ERS 60                                    Problem Set 3

 

Problem 1

 

Other than light and water, nitrogen is often the most limiting nutrient for plant growth in many ecosystems.  A simplified model of N cycling in two forested ecosystems (from Young and Giese, 1990) is shown in the compartment model on the next page (four compartments: vegetation, forest floor, mineral soil, and available).  Ecosystem (a) is a deciduous forest in the northeastern US; ecosystem (b) is a coniferous forest in the northwestern US.  Values next to arrows are transfers between compartments each year (kg/ha/yr); values in boxes are amounts of stored N (kg/ha); values in parentheses are changes in stored N each year (kg/ha/yr).  One way to calculate the rate at which N is cycled or transferred between different parts of the ecosystem is to divide the sum of transfers into the compartment by the mass of N in the compartment.  The result is a turnover rate (or rate constant) with units of yr‑1 (per year) for each compartment. 

 

EXAMPLE: Deciduous forest mineral soil                     12.2 kg/ha/yr

                                                                                    3400 kg/ha       =          0.0036/yr

 

This means 0.36% of the N in the mineral soil of the deciduous forest is turned over each year.  Put another way, it would take about 278 years (i.e., 1/0.0036) for complete turnover of the N in the mineral soil (this is referred to as the mean residence time).

 

Calculate the rate constants and mean residence times for the remaining compartments in both forests and show your results in a table. 

 

Which step is the rate-limiting step (i.e., the slowest rate, the one that determines the overall rate) for the N cycle in each forest?  Which is the fastest step?

 

Explain why the input of N by precipitation is so much greater in the deciduous forest.

 

Based on the values shown for the two forests, is there a net gain, a net loss, or a steady state amount of N in the two ecosystems?  EXPLAIN.


Problem 2

 

In contrast to ecosystems that are not harvested, wherein nutrient cycling is generally quite conservative, most agricultural ecosystems are net exporters of nutrients inherent in soil.  Consider the following barley production system:

 

--         Soil contains 0.5% “available” Ca in the rooting zone to a depth of 50 cm.

--         Soil bulk density is 1.25 g/cm3.

--         Total plant production (biomass) is 5500 kg/ha/yr (dry-weight basis).

--         Total grain yield is 2700 kg/ha/yr (dry-weight basis).

--         Ca content of the grain is 0.05%.

--         Ca content of the whole plant is 1.4%.

 

1 ha = 10,000 m2

 

If only the grain is removed from the field, how many years will the “available” Ca supply in the soil last (assuming yields don’t decrease as Ca content decreases)?

 

If the entire barley plant is removed, how long will the Ca last (same assumption as above)?

 

What sources of Ca, other than the addition of fertilizer, could contribute to the Ca nutrition of the barley?