to discovery, we aimed to build research skills, improve scientific literacy, and create an understanding of the often difficult-to-grasp concept of the nitrogen cycle. These objectives were achieved through an
introductory lecture followed by a long-term research experiment
using legumes in a high school AP Environmental Science class.
The introductory lecture about basic legume biology formed a
foundation of common, shared knowledge that we feel is imperative to the success of active-learning strategies. Background reading
may be assigned as an alternative to the introductory lecture (e.g.,
textbook passages or the background document we provide; see
Supplemental Material S1). Instructors should adapt the introductory material on the basic role of legumes to the context of their
class subject (e.g., evolution, agriculture, microbiology). We provided our class an overview of biotic and abiotic players in the
nitrogen cycle, then focused on how plants and legumes specifically
might play a different role from other plants in the nitrogen cycle.
Our lecture introduced students to common legumes, such as the
crop plants Phaseolus vulgaris (common bean) and Cajanus cajan
(pigeon pea, which is common in many Hispanic, African, and
South Asian communities) and the common weeds Medicago poly-morpha and M. lupulina (a table of common legumes, suited to various climates and cultural contexts, is provided in Supplemental
Material S1). In this lecture, we also outlined the constraints that
students would have in designing their experiment – mainly time
frame and available resources (e.g., scissors, buckets to collect soil,
trowels or shovels, plastic bags, Sharpies, and tape for labeling).
Having established an intentionally incomplete but basic
understanding of legume biology, we guided small-group discussions to develop research questions and hypotheses surrounding
plant growth, soil type, soil treatments, and nitrogen levels. Our
role here was to remind students that their hypotheses needed to
be testable within the available time frame and using only the available supplies/materials.
By brainstorming research questions and hypotheses in small
groups, students learn valuable communication skills and acceptance
of other viewpoints that differ from their own (Hess, 2009). This
process also fosters critical-thinking skills as students discuss how
to connect the appropriate research method to address their specific
research questions (Driver et al., 1994). Further, students improve
their ability to explain their thoughts and support their ideas with
reason when they must communicate their ideas to others (Osborne,
2010). The role of the instructor during this process is not to implant
research ideas, but to make sure that students’ hypotheses are test-
able hypotheses that require reasonable research methods. The
instructor can help students decide on reasonable hypotheses by ask-
ing follow-up questions and reminding students of the time con-
straints and project supplies. The discussion was concluded by
bringing the whole class together to share their hypotheses and
research approaches. Together, students agreed upon select hypoth-
eses and research methods to move forward with as a class. Below is
a list of the key hypotheses and their connections to class content
areas, listed as “Discovery,” each supported by “Evidence” from the
experiment. Throughout the process – from development of hypoth-
eses to careful scientific observations to interpretation of their
experimental results – students were encouraged to discover deeper
aspects of legume biology that were not discussed in the introduc-
tory lecture and make connections to the nitrogen cycle (see below).
Key student-driven hypotheses and evidence-based connections to
concepts by discovery are as follows:
• Hypothesis 1: Legume development is significantly affected by
the presence or absence of soil bacteria: legumes that grow in
sterile soils (A) have less leaf chlorophyll and (B) are smaller
than plants growing in fresh soils with soil bacteria.
• Evidence: Data in Figure 3A, B (and Supplemental Material S5).
• Discovery 1: Legumes rely on symbiotic bacteria called rhizobia
to convert atmospheric dinitrogen (N2) to ammonia (NH3), a
form that the plants can use to make proteins and other biological molecules (Zahran, 1999). Chlorophyll content is commonly used as a surrogate for leaf nitrogen level because the
two are highly correlated. In legumes, leaf nitrogen level is
affected by rhizobial association. Thus, higher chlorophyll levels are consistent with more productive rhizobial associations.
• Hypothesis 2: Legume development is significantly affected by
the presence or absence of soil bacteria: legumes that grow in
sterile soils have fewer root nodules than plants growing in
fresh soils with soil bacteria.
• Evidence: Data in Figure 3C (and Supplemental Material S5).
• Discovery 2: To establish symbiosis, rhizobia infect legume roots
to form nodules that contain differentiated bacteria; then they
begin the mutual exchange of beneficial substrates: the plant
provides molecules produced by photosynthesis to the bacteria,
while the bacteria provide nitrogen in the form of ammonia to
the plant (Okazaki et al., 2013).
Details of Activity
Understanding how to design an experiment is an essential skill for
science students. Students can have difficulty understanding how
to design an experiment with the appropriate methods to effectively
test their hypothesis. Following the classroom discussion, we created a summarizing PowerPoint presentation that clarified the conclusions of the previous classroom discussion and reviewed the
steps in the experiment (see Supplemental Material S2). During this
explanatory process, the teacher should emphasize the importance
of randomization, sampling techniques, replication, and potential
sources of error. In order to create a more robust experiment, we
made slight changes to the students’ selected research methods that
reflected these concepts (it is also worth mentioning that any experiment can be expanded or reduced in size by changing the number
of species, soils, and replicates, in order to fit the needs and capability of each class). We continually emphasized to students why
this research design was selected and how our changes helped contribute to answering the question. During data collection, students
were exposed to process-based learning by following step-by-step
instructions (see Supplemental Material S2). Our general procedure
in conducting the experiment was as follows.