Answers to Activity 2

Note: Answers to the simulations are hot linked below.

Activity 2 Nutrient Cycling Simulation Model

By manipulating the nutrient cycling model, students should gain insight into a variety of human interactions with the environment. Three distinct (and in some respects extreme) environments are chosen. Below are links to the summary tables of each simulation and the respective embedded graphs for:

The model values used to generate the graphs are contained in the summary spreadsheets. The values for year 270 are summarized in the Year 270 Summary table below.

Where computer facilities are not available for the demonstration or the hands-on use of the NUTCY4 model, the outputs for each of the scenarios can be used instead. Instructors may make print-outs of the table containing the initial conditions in each biome, run the simulation for each to obtain the associated graphs for each biome, and then provide students with these and the tables of the human-altered cases to use as a basis to answer the questions. For the Selva3 case it may be preferable to use the line graph rather than the histogram because the impact is more obvious there.

Notes on the simulations

Students should have taken notes like the following while running the simulation. They form the basis for their final assessment (summary paper).


Litter storage rates stabilize after a couple of years, soil storage rates stabilize after 15-20 years, and biomass storage values stabilize after about 120 years. The first rate reflects the time it takes under tropical climate conditions for litter to be produced, decomposed, and taken up via the soil. The time it takes for the nutrient storage in soils to stabilize reflects the time it takes to establish a dynamic equilibrium of edaphic (soil) processes, including physical processes, biochemical processes, and the establishment of a viable soil fauna and flora in the face of fast-growing biomass. Finally, nutrient stabilization in biomass takes the longest because tropical tree species need this amount of time to reach full maturity and for the ecosystem to establish itself as a mature, self-organizing web of physical and biotic interactions in dynamic equilibrium.

Students should be able to predict that because most of the nutrients are stored in the biomass, and not in the soil, that in the case of a large-scale deforestation, most of the fertility of this biome would be lost. (You may want to compare this to a temperate forest situation where nutrients are largely stored in the soil.)

Selva2 This simulation shows what students predicted for a deforestation of Selva. The invoked change results in a severe drop in biomass and a drop in uptake rates because the grasses do not extract as much water and nutrients from the soil as do large trees. Similarly, the standing biomass is almost completely lost to fallout on an annual basis. This is why the fallout rate is more than 99%. It reflects the character of a grassland where grazing and leaf decomposition take away most of the standing biomass.
Selva3 Because the soil depletion is not total and a plot is commonly only a few tens of meters across, which facilitates recolonization by neighboring species, the natural recovery after a slash-and-burn and farming episode is fairly rapid. In comparison to the Selva2 situation where the biomass is removed completely and continuously, biomass nutrient storage is able at least to approach pre-cutting values. But the simulation of repeated slash-and-burn cycles shows that biomass storage doesn't quite reach pre-cutting levels. A shortening of the fallow-period would result in an ever smaller nutrient recovery, i.e., an eventual loss of fertility.
Steppe Soil storage of nutrients is highest in this biome, with litter and biomass falling vastly behind. The storage differences reflect both the relatively dry climate, which doesn't allow much soil leaching and slows down litter decomposition, and the biomass that in and of itself isn't able to store large amounts of nutrients, but which produces biomass year after year and then accumulates as litter and eventually as a nutrient reservoir in these most fertile soils. Students should be able to conclude that cultivation of a steppe soil would result in a quick nutrient loss whose speed is determined by the uptake of nutrients by grain, the increase of leaching and erosion, and the lack of supply of litter for decomposition, hence replenishment of soil fertility.
Steppe2 The cultivation of corn affects the steppe in several ways: as predicted at the end of the Steppe simulation, there is a quick loss of nutrient storage from the soil. The initial boost in soil storage is simply the result of applying water (irrigation) and fertilizer and plowing the soil, all of which combine to accelerate the decomposition process. The initial boost in litter storage reflects the plowing under of grasses. Both erosion and leaching increase with a more open soil (corn is renowned for its accompanying erosion because it covers the ground so incompletely), but are still much less than in the humid tropical climates (much less rainfall). Fallout rates close to 100% reflect the annual harvesting of corn (total removal of biomass), and the increase in the uptake rate shows that corn needs more nutrients than grasses.
Steppe3 The pre-1950s cultivation and crop rotation shares some of the characteristics described above but generally seems to be more conserving of soil fertility. Litter and biomass storage values hardly differ after 270 years of this type of cultivation, but the soil fertility has declined, if more slowly than in Steppe2. Because average soil coverage is higher, leaching and runoff are less than in Steppe2. Similarly, biomass uptake is an average over a four-year crop rotation that includes lower-yield (less nutrient-intensive) crops. Finally, the return of some nutrients via manuring also slows down the loss of nutrients from the soil.
Tundra In this biome, litter stores most of the nutrients, with soil storing much less and only little more than biomass. Plants, as mentioned on the student worksheet, grow slowly and generally are small and low to the ground. Soils are mostly frozen, and generally thin and poor in nutrients, allowing soil processes to proceed only slowly. Litter decomposition is restricted by temperatures and hard plant material, which explains the relatively large accumulation of nutrients in litter. The most important effect of climate change for the tundra may be the increase in temperatures which will enhance plant growth, evapotranspiration, litter decomposition, and nutrient leaching from the thawing ground. Thus, students could predict an increase of nutrient storage in biomass along with a large loss of nutrients from the soil. Because litter decomposes faster, the storage there will decrease as well.
Tundra2 Interestingly, the changes predicted at the end of Tundra hold only for the first few years. Over time, litter storage of nutrients and biomass accumulation all increase, if slowly. This can be explained by a general amelioration of the growing conditions for plants given higher temperatures. Soil processes accelerate, leading to more fertile and better drained soils. Plant growth is stimulated by increased soil fertility and a longer growing season (light conditions stay the same, but frost leaves sooner and comes later in the year). And the increase in biomass translates to larger litter amounts. In short, assuming current species can adapt rapidly enough (as this model does), global warming is expected to increase the productivity of the tundra biome.
In the short essay that concludes this activity, students should summarize and discuss these observations and explanations. Check their essays for:

Table 2: Year 270 Summary (Results)































































The following pages are print-outs of the year-270 summary graphs that students should produce in this activity.