<P> Soil moisture is another important factor influencing soil respiration . Soil respiration is low in dry conditions and increases to a maximum at intermediate moisture levels until it begins to decrease when moisture content excludes oxygen . This allows anaerobic conditions to prevail and depress aerobic microbial activity . Studies have shown that soil moisture only limits respiration at the lowest and highest conditions with a large plateau existing at intermediate soil moisture levels for most ecosystems . Many microorganisms possess strategies for growth and survival under low soil moisture conditions . Under high soil moisture conditions, many bacteria take in too much water causing their cell membrane to lyse, or break . This can decrease the rate of soil respiration temporarily, but the lysis of bacteria causes for a spike in resources for many other bacteria . This rapid increase in available labile substrates causes short - term enhanced soil respiration . Root respiration will increase with increasing soil moisture, especially in dry ecosystems; however, individual species root respiration response to soil moisture will vary widely from species to species depending on life history traits . Upper levels of soil moisture will depress root respiration with the exception of wetland plants, which have developed specific mechanisms for root aeration . Soil moisture levels regulate root respiration, which is essential for nitrogen uptake . </P> <P> Nitrogen directly affects soil respiration in several ways . Nitrogen must be taken in by roots in order to promote plant growth and life . Most available nitrogen is in the form of NO, which costs 0.4 units of CO to enter the root because energy must be used to move it up a concentration gradient . Once inside the root the NO must be reduced to NH . This step requires more energy, which equals 2 units of CO per molecule reduced . In plants with mycorrhizal symbionts, which fix atmospheric nitrogen, the energetic cost to the plant to acquire one molecule of NH from atmospheric N is 2.36 CO . It is essential that plants uptake nitrogen from the soil or rely on symbionts to fix it from the atmosphere in order to assure growth, reproduction and long - term survival . </P> <P> Another way nitrogen affects soil respiration is through litter decomposition . High nitrogen litter is considered high quality and is more readily decomposed by microorganisms than low quality litter . Degradation of cellulose, a tough plant structural compound, is also a nitrogen limited process and will increase with the addition of nitrogen to litter . </P> <P> Different methods exist for the measurement of soil respiration rate and the determination of sources . The most common methods include: The use of long - term stand alone soil flux systems for measurement at one location at different times; survey soil respiration systems for measurement of different locations and at different times; and the use of stable isotope ratios . </P>

Indicate three functions of carbon dioxide in soil