Role of mineral-nitrogen in residue decomposition and stable soil organic matter formation

The role of mineral fertilizer-N inputs and N deposition to agricultural and natural ecosystems can affect plant residue decomposition and soil C processes. Yet it is still unclear whether residue-N or mineral-N is preferentially stabilized during the formation of soil organic matter (SOM). We undertook a 90-d incubation of 13C15N-labeled rice (Oryza sativa L.) straw residue and 15N-(NH4)2SO4 under standard temperature and moisture conditions to determine: (i) the role of mineral-N as an N source for stable SOM in the presence of residue-N; and (ii) whether mineral-N inputs can enhance sequestration of residue-C. Soil C respiration was measured frequently and organic matter (OM) was fractionated into particulate organic matter (POM), humic acid, fulvic acid, and humin before and following the incubation. Stable C and N isotopic analyses were performed on CO2-gas samples and SOM fractions. We found significantly greater residue-C was transformed into humin-C with mineral-N input suggesting that mineral-N enhances residue decomposition and favors SOM formation. We found no preferential transformation of mineral-N over residue-N into SOM, but the two N sources together interact to alter each other's rate of accumulation in stable SOM. Our results suggest that mineral-N inputs have a positive impact on the transformation of residue C into more stable SOM and that the combined addition of mineral N and residue enhance SOM formation.