Sources of Soil Organic Matter

A major factor influencing SOM dynamics is the quantity and quality of OM entering the soil. Climatic effects on OM production are covered in several other chapters in this volume, but here we look at the implications for the soil. The quantity of litter input to the soil is rather unimportant because soil storage is approximately proportional to the litter input, although the slow turnover of SOM means that long periods are required for it to reach a new steady state. On a global scale, the stability of the CO2 concentration ([CO2]) during the last few thousand years before industrialization shows that litter input and respiration costs have been approximately balanced for a long time. The problems of balancing the global C budget (the non-identified terrestrial sink) show that this is currently not the case (Schimel, 1995). On a local level, it is normal that these flows do not balance, i.e. there is either an increase in SOM due to a higher input than output through mineralization, or vice versa.

Changes in the quality of the litter input can come mainly from two sources. Firstly, a climatic change can lead to changes in allocation in the plant; hence the relative contribution of plant components to litter input will change. Lawlor and Mitchell (1991) and Rogers et al. (1996) have reviewed the literature for effects of elevated [CO2] on root-to-shoot ratios (R:S) and found a variety of responses. About 75% of the observations in the study by Rogers et al. (1996) showed changes in R:S of less than 30%, with a mean of +11%. The reason for this variability is not clear, but interactions with other factors, mainly nutrition (cf. Joffre and Agren, 1999), can be suspected. Furthermore, a majority of the available root results only estimates the amount of structural root C and ignores the soluble C, thus underestimating the C input to the soil via the root system. Whether or not the elevated [CO2] may result in a different ratio between structural and soluble root C compared with today needs to be studied further (Rogers et al., 1994). If labile C in the soil increases under elevated [CO2], it may influence soil N availability, because N mineralization may change (Davidson, 1994). However, there are divided opinions about whether this input significantly stimulates soil processes or influences soil communities (Pregitzer, 1993; Curtis et al., 1994).

The second cause of changes in litter quality is a change in the chemical composition of a given litter type. Cotrufo et al. (1998) reviewed 75 published studies of effects of elevated [CO2] on N concentration in plant tissues. They found an overall 14% decrease in N concentration, but with great variability between plant functional types. C4 and N-fixing plants responded considerably less than C3 plants. The implications of these results for litter quality are not immediate. Firstly, the observations are made on live tissue, and it is not known how the retranslocation of N during senescence will interact with this change in live-tissue N concentration. Secondly, although the C:N ratio has been widely used as an index of decomposibility (Melillo et al., 1982), it is not unchallenged (Joffre and Agren, 1999). Other more subtle changes in the chemical composition may play a more important role. For example, a small change in lignin concentration can be more important. The general conclusion today is that changes in [CO2] per se will probably have minor effects on litter decomposition rates (Norby and Cotrufo, 1998).

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