1 UIUC ATMOS 397G Biogeochemical Cycles and Global Change Lecture 18: Nitrogen Cycle Don Wuebbles Department of Atmospheric Sciences University of Illinois, Urbana, IL March 20, 2003

2 UIUC Biospheric Processes that Transport Nutrients

3 UIUC Simplified Ecosystem Nutrient Cycle Model. In the example below only the primary production trophic level is shown. Harvest by humans or other animals occurs if the material is removed from the site; otherwise the consumed portion of vegetation is returned to the system as fallout. The circles represent ecosystem nutrient reservoirs (storage compartments). Two independent inputs are involved: (1) the nutrients provided through soil development from the soil parent material (weathered rock) and (2) nutrients contributed from the atmosphere to the surface with precipitation or as dry atmospheric deposition (dustfall). Nitrogen, for example, can volatilize and then flow to and from the atmosphere. To simplify the model, we can consider respiration as a positive net input. The ecosystem would collapse if the balance was sustained as negative. All other transfers in the model are set as annual rates (percent) multiplied by the storage in the contributing nutrient reservoir. Harvest, erosion, and leaching represent losses to the local ecosystem. In many managed ecosystems, there is additional human input of nutrients through fertilization.

4 UIUC The model above can be expressed as a series of equations for each annual time step and for each nutrient storage compartment in the simulation model we use in this activity. L 1 = (L 0 + (B 0 * f) + n + r) - ((L 0 * d) + L 0 * e). S 1 = (S 0 + (L 0 * d) + w) - ((S 0 * u) +S 0 * l). B 1 = (B 0 ) (S 0 * u) - ((B 0 * f) + B 0 * h). Where: d = decay, e = erosion, f = fallout, h = harvest, l = leaching, n = nutrients applied, r = respiration, u = uptake, w = weathering, and for time period 1, L 1 = litter, S 1 = soil, B 1 = biomass. Loss rates from a compartment cannot total more than 100%. If all nutrient storage compartments started (at time 0) with 33 units and the transfer rates were as follows; d = 0.9, e = 0.05, f = 0.05, h = 0.0, l = 0.2, n = 0, r = 9, u = 0.7, and w = 0.01, then at time 1 we would get: L 1 = (33 + (33* 0.05) ) - ((33 * 0.9) + 33 * 0.05) == = 12.3 S 1 = (33 + ((33 * 0.9) ) - ((33 * 0.7) + 33 * 0.02) = = 6.01 B 1 = (33 + (33 * 0.7) - ((33 * 0.1) + 33 * 0.0) = = 52.8 In this example, the high transfer rates result in a rapid adjustment, i.e., it would take only a brief period before the nutrient storage in the different compartments would stabilize. From t=0 to t=1

5 UIUC Integrative Biosphere Models Comparison of Net Carbon Storage During the 1980s

6 UIUC Simplified Model of the Biosphere From Scott Denning

7 UIUC Integrated Biospheric Simulator (IBIS) t ~ minutes to hours t ~ days to weeks Land Surface Module Belowground Carbon & Nitrogen Cycling Module Vegetation Dynamics Module Biomass Production: GPP, total respiration, NPP Aboveground Carbon Cycling Plant Physiology: photos. & leaf respiration, stomatal conductance Soil Physics: energy and water balance Canopy Physics: energy & water balance, aerodynamics ATMOSPHERE (prescribed atmospheric datasets) Vegetation Phenology Module: budburst & senescence GPP, foliage respiration, C:N ratios Vegetation structure & biomass Daily LAI temperature, photosynthesis Canopy nitrogen allocation t ~ years Adapted from Kucharik et al. (2000)

8 UIUC Discussion Questions How would acid rain affect nutrient availability to an ecosystem/ Do human activities (other than the contribution to acid rain) affect nutrient availability in ecosystems? If so, how?

9 UIUC In an aging Picea abias stand in Russia, the canopy becomes more open after 70 years, and understory vegetation increases in volume and importance in nitrogen cycling

10 UIUC Ecologist view of nitrogen cycle