Anthropogenic influences on American Indian agricultural soils of the Southwestern United States
This study focused on determining and assessing anthropogenic influences on soil quality in two American Indian agricultural systems of the Southwest U.S. One is a runoff system in the Zuni area of New Mexico where runoff farming has been practiced for over two millennia, and the other is an ancient rock mulch system in southeast Arizona that was abandoned over 500 years ago. Results of the Zuni study indicate that cultivation has had both positive and negative effects on soil productivity. Relative to uncultivated soils, cultivated soils tend to have slightly elevated bulk density and pH levels, and inconsistent changes in N and organic C. Soil changes at the levels found are not sufficient to indicate that cultivation caused degradation. Potential negative impacts are offset to varying degrees by thickened topsoils, co-sedimentation of organic matter and silt in fields, and organic matter coatings on peds;Extensive rock mulch features (grids, terraces, and rock piles) were built to conserve water and nutrients in the shallow rooting zone of the Safford fields of Arizona. Compared to uncultivated soils, mulched soils have elevated C, N, and available P concentrations and no evidence of soil compaction. Existing vegetation concentrated in the rock mulch features today demonstrates their effectiveness in conserving moisture and nutrients. There is no evidence that ancient rock mulch farming in Arizona caused soil degradation, and it appears that agricultural practices actually improved soil quality for crop production;An ancillary study was undertaken to measure soil changes caused by the western harvester ant (Pogonomymex occidentalis). This research aimed to determine their effect on soil productivity in the context of agricultural land use and landscape modifications. Results indicate that ant-affected soils have elevated levels of organic C, N, and available and total P, so they have a positive influence on agricultural soils. In addition to nutrient enrichment, ants help to aerate the soil and increase its hydraulic conductivity and water-holding capacity. Ant effects on surface soils extend to entire landscapes within about 2500 years, which is within the time frame of agricultural practices in the Zuni area.