Ecosystem Budget Approach. Stand Growth

Biogeochemical processes that take place in terrestrial ecosystems can be described as fluxes into, out of or within ecosystem components and compartments, but also as size changes of element or water pools present in ecosystems. Often element or nutrient pools are also called stocks—for example, soil carbon stocks. Pools or stocks can be compared with the amount of money in a bank account, while fluxes in this analogy would be all transfers—that is, deposits and withdrawals from this bank account.

Thus, if the withdrawals exceed the deposits, the pool is decreasing in size or even disappears; if the deposits exceed the withdrawals, the pool is increasing in size. One can also calculate the net budget of all gains (deposits) and losses (withdrawals). This net budget (the sum of all transfers) can be either positive (gains > losses) or negative (gains < losses), depending on the fluxes (transfers). If one considers the CO₂ budget of an ecosystem, a positive net budget represents the condition when assimilation exceeds respiration; a negative net budget represents the condition when assimilation is lesser than respiration.

If one considers the C budget, additional fluxes need to be considered— for example, methane uptake and loss, volatile organic carbon losses, dissolved organic and inorganic losses, etc. Often these fluxes have a special sign convention, depending on the discipline in which they are used. So plant ecophysiological calculate with positive numbers when assimilation is studied, while micrometeorologists calculate with negative numbers when they consider assimilation, since CO₂ taken up by plants is lost from the atmosphere. In all disciplines, the following units are used to describe biogeochemical processes:

- Pool or stock size (amount of an element or substance): grams per square metre
- Unidirectional element or substance flux rate: grams per square metre per unit of time
- Net element or substance budget: grams per square metre per unit of time, often partitioned into losses and gains, also called sources and sinks

Ecosystem budget studies have become increasingly frequent in recent decades, although tracing and measuring all processes within an ecosystem will hardly be possible, despite the application of sophisticated new methods. Mechanisms regulating the size of the flux include regulation by the substrate (i.e. feedforward regulation), regulation by the products (i.e. feedback regulation), branching, modulation and co-limitation (Parts I and II). The details of selected biogeochemical fluxes are described in Chap. 16.

Stand Growth. Growth of plant stands depends not only on assimilation of CO₂ and the availability of water but also on the turnover and availability of nutrients. This means that to describe stand growth, the net budgets of carbon, water and nutrients need to be quantified. Water and CO₂ may be regarded in ecosystems as renewable resources from an indefinitely large atmospheric pool (with certain exceptions, such as deserts). This is different for nutrients. Generally, nutrients are released during decomposition of organic matter or during weathering of primary minerals or bedrock, and they are then reversibly absorbed to the soil ion exchange system or to soil organic matter (SOM).

The ash content of biomass shows the amount of nutrients taken up by plants from the soil (neglecting above-ground uptake from deposition). These nutrients become available by decomposition of organic biomass when it is not exported (for example, by harvesting) but remains as dead biomass (litter, debris) in the system. Ulrich (1987) has formalised the connection between the availability of CO₂, water and nutrients, and their incorporation into organic substances, with an equation of substance and energy balance for organic matter in ecosystems:

where M⁺, A^- and H⁺ are cations, anions and hydrogen ions, and the coefficients a, x, y and z are stoichiometric coefficients in the soil solution. To maintain high production rates (of organic matter containing these substances) in managed systems, the use of elements from the environment owing to harvesting must be replaced by fertilization.