Monday, November 13, 2006 - 10:45 AM

Soil Aggregates Control N Cycling Efficiency in Long-Term Conventional and Alternative Cropping Systems.

Angela Y. Y. Kong, Steven Fonte, Chris van Kessel, and Johan Six. Univ of California, One Shields Ave, Dept of Plant Sciences, Davis, CA 95616

We present novel data illustrating how soil aggregates control N dynamics and availability for plant uptake by using 15N-labeled cover crop residue and synthetic fertilizer across conventional and alternative cropping systems.  The experiment was conducted in long-term maize-tomato rotations: conventional (synthetic N only), low-input (reduced synthetic and cover crop-N), and organic (composted manure- and cover crop-N).  Soil and N2O samples were collected during the maize growing season and the soil samples were separated into three aggregate size classes.  Crop fertilizer-N use efficiency was not different among the three cropping systems.  The majority of synthetic fertilizer-derived 15N in the conventional system (CMT) was associated with the silt-and-clay fraction, which showed shorter mean residence times (2.6 months) than cover crop-derived 15N in the silt-and-clay fraction in the organic (OMT) (18.3 months) and low-input (LMT) (14.5 months) systems.  The latter results, plus greater N2O fluxes and low fertilizer-N recoveries in the soil and crop, suggest that rapid aggregate-N turnover induced greater losses of N and reduced the amount of synthetic fertilizer-N retained in the CMT system.  Also, N-fertilizer type influences SOC and soil N sequestration as the OMT system sequestered 5.7 MgSOCha-1 and 0.59 MgNha-1 after 11yrs of continuous organic amendments (~0.47 MgNha-1yr-1), whereas SOC and N levels were merely maintained in the CMT system, which received 0.29 MgNha-1yr-1 of synthetic fertilizer-N.  In contrast, SOC and N levels declined in the LMT system after the 11yrs of cropping, despite showing intermediate rates of aggregate-N turnover and the highest yield N-1 applied relative to the CMT and OMT systems.  The application of cover crop-N, alternating with synthetic fertilizer-N, accelerates aggregate-N turnover, thereby, leading to tradeoffs in the LMT system, between the benefits of organic amendments to SOC and N sequestration and N availability for plant uptake versus N susceptibility to loss.