Soil-specific effects of urea addition on mineralization of aromatic and proteinaceous components of humic-like substances in three agricultural soils

Abstract

Humic substances (HS) are the most abundant components of soil organic carbon (SOC). Intensive use of inorganic N fertilizers in agricultural soils can alter stability and turnover of HS and hence C sequestration. Using specif- ically 14 C-labeled humic-like substances ( 14 C labeled in the aromatic or the proteinaceous component) as model HS, the effects of urea addition (0.1 and 0.2 mg N g �1 soil) on miner- alizationandtransformationofthearomaticandproteinaceous components of HS in three soil types (Anthrosols, Fluvisols, and Chernozems) were investigated. Our results showed that the urea addition had no effect on the mineralization of the aromatic and the proteinaceous components in the Anthrosol, whereas it significantly (P<0.05) inhibited (13.1-17.7 %) the mineralization of the two components in the Fluvisol. Urea addition (0.2 mg N g �1 soil) significantly (P<0.05) stimulated the mineralization of the aromatic (8.1 %) and the proteina- ceous (9.0 %) components in the Chernozem. The amounts of 14 Cindissolvedorganicmatter(DOM)inthethreetestedsoils were reduced after urea addition, probably because specific microbial species incorporating humic-like substances into DOM were inhibited by urea. The soil-specific humic-like substancesmineralizationafterureaadditionisconsistentwith the reported SOC change patterns of soil, indicating that the variable effects of urea addition on certain components of SOC could be partly responsible for changes in SOC contents induced by inorganic N fertilizers addition. Keyword 14 C.Decomposition .Transformation .N addition .Soilorganiccarbon