Degradative stages

As decomposition proceeds, all the contained energy is eventually lost in respiration along with much of the carbon as CO2 while the nutrients are released in an inorganic state, potentially available to the plant subsystem, ready to start the cycle again. But the availability of these nutrients to plants is not just a straightforward slow trickle.

When fresh litter arrives on the soil surface it has a high carbon to nitrogen ratio, that is, it is rich in carbon (C) but comparatively short on nitrogen (N), and usually other nutrients such as phosphorus (P). Living plant material has a C/N ratio of 50-100 but due to resorption of valuable nutrients, the litter produced has a ratio nearer 100-200. The microbes doing the decomposing, and to a certain extent the soil animals physically chewing up the litter, will soak up all the N and P being released by decomposition for their own use and lock up the nutrients in their cells. More than that, however, their need is often so great that they will also pull in nutrients from surrounding areas and lock these up as well. So rather than resulting in mineralization, the microbes at this stage actually cause immobilization. This is why wood chips (which have a very high C/N ratio) used as a garden mulch can result in plants showing classic nutrient shortage symptoms; they are being outcompeted for nutrients by microbes. (Note that bark chips have a much lower C/N ratio and do not cause anywhere near the same problem!) This explains why gross nitrogen mineralization rates are often an order of magnitude higher than net rates; most of the mineralized nitrogen is absorbed directly back into the microbial pool. Carbon is progressively used up in respiration and lost as CO2, but N, P and other nutrients are conserved and reused. This cycling of nitrogen as it is taken up by microbes and released as they die to be taken up by other microbes is referred to as the microbial nitrogen loop. It is this loop that keeps the nitrogen in constant use and unavailable to plants. But as carbon is progressively used up, the ratio between C and the nutrients declines, reaching a C/N ratio of 10-20 in decomposed soil organic matter. Eventually the concentration of nutrients is sufficient to meet the needs of the decomposer organisms and at this point as microbial cells die, nutrients will be released from the loop and made available for uptake by plants. In any given substrate-microbe complex the processes of immobilization and mineralization occur simultaneously, with the difference between the two processes (net mineralization) determining when nutrients are released into the soil. From this it can be seen that when a fresh batch of litter arrives on the forest floor there is a variable lag before the carbon has been reduced sufficiently to allow nutrients to be freed for plant growth, the process being regulated by the microbial community (see Agren et al., 2001 and Attiwill and Adams, 1993 for more detail). Thus, the dead organic matter is the main bottle-neck controlling the availability of nitrogen to plants.

This immobilization when carbon is overabundant may go some way in explaining why N experimentally added to soils, especially where there is a lot of organic matter such as in podzols, has little effect on decomposition (see data from Prescott, 1995 below); extra N is soaked up by an expanding microbial community. Fogg (1988) points out that a large number of experiments have found that adding N to soils as fertilizer may actually slow decomposition. The reasons are not completely known but may well involve:

(a) changes in the composition of the microbe community through competition;

(b) N (as ammonia) suppressing the production of enzymes needed for breaking down lignin and other recalcitrant compounds; and

(c) ammonia and proteins reacting with organic matter to form recalcitrant material.

Part of the problem of interpreting the numerous nitrogen addition experiments results from the different forms of nitrogen added at different rates and frequencies, and complications caused by shortages of other nutrients.

The section above describes what happens to each input of litter. Decomposition can also be looked at over a longer period as conditions within a forest change, but it is much more difficult. As an example of this, W.W. Covington (1981) investigated what happens to forest floor organic matter over a 200-year period when a hardwood forest is felled by clear cutting. He looked at 14 hardwood stands that had been felled at various times through the past 200 years in the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire and made the assumption that the pattern of soil organic matter changes between different-aged sites would mirror what would happen on one site over the same length of time, i.e. that

■ Measured by Covington in 1974 + Measured by Federer in 1979-80

20 40 60 80 100

Years since harvest

Figure 7.2 The amount of forest floor organic material (in t ha-1) over a 200-year period following the felling of trees in hardwood stands in the White Mountains of New Hampshire. The graph is based on an original study conducted by Covington (1981) with additional stands added from Federer (1984). (Redrawn from Yanai et al. 2003. Ecosystems 6, Fig. 1. With kind permission of Springer Science and Business Media.)

the situation found on a site felled at 10 years occurred at the tenth year in a 200-year-old site. The resulting Covington curve (Fig. 7.2) suggests that the amount of organic matter in the soil declines sharply after clear-felling, with 50% (some 40tha-1) lost in the first 20 years. This curve has become very influential in modelling carbon losses from forestry operations and in larger models of global carbon budgets, and yet may not be wholly true. Federer (1984) replicated the study and suggests that a more reasonable estimate of organic matter loss solely due to opening up the stands is 36% or approximately 30tha-1. Yanai et al. (2003) in turn revisited Federer's plots 15 years later and found that individual plots had not moved along the Covington curve; some had lost organic matter and others had gained it but not in any pattern relating to time since clearing.

It was originally suggested that the loss of organic matter was due to (a) reduced litter input while the new trees that started growing were still young, and (b) increased decomposition of soil organic matter. The decomposition of surface litter would have been quite slow because although exposed to the sun and so warmer, it would also have readily dried in the open conditions. But the organic matter below would have been warm and moist (protected by the mulch of dry litter and with extra water that would have been taken up by the trees in an intact forest), excellent conditions for rapid decomposition. However, few studies have found these two assumptions to be reliably true. It seems much more likely that organic matter loss after tree felling may have been due to mixing of the organic matter into the mineral soil by mechanical disturbance of the felling operation, thus accelerating decomposition. If this is true, Covington's curve may be more of a reflection of changing forestry practices over time rather than natural changes in decomposition. This has very important implications for using Covington's data in predicting carbon budgets for natural forests where there is no mechanical disturbance. It also demonstrates the great difficulty in finding ways to accurately test and understand how whole forests work.

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