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Effects of Disturbance on Nutrient Stocks in the Soil

To try to better understand nutrient dynamics following disturbance in the tropics, an 8-year study of nutrient dynamics before and after slash-and-burn agriculture was carried out in the lowland rainforest near San Carlos de Rio Negro, in the Venezuelan Amazon (Jordan 1989; Montagnini et al. 2000). While other types of disturbances in tropical forests have different intensities, sizes, and durations, and the quantities of nutrient loss and recovery are different, the overall pattern of nutrient dynamics may be similar, as evidenced by studies of nutrient dynamics in tropical pastures presented later in this section.

Shifting agriculture has been practiced in the tropics for many centuries. Shifting (also called "swidden", "slash-and-burn") agriculture is the predominant land-use practice on a large portion of the arable soils of the world and provides sustenance for many millions of people (Andriesse and Schelhaas 1987). Traditional shifting agriculture uses long forest fallows between short periods of farming. Long fallows make the traditional technique sustainable but also require extensive amounts of land. When land is scarce, farmers shorten forest fallows and lengthen agricultural periods, resulting in soil nutrient depletion, reduced crop yields, and increased weed invasion. Similar patterns are reported in the tropics worldwide, in spite of differences in ecological and socioeconomic conditions.

There have been many studies to try to explain decreases in productivity of shifting agriculture after relatively short periods of farming. There have been at least two major hypotheses to explain the sharp decreases in growth of crops like corn and rice after 2 or 3 years of shifting cultivation. One is that the decline in crop yield is due to competition from vigorous weeds that have invaded the site (Uhl et al. 1982). The other is that the decline is caused by nutrient losses due to leaching of calcium and potassium and volatilization of nitrogen (Nye and Greenland 1960). A third hypothesis combines the first two: there is substantial nutrient loss, but weedy species are more effective in scavenging low levels of nutrients in the soil.

Average temperature at the San Carlos site was 26 °C and the soil was classified as an Oxisol. Total standing stocks of calcium, potassium, nitrogen, and phosphorus were measured in the undisturbed forest and in the soil. Then an area of about a hectare was cut and burned by local farmers, and planted with manioc (Manihot esculenta), plantain (Musa paradisiaca), and cashew (Anacardium occidentale). The plot was farmed for 3 years, and then abandoned. Total stocks in the crops and weeds (successional vegetation) were measured yearly. Production of manioc roots declined from 1,465 kg/ha in the first year to 700 kg/ha in the third year. Biomass of the other crops was negligible, but biomass of weeds increased from 300 kg/ha in the first year to 990 kg/ha in the third.

Nutrient stocks at the experimental site as a function of time are shown in Fig. 2.11. In the undisturbed forest, most of the calcium and potassium was held in the biomass of the above-ground vegetation. Cutting and burning of the biomass converted most of the wood to ash, and, as a result, much of the calcium and potassium was quickly leached into the mineral soil and held temporarily on the surface of clays in an "exchangeable" form (easily replaced by hydrogen ions). The amounts in the mineral soil gradually decreased, and a year after abandonment, levels were close to those in the undisturbed forest. However, the total amounts in the soil at the time of abandonment were still greater than the total amounts in the crops during the first 2 years of cultivation, and were greater than the amounts in the soil of the undisturbed forest. Therefore it is unlikely that a shortage of calcium and potassium caused the decline in crop production.

The dynamics of nitrogen was different from the cations. Since most of the nitrogen is held in the organic matter of the soil, it was little affected by the cut and burn. The cultivation of the plot may have resulted in oxidation of the soil organic matter with a consequent loss of nitrogen, due to volatilization (Vitousek 1981). However, the quick recovery of nitrogen in the soil stocks during the third year of cultivation suggests a statistical anomaly. In any case, it would seem that lack of nitrogen was not a cause of decline of crop productivity.

In the undisturbed forest, over 83% of the 300 kg/ha stock of phosphorus was in the soil. However, little was available for uptake by roots, because most of it was bound by the iron and aluminum in the mineral soil. When the burned remains of the trees and the decomposing organic matter on the soil surface were almost gone, and crop production had declined, the local farmers gathered leaf litter from the surrounding forest and piled it around the stems of the manioc. As a result, plants resumed vigorous growth. An analysis of soil samples for phosphorus using a fractionation method that separated available from total stocks (Potter et al. 1991) was carried out. Soils were collected from undisturbed sites, from sites where leaf litter had been piled on bare soil, and from bare soil. Results showed that available forms of phosphorus were much higher where soil organic matter was present. A probable explanation is that organic acids leach from decomposing organic matter and react with iron and aluminum in the soil, chelating them, and thereby preventing them from reacting with phosphorus (Ae et al. 1990). As a result, phosphorus remains soluble and available to plant roots.

The conclusion was that the decline in crop productivity was due to neither nutrient loss through leaching and volatilization nor weed competition. The productivity decline was rather due to the fixation of phosphorus by iron and aluminum, following the disappearance of organic matter on and in the soil. Addition of more organic matter to the soil, as done customarily by the local farmers, resulted in leaching of organic acids, which in turn kept phosphorus available.

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