Lignin from rice straw Kraft pulping: Effects on soil aggregation and chemical properties

Lignin contained in pulping liquor that is generated during the pulping process for papermaking is a disposal problem for the pulp and paper industry. Separating lignin and other organic components from pulping liquor with inorganic acids may improve its applicability to fields as a beneficial soil amendment while offering a potential disposal alternative. Sulfuric acid-precipitated lignin from rice straw pulping liquor applied at rates of 1.67 and 3.34 g C kg−1 soil was incubated to evaluate its effects on soil properties over 8 weeks of incubation. Addition of this acid-precipitated lignin at these rates decreased soil pH by 0.24–0.53 units over 8 weeks of incubation, suggesting that this sulfuric acid-precipitated lignin from pulping liquor may have potential as a soil acidifying agent. Soil electrical conductivity (EC) only increased by up to 0.36 dS m−1, but highest EC levels were less than 4 dS m−1, indicating that lignin applied at both rates would not cause salinity problems. Application of this lignin increased soil organic C by 1.46 and 3.13 g C kg−1, and total soil N by 0.07 and 0.17 g N kg−1 over the incubation period. Lignin improved the macroaggregation of >2 mm size fraction, and increased wet microaggregate stability of >2 mm and 0.5–0.25 mm aggregates compared to a nonamended control. The results of this study suggest that this acid-precipitated lignin from pulping liquor may have potential as a beneficial soil amendment.