IMPACTS OF CROPPING INTENSITY ON CARBON AND NITROGEN MINERALIZATION UNDER NO-TILL DRYLAND AGROECOSYSTEMS

Imposing no-till and lower fallow frequency on soils previously managed under tilled and frequent fallow systems may alter soil organic C and N concentrations and activity (potential mineralization). This study was conducted to determine the effect of cropping intensity (number of crops/unit time) on surface soil (0-5 cm) C and N activity after 3.5 yr of no-till management. The effect was examined across three soil catenas in the West Central Great Plains that were previously managed under tilled and alternate crop-fallow systems for > 50 yr. Production systems included the less intensive wheat (Triticum aestivum L.)-fallow (WF), and the more intensive wheat-corn (Zea mays L.)-millet (Panicum miliaceum L.)-fallow (WCMF). After 3.5 yr of no-till, potential C and N mineralization, C turnover, and relative N mineralization were 61, 39, 36, and 43% greater under WCMF than WF, respectively. Footslope soils had greater potential C and N mineralization than summit or backslope soils, but lower C turnover and relative N mineralization, which was probably due to long-term accumulation of recalcitrant C and N compounds. Differences in potential soil C and N activity between cropping systems were due to greater surface organic C concentrations under WCMF (mean = 10.88 g kg-1) than WF (mean = 9.60 g kg-1), which were related to cumulative plant residue additions over the 3.5-yr-study period (mean = 9.01 and 7.04 Mg ha-1 for WCMF and WF, respectively). It appears that potentially active surface soil organic C and N are very sensitive to change in cultural practices, and are increased by greater cropping intensity under no-till management.