If working out what will happen to vegetation due to direct C02 fertilization is a challenge, figuring out its effects on soil carbon is even harder to do. Soil organic carbon is the largest reservoir of organic carbon in terrestrial ecosystems, exceeding vegetation two to three-fold. But, it is indirectly a product of the growth of living plants, plus what happens to them after death.
Various C02 fertilization studies have found that plants invest proportionately more in roots at high C02 levels (which may be explained as being an adaptation to bring in nutrients which become more limiting). Both closed-chamber and FACE experiments (e.g., that Tennessee sweetgum stand, once again) have found a large increase in the rate of turnover of fine roots at higher C02 concentrations. Roots supply carbon directly to the soil when they die, and an increase in the size and rate of turnover of root systems seems likely to increase the amount of organic matter in soils.
As well as a change in the rate of supply, the detailed composition of plant materials may alter under increased C02, making a difference to decay rates and organic matter in soils. Many studies have shown a decrease in nitrogen content of plant tissues under increased C02. Nitrogen generally seems to be a limiting factor in the rate of decay of plant materials by fungi and bacteria, so less nitrogen in these tissues should mean they decay more slowly, perhaps adding to the carbon store in soils. However, one study which looked at the breakdown of the less nitrogen-rich plant parts that had grown at increased C02 found no effect on decay rate.
Even if plant materials are lower in nitrogen, it is possible that, because of their greater rate of supply to the soil and litter layer (due to increased growth rates), they will encourage the growth of a specialized microbial and detritivore community. This will be able to break them down quicker overall, because the right organisms are always on hand—the decomposer populations are "primed" with a continual supply of material to feed off. According to this hypothesis, there will be less carbon ending up in soils in a high-C02 world.
Where C02 fertilization experiments have looked at soil carbon, in some cases they have found that it increased, while in other cases it decreased. 0ne set of chamber experiments with sets of wild tropical plants growing together in artificial communities found a decrease in soil carbon at doubled C02, which is what the "priming" hypothesis predicts. Another study comparing soil changes under soy (C3) and sorghum (C4) found that soil carbon decreased under soy but increased under sorghum at high C02. This latter experiment contrasts two very different photosyn-thetic metabolisms, and it is not clear how more subtle differences in metabolic and growth characteristics might affect the soil carbon response to raised C02. However, this degree of complexity does not bode well for understanding future responses of soil carbon to increased C02.
Another effect that might turn out to be important under increased C02 is the enhancement of chemical weathering, which itself acts as a C02 sink (Chapter 6). If plants produce more roots and more root exudates under increased C02 (remember that various studies show they do put more into fine roots when C02-fertilized), then this might promote the fungal and bacterial activity that breaks down minerals in the soil. This will act as an increased global sink of C02: a negative feedback on C02. There is a need for studies which address this question. Some preliminary studies do show that under increased C02, chemical weathering is enhanced.
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