Kong, Angela (Univ. of California, Davis, Dept. of Plant Sciences, PES Bldg., One Shields Ave., Davis, CA, 95616; Phone: 530-754-7537; Fax: 530-752-4361; Email: aykong@ucdavis.edu)

Effect of Organic and Mineral Nutrient Sources and the Role of Aggregate Dynamics on Carbon Sequestration Across Conservation and Standard Tillage Farming Systems

A. Y. Kong*, S. J. Fonte, J. Six, D. C. Bryant, C. Van Kessel

'Developing sustainable cropping system management practices that enhance agricultural sustainability, reduce negative environmental impacts, and attenuate anthropogenic greenhouse gas (GHG) emissions rests to a large extent on our understanding of C turnover, N synchrony, and C and N storage in soils. This study focused on 1) elucidating the linkage between organic resources (OR), mineral fertilizer (MF) additions, tillage and aggregate dynamics and 2) how this linkage controls the cycling of N and the sequestration of C in Mediterranean cropping systems. Soil and GHG (CO₂ and N₂O) samples were collected from standard (ST) and minimum tillage (MT) treatments within organic (OMT), conventional (CMT), and low-input (LMT) maize-tomato cropping systems. Soil samples were separated into three aggregate size classes (macroaggregates: greater than 0.25mm, microaggregates: 0.053-0.25mm, and silt&clay: less than 0.053mm) and into three SOM fractions within the macroaggregate fraction (cPOM: greater than 0.25mm, mM: 0.053-0.25mm, and silt&clayM: less than 0.053mm). We calculated turnover of ¹³C- and ¹⁵N-labeled legume residues (OMT and LMT systems) and mineral fertilizer (CMT system) among the fractions and found faster turnover of ¹⁵N in the CMT systems than the OMT and LMT systems. Higher SOC, SON, and aggregate stability were observed in the OMT than the CMT and LMT systems, regardless of tillage practices. However, faster turnover of ¹⁵N was measured in the CMT systems than the OMT and LMT systems. Preliminary measurements indicated significant crop treatment effects on N₂O fluxes. Our results suggested that the LMT systems under CT led to an optimal rate of C and N cycling.