Carbon and nitrogen stable isotope ratios can estimate anionic polyacrylamide degradation in soil

Abstract

Water-soluble anionic polyacrylamide (PAM) is applied to millions of hectares worldwide as a highly effective erosion-preventing and infiltration-enhancing polymer, when applied at rates of 1 to 2 kg ha−1 (i.e., 1 to 10 g m−3) in furrow water. PAM degradation has not directly been measured in soil. We tested the ability of stable isotopes of C and N at natural abundance to estimate PAM degradation rates. Values of δ13C were related to anionic PAM concentration in a positive curvilinear relationship in a low-C, low-N, Durinodic Xeric Haplocalcid (Portneuf series) soil. The other soils with higher organic C or N concentrations did not show significant relationships between PAM concentrations and δ13C values. The δ15N values were not related to anionic PAM concentration in any of the soils tested. When 2691 active ingredient (ai) kg PAM ha−1 was applied to the Durinodic Xeric Haplocalcid soil, 49% and 74% of the PAM was degraded after 6 and 12 yr respectively. When 5382 kg ai PAM ha−1 was applied to the Durinodic Xeric Haplocalcid soil, 13% was degraded after 6 yr, and 73% of the PAM was degraded after 12 yr. We calculated PAM degradation rate based on δ13Cfor the Durinodic Xeric Haplocalcid soil to be 9.8% yr−1. Further testing using labeled PAM is necessary to estimate degradation rates in higher C soils, and to determine what portion of the C is released from decomposing PAM is emitted to the atmosphere, incorporated into soil organic matter and living microbial biomass.