Role of Mineral-Nitrogen in Residue Decomposition and Stable Soil Organic Matter Formation

of plant residue C and N. Application of 15N-labeled fertilizer or maize residue together resulted in more than The role of mineral fertilizer-N inputs and N deposition to agricultwice as much 15N recovered from fertilizer than from tural and natural ecosystems can affect plant residue decomposition and soil C processes. Yet it is still unclear whether residue-N or residue in the SOM (Dourado-Neto et al., 2001). mineral-N is preferentially stabilized during the formation of soil organic At the least, residue-N will affect mineral-N transformatter (SOM). We undertook a 90-d incubation of 13C15N-labeled rice mations and vice versa in the soil through the microbial (Oryza sativa L.) straw residue and N-(NH4)2SO4 under standard biomass (MB). Inorganic-N availability may be regutemperature and moisture conditions to determine: (i) the role of lated by organic-N availability through the regulation mineral-N as an N source for stable SOM in the presence of residue-N; of proteolytic enzymes (Smith et al., 1989). Azam et al. and (ii) whether mineral-N inputs can enhance sequestration of resi(1985) found that in the presence of legume residues, due-C. Soil C respiration was measured frequently and organic matter (OM) was fractionated into particulate organic matter (POM), humic more ammonium sulfate-N (19%) was transformed into acid, fulvic acid, and humin before and following the incubation. humic compounds than applied without legume residue. Stable C and N isotopic analyses were performed on CO2–gas samples The authors attributed this to greater microbial activity and SOM fractions. We found significantly greater residue-C was when plant residue was added. Microbial preference for transformed into humin-C with mineral-N input suggesting that minthe N source that is immediately available and can be eral-N enhances residue decomposition and favors SOM formation. assimilated without prior chemical transformation may 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 lead to greater uptake of mineral-N than residue-N. Since rate of accumulation in stable SOM. Our results suggest that mineral-N microbial by-products are thought to be a major contribinputs have a positive impact on the transformation of residue C into utor to stable SOM formation (Nelson et al., 1979; Stemore stable SOM and that the combined addition of mineral N and venson, 1994), mineral-N may be preferentially stabiresidue enhance SOM formation. lized over residue-N. Changes in the POM and mobile humic acid fractions N inputs to ecosystems play an important can show shifts in C dynamics under different treatrole in the decomposition of plant residues (Carments. The POM fraction has been shown to be a more reiro et al., 2000; Fog, 1988; Lueken et al., 1962). Inputs sensitive indicator of the effects of mineral-N additions of mineral-N to agricultural and natural systems have on soil C and N than total SOM (Malhi et al., 2002). increased as a consequence of anthropogenic activities. Recent research has also shown that the N content of How increased N-inputs affect litter decomposition and NaOH-extractable mobile humic fractions derived from SOM dynamics is still poorly understood and requires fertilizer-N are affected by crop residue management further study (Adams, 2003; Neff et al., 2002). Few studpractices (Bird et al., 2003; Devevre and Horwath, 2001). ies have looked at the simultaneous fates of decomposThe POM fraction was shown to cycle the greatest amount ing residue-C and -N (Bird et al., 2003), and fewer still of fertilizer-N followed by humic and fulvic acids, and have looked at the effect of added mineral-N on resithen humin (Bird et al., 2002). Since humic fractions are due-C and residue-N transformations. transformed during NaOH-extraction from soil, these Plant residue-N is the main source of N in stable SOM pools are not a direct measure of original SOM. Humic (Angers et al., 1997; Bird et al., 2003), especially in fractions are chemically extracted without any relationnatural systems. But, it is not well understood how N ship to soil structure or biological activity. Nevertheless, from residue versus mineral sources not associated with NaOH-extractable humic fractions are useful to characC, such as fertilizers and deposition, are transformed terize OM pools representing a range of turnover rates into stable SOM when both are present. Past research and recalcitrance. suggests that there is an opportunity for mineral-N to The objectives of this study were to determine: (i) be preferentially sequestered in stable SOM over resithe role of mineral-N as an N source for stable SOM-N due-N. Bird et al. (2003) reported divergent pathways of residue C and N during the formation of SOM sugin the presence of residue-N; and (ii) whether mineral-N gesting an uncoupling of processes that affect the fate inputs could potentially increase the movement of residue-C into more stable fractions of SOM-C. We hypothesized that mineral-N contributes more to stable SOM K.K. Moran, J. Six, and C. van Kessel, Dep. Plant Sciences, Univ. of California, Davis, CA 95616; W.R. Horwath, Dep. of Land, Air and formation than residue-N. To test this hypothesis, a 90-d Water Resources, Univ. of California, Davis, CA 95616. Received 7 incubation study using 13C15N-labeled rice residue and Sept. 2004. *Corresponding author ([email protected]). N-(NH4)2SO4 was conducted. Published in Soil Sci. Soc. Am. J. 69:1730–1736 (2005). Soil Biology & Biochemistry doi:10.2136/sssaj2004.0301 Abbreviations: MB, microbial biomass; mSOM, mineral-associated soil organic matter; OM, organic matter; POM, particulate organic © Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA matter; SE, standard error; SOM, soil organic matter. 1730 Published online September 29, 2005