Ann Arbor, Michigan
Forests represent 65% of the net primary productivity/carbon fixing on the Earth's land surface and so have enormous implications for carbon dioxide (CO2) concentrations. To understand forests, we need to know how both plants and soil microorganisms react to CO2, how plants affect microbes, and how microbes affect plants. Soil microorganisms contain 1.5% of the carbon (C) and 3% of the nitrogen (N), but microbial activity of the soil decomposes organic matter, which releases CO2, so there's a large potential to have an impact on carbon and nitrogen cycles.
The greatest potential for change comes through the indirect effects of plant production and changes in microbial activity in the soil. Plant production is an important driver of microbial activity. Elevated CO2 has the potential to affect these processes, both directly and indirectly, but direct effects are likely to be very few. The concentration of CO2 in the soil is 8-100 times that in the current atmosphere, so the relatively small increase from 350 to 700 parts per million of atmospheric concentration is likely to have only a minor effect. Many soil microorganisms are very tolerant to changes in CO2.
There is a link between plant productivity and soil microbial activity. Plant productivity is limited by the rate at which N is released by microbial activity. In turn, plants need N. When rates of N release outstrip rates of availability, then availability begins to control the balance between release and uptake. The amount of C allows for net growth of microbial populations. Elevated CO2 levels have the potential to increase soil C availability, causing potential fertilization effects.
Zak's research addressed the relationship between net primary productivity (NPP), soil C availability, and microbial biomass at study sites throughout the United States, in a wide range of biomes with a wide range of NPP and soil characteristics. A significant relationship between NPP and soil texture accounts for two-thirds of the variation in US sites. There is also a significant relationship between NPP and microbial biomass (amount of C in the microbes).
Zak says plant production is significantly related to microbial biomass which accounts for 41% of the variation. In ecology, a 41% regression is quite large, relatively. Zak's results show that elevated CO2 will increase plant photosynthesis and so the soil availability of labile carbon. Moreover, an attendant increase in protozoan activity will increase N availability. This demonstrates the "priming" effect of elevated CO2 - increased N, C, microbial biomass, etc., initiate a positive feedback loop.
Is there a possible negative feedback? A research group from the United Kingdom (Diaz and Grime) says it could happen that nitrogen could decrease in availability with elevated CO2, and plant production could grind to a halt - surprise!
CO2 and N variations occur naturally on a seasonal course in ecological systems; both elevated CO2 and elevated nitrogen availability are potent determinants of plant photosynthesis.
This observed increase in photosynthesis increased total biomass of plants by 50% at high N availability and by 25% in low N availability. It also revealed that plants are taking most of the productivity increase and putting it into the roots to forage for more water and N. The increase in root biomass seems to come from increased production and mortality of roots, with the rate of increase outstripping rate of mortality. At elevated CO2 and high N availability, plants allocate most C below ground to obtain nutrients. They become more efficient in their use of N to form plant tissue.
Elevated CO2 caused no significant increase in soil C availability and no difference in the metabolic rate of C. We need to understand throughput; how is it turning over? There is no negative feedback in litter chemistry in field experiments; this only occurs in pots where root bound plants lose fine root carbon.
Conclusions
Elevated CO2 caused no significant increase in soil C availability and a negative feedback in plant-soil systems unlikely. If plants forage by increasing productivity and mortality of roots, it doesn't make sense that they would turn off the resource for which they are foraging.
How common is it to observe below ground biomass increase? Out of 28 studies of 17 tree species in an environment with elevated levels of CO2, 27 report a below ground biomass increase, 0 report a decrease and 1 reports no change. So we know that increased CO2 in the atmosphere will cause an increase in below ground biomass (roots). see (Rogers et al.)
How does that root increase translate into changes in microbial activity? It is the fuel that drives microbial activity. The mean response is a 24% increase of microbial activity in soil. So increased C availability increases both microbial activity and biomass. How does it affect community? There is no change in composition of the community of soil microbes (based on lipid analysis). Are there changes in N availability? Yes, with a mean response of 76%, a small net increase or no net change seems likely. There is no evidence of decline - no negative feedback.
Temporary increases in NPP are likely to occur in increased CO2. What does this mean for ecosystem carbon storage? The initial slope of the growth curve is more steep but we don't know what will happen out into the future. Short term studies with short-lived plants gives us little insight into what will happen down the road. In particular, questions in temporal scale could hold surprises. Will we get to the same place more rapidly or will we get to a different place?
One important difference between experiments and reality is that in reality, CO2 is increasing gradually in the environment compared to experiments that go from ambient levels to doubled levels immediately. Growth is a multivariant response and therefore it is difficult to sort out. We could also use paleodata on trees, to see how changes in CO2 that have already occurred have affected growth.
Zak disputes the claim that there could be a negative feedback on vegetation growth in the short term due to nutrient availability. The majority of evidence suggests a positive feedback on photosynthesis and a negative feedback on ambient CO2. We do not have enough information to discern long-term impacts.