I

77 78 79 80 81 82

Year

Crops

Successional vegetation Cumulative harvested Cumulative rainfall-leaching Cumulative fixation-denitrification

Trunks of primary forest IIIII^ Roots of primary forest I R I Leaves of primary forest Organic matter in root mat I M I Humus in soil I H I Soil exchangeable l/V/l Soil total M

Fig. 2.11. Stocks and cumulative losses of potassium, calcium, and nitrogen as a function of time in the experimental plot at San Carlos de Rio Negro, Venezuela. (Adapted from Jordan 1985, with permission of John Wiley and Sons Ltd., publisher)

Studies in other regions of the Amazon Basin following conversion of forest to pasture suggest a similar pattern (Fig. 2.12). When phosphorus is supplied to the pasture, productivity can remain relatively high (Fig. 2.13), but if it is not, phosphorus availability declines (Fig. 2.12). A common practice in the Amazon when pasture production declines is to burn the shrubs that have invaded the pasture, thus adding a small pulse of nutrients to the soil (Hecht 1984). However, if nitrogen is not replenished, the stocks gradually decline. Although stocks of nitrogen are slow to diminish, they are also slow to be replenished following abandonment of the pasture.

Because of the low nutrient-holding capacity of the soils at San Carlos de Rio Negro, the time scale of the nutrient dynamics may not be typical for all lowland rain forests. Nevertheless, the pattern should not vary. Potassium, a

Amazon forest Burning

Amazon forest Burning

Fig. 2.12. Changes in available phosphorus following conversion of forest to pasture in the Amazon region of Brazil on various soil types: line without symbols yellow La-tosol (Oxisol), very heavy texture; line with circles red-yellow podzolic (Ultisol), medium texture; line with triangles dark red Latosol (Oxisol), medium texture. Levels of P for sustainable pasture: dotted and dashed line standard level; dashed line critical level. (Adapted from Jordan 1985, with permission of John Wiley and Sons Ltd., publisher)

Fig. 2.12. Changes in available phosphorus following conversion of forest to pasture in the Amazon region of Brazil on various soil types: line without symbols yellow La-tosol (Oxisol), very heavy texture; line with circles red-yellow podzolic (Ultisol), medium texture; line with triangles dark red Latosol (Oxisol), medium texture. Levels of P for sustainable pasture: dotted and dashed line standard level; dashed line critical level. (Adapted from Jordan 1985, with permission of John Wiley and Sons Ltd., publisher)

Fig. 2.13. Model of the dynamics of the animal-forage grass-soil system in an Amazon Oxisol, with and without the addition of phosphorus and nitrogen-fixing species. Line with open circles Traditional pasture at optimum grazing pressure; line with solid circles traditional pasture at grazing pressure above optimum; line with triangles improved pasture at optimum grazing pressure; solid line improved pasture at grazing pressure above optimum. Improved pasture means grass and legume plus phosphorus; traditional pasture means grass (Panicum maximum). (Adapted from Jordan 1985, with permission of John Wiley and Sons Ltd., publisher)

Fig. 2.13. Model of the dynamics of the animal-forage grass-soil system in an Amazon Oxisol, with and without the addition of phosphorus and nitrogen-fixing species. Line with open circles Traditional pasture at optimum grazing pressure; line with solid circles traditional pasture at grazing pressure above optimum; line with triangles improved pasture at optimum grazing pressure; solid line improved pasture at grazing pressure above optimum. Improved pasture means grass and legume plus phosphorus; traditional pasture means grass (Panicum maximum). (Adapted from Jordan 1985, with permission of John Wiley and Sons Ltd., publisher)

monovalent cation, is easily leached, and will decrease rapidly. Calcium will be also leached but more slowly since it is a divalent cation and is held more firmly in the soil. Nitrogen, in contrast to other nutrients, has its greatest concentration in the soil. Clearing and burning of the aboveground biomass have little immediate effect on stocks of nitrogen in the soil. It is lost slowly, but once it is gone, it is slow to recover. Phosphorus is often a critical element, and its loss due to fixation will vary with soil type (Brady and Weil 2001).

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