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Title:   Author  item
                                            WANG, JUN - Northwestern Universityitem
                                            Sainju, Upendra
  Submitted to: Soil Science
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 4/21/2014
                                    Publication  Date: 7/1/2014
                                    Publication URL: http://handle.nal.usda.gov/10113/59640Citation: Wang, J., Sainju, U.M. 2014. Aggregate-associated carbon and nitrogen affected by residue placement, crop species, and nitrogen fertilization. Soil Science. 179(3):153-165.Interpretive Summary: Carbon and N sequestration as a result of crop residue returned to the soil or through amendment application primarily occur in aggregates because soil is composed of various aggregate size classes. Mineralization of soil organic carbon (SOC) and total nitrogen (STN) is often protected by aggregates which reduce microbial access to the substrates that bind them. High variability in soil and climatic conditions results in non-significant difference in soil aggregate-associated carbon (C) and nitrogen (N) in the field. These changes may occur rapidly during a crop growing season under controlled soil and environmental conditions in the field.  
We evaluated the effects of crop species (spring wheat, pea, and fallow), N fertilization rate (0.11 and 0.96 g N pot-1), and residue placement (no residue, surface placement, and incorporation into the soil) and rate (2.5 and 5.0 g kg-1) on soil aggregation and C and N contents during a growing season under controlled soil and climatic conditions in the greenhouse. Soil samples collected from the field were grown with crops in the greenhouse and analyzed for aggregation, SOC, STN, particulate organic C (POC), and particulate organic N (PON) contents in aggregates. 
Residue C and N losses, proportion of macroaggregates (>0.25 mm), and soil C and N contents in microaggregates (<0.25 mm) were higher in surface residue placement (2.5 g kg-1) under pea with 0.11 g N pot-1 than the other treatments. The SOC and STN were greater in surface residue placement (5.0 g kg-1) under wheat with 0.96 g N pot-1 in large and intermediate macroaggregates (8.00-4.75 and 4.75-2.00 mm, respectively), PON was greater in surface residue placement (2.5 g kg-1) under pea with 0.11 g N pot in large macroaggregates, but POC was greater in residue incorporation (2.5 g kg-1) under fallow with 0.96% g N pot-1 in intermediate macroaggregate than the other treatments. 
Under controlled soil and environmental conditions, soil C and N levels in aggregates changed rapidly during a crop growing season. Surface residue placement increased soil aggregation and C and N storage with concurrent losses of residue C and N, but residue incorporation increased coarse organic matter fraction. Farmers may benefit by using these techniques to measure soil quality and health as affected by management practices.
                                Technical Abstract: High variability in soil and climatic conditions results in limited changes in soil aggregate-21 associated C and N levels as affected by management practices during a crop growing season in 22 the field. We evaluated the effects of crop species (spring wheat [Triticum aestivum L.], pea 23 [Pisum sativum L.], and fallow), N fertilization rate (0.11 and 0.96 g N pot-1), and residue 24 placement (no residue, surface placement, and incorporation into the soil) and rate (2.5 and 5.0 g 25 kg-1) on soil aggregation and C and N contents during a growing season under controlled soil and 26 climatic conditions in the greenhouse. Soil samples collected from the field were grown with 27 crops in the greenhouse and analyzed for aggregation and soil organic C (SOC), total N (STN), 28 particulate organic C (POC), and particulate organic N (PON) contents in aggregates. Residue C 29 and N losses, proportion of macroaggregates (>0.25 mm), and soil C and N contents in 30 microaggregates (<0.25 mm) were higher in surface residue placement (2.5 g kg-1) under pea 31 with 0.11 g N pot-1 than the other treatments. The SOC and STN were greater in surface residue 32 placement (5.0 g kg-1) under wheat with 0.96 g N pot-1 in large and intermediate 33 macroaggregates (8.00-4.75 and 4.75-2.00 mm, respectively), PON was greater in surface 34 residue placement (2.5 g kg-1) under pea with 0.11 g N pot in large macroaggregates, but POC 35 was greater in residue incorporation (2.5 g kg-1) under fallow with 0.96% g N pot-1 in 36 intermediate macroaggregate than the other treatments. Under controlled soil and environmental 37 conditions, soil C and N levels in aggregates changed rapidly during a crop growing season. 38 Surface residue placement increased soil aggregation and C and N storage with concurrent losses 39 of residue C and N, but residue incorporation increased coarse organic matter fraction.


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