III.11 Terrestrial Carbon and Biogeochemical Cycles
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III.11 Terrestrial Carbon and Biogeochemical Cycles R. A. Houghton OUTLINE 1. The production equations (carbon) 2. To what extent do C, N, and P limit photosynthesis in terrestrial ecosystems? 3. To what extent do C, N, and P limit NPP? (What determines RsA?) 4. To what extent do C, N, and P limit NEP? (What determines RsH?) 5. To what extent do C, N, and P limit the amount of carbon in vegetation and soil? 6. Disturbances limit C accumulation Two modes of explanation account for the accumulation of carbon in terrestrial ecosystems: metabolism and demography . Carbon, nitrogen, and phosphorus (as well as temperature and moisture) affect the metabolic processes that control the rate at which ecosystems fix and accumulate carbon in organic matter. Whether they also control the total amount of carbon that can be held in the biomass and soils of ecosystems is less clear. An alternative explanation is that maximum carbon storage is limited, at least in forests, by disturbances, both natural and anthropogenic, that initiate changes in demography. GLOSSARY autotrophicrespiration. Themetabolicprocessbywhich primary producers (green plants) convert sugars to carbon dioxide, releasing energy. denitrification. A process by which nitrate and nitrite are reduced to ammonium. global carbon balance. The total sources of carbon from fossil fuels and land-use change must sum to the total sinks (accumulations) of carbon in the atmosphere , oceans, and land. gross primary production. The amount (or rate) of organic matter (sugars) produced from CO2 by green plants through photosynthesis. heterotrophic respiration. The metabolic process by which consumers (heterotrophs) convert sugars to carbon dioxide, releasing energy. net ecosystem production. The amount (or rate) of organic material produced by green plants after both autotrophic and heterotrophic respiration. net primary production. The amount (or rate) of organic material produced by green plants after (autotrophic) respiration. nitrification. A process by which ammonium is oxidized to nitrite, and nitrite to nitrate. nitrogen fixation. A process by which molecular nitrogen is reduced to form ammonium. A major impetus for studying carbon and biogeochemical cycles has always been propelled from the applied sciences of agriculture and forestry: How can yields be increased? Ecologists have also investigated the cycles of C, N, and P to understand how ecosystems function. Most recently, a third motive has emerged for studying the biogeochemistry of ecosystems: carbon management. How much carbon is (can be) sequestered in the vegetation and soil of ecosystems? Which ecosystems are best at sequestering carbon? How rapidly does this process occur? And what factors limit both the rate of accumulation and the total amount? The global carbon balance suggests that terrestrial ecosystems have been accumulating carbon for several decades, but the reasons for this are not entirely clear. Candidate explanations include (1) ecosystem responses to changing environmental conditions [for example, increasing concentrations of CO2 in the atmosphere, increased mobilization of nutrients (nitrogen, phosphorus ) from human activities, changes in climate] and (2) recovery of forests from past harvests, abandonment of farmlands, and fire exclusion. The mechanisms responsible for current carbon sinks on land are important to understand, first, because the environmental variables driving these mechanisms may be different in the future, thereby either enhancing or diminishing current rates of sequestration , and, second, because an understanding of the mechanisms should indicate the types of management likely to increase terrestrial carbon sequestration. What is it that limits the rate of carbon sequestration? The focus of this chapter is to explore these limits in the context of biogeochemical cycles. The concept of limiting factors is an extension of the Law of the Minimum, attributed to Liebig but recognized by others before Liebig’s formulation in 1840 (Browne, 1942). The Law of the Minimum states that, if all of the mineral nutrients but one are available in the quantities required for the growth of a plant, the deficiency of that one nutrient will prevent growth. The concept of limiting factors in ecology includes not only biogeochemical factors but all resources, including water and light, as well. This chapter focuses on the biogeochemical factors. Furthermore, the concept of limiting factors applies not only to plant growth but also to the decomposition of organic matter. 1. THE PRODUCTION EQUATIONS (CARBON) The processes governing carbon accumulation occur over minutes to hours as leaves fix atmospheric CO2 and over thousands to millions of years as soils develop during primary succession. The exchanges of carbon between terrestrial ecosystems and the atmosphere are described by the following chemical equation: 6CO2 þ 6H2O . C6H12O6 þ 6O2: (1) In photosynthesis, carbon...