Soil hydraulic response to land-use change associated with the recent soybean expansion at the Amazon agricultural frontier

Abstract

Clearing for large-scale soy production and the displacement of cattle-breeding by soybeans are major features of land-use change in the lowland Amazon that can alter hydrologic properties of soils and the runoff generation over large areas. We measured infiltrability and saturated hydraulic conductivity (Ksat) under natural forest, pasture, and soybeans on Oxisols in a region of rapid soybean expansion in Mato Grosso, Brazil. The forest-pasture conversion reduced infiltrability from 1258 to 100 mm/h and Ksat at all depths. The pasture-soy conversion increased infiltrability from 100 to 469 mm/h (attributed to shallow disking), did not affect Ksat at 12.5 cm, but decreased Ksat at 30 cm from 122 to 80 mm/h, suggesting that soybean cultivation enhances subsoil compaction. Permeability decreased markedly with depth under forest, did not change under pasture, and averaged out at one fourth the forest value under soybeans with a similar pattern of anisotropy. Comparisons of permeability with rainfall intensities indicated that land-use change did not alter the predominantly vertical water movement within the soil. We conclude that this landscape is well buffered against land-use changes regarding near-surface hydrology, even though short-lived ponding and perched water tables may occur locally during high-intensity rainfall on pastures and under soybeans.

Leaching losses from Kenyan maize cropland receiving different rates of nitrogen fertilizer

“…Soil type controls nutrient soil solution flux by the degree to which it retains ions and inhibits fluid flow, particularly in response to seasonal and pulsed rainfall. Clay-rich soils tend to have lower leachate loss rates than coarser textured soils and because of greater residence time of nutrients in soil solution and lower total flux (Simmelsgaard 1998), but aggregation of clays, particularly weathered clays of tropical soils, can also lead to very high infiltration rates Scheffler et al 2011). Clay-rich soils also will typically retain more base cations than sandy soils of the same mineralogy because clays contribute to fixedcharge cation exchange capacity (Brady and Weil 2007).…”

The fate of phosphorus fertilizer in Amazon soya bean fields

“…We additionally hypothesized that the high P-binding capacity of soils would keep P surpluses in the upper soil, and that P transport deeper in the soil column would be relatively small [34]. Based on our previous measurement of low dissolved P losses from soya bean streams [35] and minimal occurrence of overland flow [36,37], we hypothesized that losses to streams would be low.…”

“…In fact, with no movement of P below 20 cm and with more than 20 m of soil and almost exclusively vertical flow paths [36,37], it does not appear that soya bean cultivation is increasing the risk of P losses to surface waters.…”

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“…Estes resultados estão de acordo com diversos trabalhos que demonstraram que há maior geração de escoamento superficial em áreas de cultivo em relação a áreas sob floresta (e.g. Zimmermann et al, 2006;Chaves e Piau, 2008;Zimmermann e Elsenbeer, 2008;Scheffler et al, 2011;Hassler et al, 2011).…”