Molecular and Isotopic Analysis of Mobilized Organic Matter in Overland Flow, Tile Drainage, and Basin Discharge from Leary Weber Ditch, Indiana

Keith D Crooker, Purdue University

Abstract

Few studies integrate land cover, soil physical structure, and aquatic physical fractions when investigating the fate of agricultural carbon in watersheds. In crop systems involving soy and corn rotation (C3 and C4 plants respectively), the differences in stable carbon isotopes and lignin chemistry enable tracking of mobilized plant carbon from senescent surface residue to soil and ultimately to aquatic systems during precipitation events. In a small 3.6 km2 agricultural watershed in central Indiana, we investigated the export of organic matter (OM) via overland flow and tile drain seepage over a 16-month period. Alternating years of corn and soy rotation were investigated and the fate of plant carbon was tracked from the upper 0-5 cm active layer of surface soil in a 2-hectare subplot, to colloidal (0.1–0.7 µm) and dissolved (0.1µm–1,000 Dalton) aquatic size fractions using lignin phenol chemistry and the natural abundance of 13C. Soils were fractionated into macroaggregate (>250 µm), microaggregate (53–250 µm), and silts plus clay (<53 >µm) size fractions, and using a simple two-member isotope mixing model, corn-derived soil organic carbon was found to range from 51, 39, to 33%. Aquatic OM collected at the outlet of the watershed showed that relative contribution of C4 (corn) varied from 0 to 85% depending upon discharge. Corn-derived lignin was also found to be dominated by cinnamyl and syringyl biomarkers. There was observed trend of increasing corn residue contribution with discharge in overland flow, and the carbon mobilized from subterranean flow tile drains had a consistently lower contribution of C4-derived carbon with respect to overland flow. Based upon the relative abundance of oxidized (acidic) to reduced (aldehydic) lignin, there was a progressive increase in oxidation state with decreasing size fraction in soils and aquatic fractions. An increased contribution of corn-derived carbon to aquatic OM relative to other C3 material (e.g. soy, relict agriculture, or historic plants) at the watershed outlet could only be detected when total stream discharge exceeded 0.531 m3sec−1. Finally, estimates indicate that the largest contribution of mobilized carbon to the watershed outlet came from surface runoff during punctuated hydrologic events.

Degree

M.S.

Advisors

Filley, Purdue University.

Subject Area

Ecology|Biogeochemistry|Environmental science

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