plots, which has been shown to accelerate rates of decomposition within the light soil carbon fraction (decadal turnover time) (Neff et al. 2002).

In general, perennial crops sequestered more carbon in soil than annual crops. No-till strategies were the most effective at increasing soil carbon in annually planted plots, and no agroecosystems exhibited further decline in soil carbon. Students are asked what benefits a farmer might gain from having increased soil organic matter. These benefits can be increased nutrient supply from the soil, increased water holding capacity, reduced risk of erosion, pH buffering capacity and potentially reduced risk of soil pathogen outbreaks. Other answers not mentioned here may also be correct.

Agricultural researchers debate whether raising a productive agricultural crop, such as corn, for many years in a row can increase soil organic carbon levels. The answer to this question likely varies depending on climate and management conditions, but it may be an interesting question to bring up with your students. Perhaps consider the question of whether or not continuous corn cropping can build organic matter in conventional vs. no-till managed fields.

The short essay assessment below asks students to consider why ecosystems with perennial crops exhibit higher rates of carbon sequestration rates in soil than ecosystems where annual crops are grown. They could list several reasons for this, and it is your discretion to determine if an answer is satisfactory. Students could provide answers that include deep root systems of perennial plants, lack of soil tillage, plant cover during winter months, cooler soil temperatures in summer due to plants covering the soil, etc.. to explain the differences between annual and perennial dominated ecosystems.

Post Lesson Assessment - Short Essay (100 - 200 Words):

Why do agroecosystems with perennial crops (alfalfa, successional, poplar trees) build soil carbon faster than agroecosystems containing annual crops (conventional, no-till, organic)? Consider what you know about perennial and annual plants and the ways they are managed to develop your answer.