Organic carbon effects on soil physical and hydraulic properties in a semiarid climate

Increasing cropping intensity in the central Great Plains of the United States has the potential to increase organic carbon (OC) stored in the soil and lead to improved soil physical properties. A Cropping Systems Study was started in 1990 at the Central Great Plains Research Station near Akron, CO. In 2005 soil samples were taken in six 95-mm increments to a depth of 370 mm to measure OC, water stable macroaggregates (water stable aggregates > 250 mu m), bulk density (rho(b)), total porosity (phi(total)), water storage porosity (phi(ws)), and saturated hydraulic conductivity (k(sat)). Samples we re collected from permanent grass plots {45% smooth brome [Bromus inermis (Leyss.)], 40% pubescent wheat grass [Agropyrons trichophorum (Link) Richt.], and 15% alfalfa [Medicago sativa (L)]}, plots in a wheat {[Triticum aestivum (L)]-corn [Zea mays (L.)]-millet [Panicum miliaceum (L)]} rotation, and pious in a wheat-fallow rotation. Increased cropping intensity, significantly increased OC, water stable macroaggregates, and k(sat), but had no significant effect on rho(b,) phi(total), or phi(ws). Permanent grass increased OC compared with the annually cropped rotations, particularly deeper in the soil. Plots in permanent grass had greater k(sat) and this may indicate greater pore continuity, and stability under saturated conditions. Organic carbon and water stable macroaggregates were poorly correlated. Water stable macroaggregates was negatively correlated with rho(b), and positively correlated with k(sat). Increasing soil OC may not immediately lead to changes in soil aggregation in a semiarid climate. Increased time and biological activity may be needed to convert the crop residues into organic compounds that stabilize aggregates and soil pore systems.