Wright, Alan (Texas A&M Univ., Dep. of Soil and Crop Sciences, Texas A&M Univ, College Station, TX, 77843-2474; Phone: 979-845-8738; Fax: 979-845-0456;

Email: awright@ag.tamu.edu)

Tillage, Cropping Sequence, and Fertilization Effects on ¹³C Abundance of Soil Physical Size Fractions

F. Dou, A.L. Wright *, F.M. Hons

Increasing soil organic C (SOC) not only plays an essential role in soil nutrient cycling, but also in mitigating the increasing concentration of atmospheric CO₂. Agricultural practices that decrease soil disturbance and increase crop residue input enhance soil C sequestration. Soil physical size fractionation coupled with ¹³C natural abundance was utilized to study the distribution of crop residue into and the turnover of SOC fractions in continuous sorghum [Sorghum bicolor (L.) Moench.] (CS) and wheat [Triticum aestivum (L.)] (CW), continuous wheat/soybean [Glycine max (L.) Merr.] (WS), and wheat/soybean-sorghum (SWS) sequences under conventional (CT) and no tillage (NT) with and without N fertilizer in a long-term (20-year) experiment. Only surface (0-5 cm) soil samples were studied. ¹³C abundance of protected and unprotected resistant organic C (ROC), protected and unprotected mineral-associated C, microaggregate C, and protected (PPOC) and unprotected particulate organic C (UPPOC) were determined. Differences in ¹³C concentrations of crop residues significantly affected ¹³C concentrations of SOC in all size fractions, with greatest differences observed in more labile pools. Carbon turnover rates increased in the sequence: ROC less than silt- and clay-associated C less than microaggregate C less than POC. Tillage effects on ¹³C values were also observed primarily in the labile pools. Compared to CT, greater ¹³C values were found in POC under NT, except in UPPOC in CS and in SWS. Compared to crop sequence and tillage, effect of N addition on ¹³C of labile pools was minimal. Our study indicated that soil physical size fractionation, in conjunction with ¹³C analysis, can improve our understanding of SOC dynamics.