Box 83 Nutrient dynamics at Hubbard Brook

The long-term studies revealed that over the last half of the previous century the undisturbed northern hardwood forest showed a long-term net retention (i.e. more came in with precipitation than left in stream water) of hydrogen (H), chlorine (Cl), N and P, and net losses of Ca, Mg, Na, K, sulphur (S), silicon (Si) and Al. Likens (2004) suggests that the net loss of calcium is attributable to acid rain, reducing the input of Ca in precipitation and increasing its leaching from soil. Certainly the amount in stream water has declined over the past 50 years. This has also been associated with a marked decline in concentration and input of Ca in precipitation and similar declines in stream water (Likens et al., 1998). This is discussed further in Chapter 11.

One of the Hubbard Brook watersheds (W2) was clear-felled in 1965-66 and herbicided for 3 years to prevent any regrowth. Stream flow went up (due to reduced evapotranspiration) and decomposition, mineralization and nitrification were greatly accelerated producing high levels of hydrogen ions and nitrate which were rapidly lost in the streams. Net losses of nitrate, Ca and K generally peaked in the second year after felling with each returning to pre-cutting levels at rates unique to each ion. Even decades after clear-cutting, differences in stream water solutes can still be seen, especially in Ca.

An ice storm in January 1998 broke off around 28-30% of the canopy at Hubbard Brook. Following this, levels of nitrate in stream water increased to 7-10 times greater than predisturbance values, peaking over the following winter (Houlton et al., 2003). This was attributed not so much to increased microbial activity (since mineralization, nitrification and denitrification stayed the same) but to a decreased plant uptake of nitrate. It is salutary to note the flux of nitrate was still 2-10 times lower than that expected after complete canopy removal due to forest harvesting.

above by very low levels of P in tropical forests on old infertile soils). Vogt et al. (1986) note the same trend in N/P ratios in litter-fall (9-13/1 in boreal and temperate forests, 22-27/1 in the tropics and subtropics) and consequently in the forest floor organic matter (10-11/1 in coniferous and temperate forests and 31-32/1 in tropical forests). Whitmore (1998) points out that such broad-brush pictures can conceal a good deal of variation; for example phosphorus appears limiting in lowland rain forest but in montane rain forests nitrogen seems to be the limiting factor.

In total amounts, subtropical, tropical and Mediterranean forests have a higher concentration of nitrogen in litter-fall than in the forest floor, implying that nutrients are rapidly mineralized and taken back by plants. In temperate and boreal forests, the opposite is true: nitrogen is in higher concentrations in the forest floor (1.13-1.96%) compared with the litter (0.63-1.12%). This implies rapid immobilization and storage in the forest floor (see Vogt et al, 1986 for a worldwide review). So boreal soils may contain a great deal of nitrogen but productivity of the forest is still nitrogen-limited because of slow rates of mineralization (hence the value of the short-circuit outlined in Section 8.4.2). Indeed, boreal forests retain nitrogen for 40-50 times (and sometimes up to 100 times) longer than do temperate deciduous forests (average turnover times are 230 years and 5.5 years, respectively). Phosphorus does not follow the same pattern: in general terms, most forests have similar amounts of phosphorus whether comparing foliage, litter or forest floor except that the colder forest floors of boreal forests have a higher concentration.

Tropical forests deserve looking at in more depth. Whitmore suggests that there are two conventional myths about tropical rain forests and nutrients. Firstly, that most of the nutrients are in above-ground biomass, and secondly that these forests have closed nutrient cycles with little or no leakage. Both Charles Darwin and Alfred Russel Wallace, whose almost simultaneous formulation of the theory of evolution was announced at a meeting of the Linnean Society in 1858, and later elaborated by Darwin (1859), were immensely impressed by the magnificence and variety of species in tropical forests. This is made very clear in Darwin's account of the almost 5-year voyage of H. M. S. Beagle first published in 1839 (Darwin, 2003). Terborgh (1992) quotes Wallace at the beginning of his chapter entitled The Paradox of Tropical Luxuriance: 'The primeval forests of the equatorial zone are grand and overwhelming by their vastness and by the display of a force of development and vigour of growth rarely or never witnessed in temperate climates.' Yet the soils of these luxuriant rain forests contain very little in the way of mineral nutrients. Figure 2.12 shows that the popular belief that most of the nutrients in a tropical rain forest are in the biomass is seldom true except for certain minerals in certain forests but nevertheless, nutrients are in overall short supply as discussed above. The solution to this paradox of a rich forest over a poor soil is solved by very rapidly recycling when organisms die or parts of them are shed.

Terborgh (1992) demonstrates just how well the trees of particular tropical forests are related to their soils, quoting data concerning the availability of nutrients in the soils of seven forests. Though nutrient availability varies by several orders of magnitude, the above-ground biomass of the least fertile site (San Carlos) is only 20% less than that of the richest (Panama) and the forest of highest biomass (Ivory Coast) is on soil of intermediate fertility. How is this accomplished? The biomass of these forests above ground varies within a factor of 2.3, variation of that below ground is far greater (nearly 12); it is their well-developed and effective root systems that allow the above-ground biomass of tropical forests on extremely nutrient-poor soils to be so high. Nutrient recovery by tree roots is very efficient, aided by mats of roots above the soil surface. In an experiment in an Amazonian forest more than 99% of radioactivity applied as calcium-45 and phosphorus-35 isotopes to the ground surface was retained in the root mat. Termites and fungi, especially decomposer basidiomycetes, destroy dead wood above ground, while root absorption is often assisted by mycorrhizas.

Figure 8.8 illustrates inorganic nutrient cycling in rain forests. Note that in tropical rain forest, 99% of water reaching the ground does so as throughfall and leaches nutrients as it goes. In the New Guinea example shown in the figure, there is great enhancement: K and Mg 9 times, Ca and P 5 times and N 4.6.

^tAIN INPUT)

Canopy leaching

THROUGHFALL

FINE LITTERFALL Dry Weight 7550 N 91 P 5.1 K 28 Ca 95 Mg 19

FINE LITTERFALL Dry Weight 7550 N 91 P 5.1 K 28 Ca 95 Mg 19

FOREST FLOOR

FINE LITTER Dry Weight 6500 N 91 Ca 96 P 5 Mg 15 K 11

Canopy leaching

THROUGHFALL

CANOPY Dry weight 310 000 N 680 Ca 1300 P 37 Mg 190 K 660

CANOPY Dry weight 310 000 N 680 Ca 1300 P 37 Mg 190 K 660

Molde Ramos Arvore

MINERAL SOIL Organic matter 410 000 N 19 000 Ca 3700 P 16 Mg 680 K 400

ROOTS Dry Weight 40 000 N 140 Ca 330 P 6.4 Mg 61 K 190

Weathering ROCK ^

MINERAL SOIL Organic matter 410 000 N 19 000 Ca 3700 P 16 Mg 680 K 400

ROOTS Dry Weight 40 000 N 140 Ca 330 P 6.4 Mg 61 K 190

STREAMFLOW

Figure 8.8 Simplified diagram of the cycling of inorganic nutrients in a tropical rain forest. Figures (in kg ha-1 or kg ha-1 y-1) are for the lower montane rain forest at Kerigomna, New Guinea. (After Edwards, 1982. From Whitmore, 1984. Tropical Rain Forests of the Far East (2nd edn). Clarendon Press/Oxford University Press.)

The extra nitrogen in the throughfall is unusually high in this forest and is believed to be due to leaching from N-fixing algae epiphytic on the leaves (Whitmore, 1998). Most of the plant roots are in the top 0.1-0.3 m of soil since this is where the bulk of nutrients are released. As already discussed, old forests on well-weathered soils are dependent upon input from outside, particularly for phosphorus, and are less closed (a closed forest shows perfect internal cycling with no nutrient losses or gains). Many rain forests are on hilly terrain where the soil is shallow and continually being rejuvenated by creep and landslip and so unless the parent material is very poor (as are the sedimentary soils in various parts of Sarawak) these ecosystems have a continual input of nutrients. Silver et al. (1994) looked at nutrient availability in the tropical montane forest of Puerto Rico and found that cation concentrations and pH increased down the hillside from ridge tops to valley bottoms while soil organic matter and available iron (affected by the pH) decreased. The calcium, potassium and phosphorus content of the vegetation was higher on the ridges and slopes than in the valley bottoms.

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