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.