ORIGINAL PAPER Soil aggregates control N cycling efficiency in long-term conventional and alternative cropping systems Angela Y. Y. Kong Æ Steven J. Fonte Æ Chris van Kessel Æ Johan Six Received: 12 November 2006 / Accepted: 5 February 2007 / Published online: 3 March 2007 Ó Springer Science+Business Media B.V. 2007 Abstract This paper presents novel data illustrat- ing how soil aggregates control nitrogen (N) dynamics within conventional and alternative Med- iterranean cropping systems. An experiment with 15 N-labeled cover crop residue and synthetic fertil- izer was conducted in long-term (11 years) 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 nitrous oxide (N 2 O) samples were collected throughout the maize growing season. Soil samples were separated into three aggregate size classes. We observed a trend of shorter mean residence times in the silt-and-clay fraction than macro- (>250 lm) and microaggregate fractions (53–250 lm). The majority of synthetic fertilizer- derived 15 N in the conventional system was asso- ciated with the silt-and-clay fraction (<53 lm), which showed shorter mean residence times (2.6 months) than cover crop-derived 15 N in the silt-and-clay fractions in the low-input (14.5 months) and organic systems (18.3 months). This, combined with greater N 2 O fluxes and low fertil- izer-N recoveries in both the soil and the crop, suggest that rapid aggregate-N turnover induced greater N losses and reduced the retention of synthetic fertilizer-N in the conventional system. The organic system, which received 11 years of organic amendments, sequestered soil organic car- bon (SOC) and soil N, whereas the conventional and low-input systems merely maintained SOC and soil N levels. Nevertheless, the low-input system showed the highest yield per unit of N applied. Our data suggests that the alternating application of cover crop-N and synthetic fertilizer-N in the low- input system accelerates aggregate-N turnover in comparison to the organic system, thereby, leading to tradeoffs among N loss, benefits of organic amendments to SOC and soil N sequestration, and N availability for plant uptake. Keywords Aggregate dynamics  Long-term cropping system  Mean residence time  Plant nutrient uptake  Soil organic matter sequestration  Soil nitrogen cycling Introduction Mounting concerns about rising greenhouse gas emissions, environmental degradation, and rural economic decline associated with modern conventional farming practices have warranted a critical need for management practices that enhance land value for producers, while promot- ing long-term agricultural sustainability and A. Y. Y. Kong (&)  S. J. Fonte  C. van Kessel  J. Six Department of Plant Sciences, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA e-mail: aykong@ucdavis.edu 123 Nutr Cycl Agroecosyst (2007) 79:45–58 DOI 10.1007/s10705-007-9094-6