Inorganic N was significantly higher under NH than under N0 and NL among N treatments at 0, 10, 45, and 200 days. Inorganic N was significantly lower under CH than under C0 and CL among C treatments. Mean residence time of the mineralizable organic N pool Soil organic carbon (SOC) is the largest organic C pool (1550 Pg of C) in terrestrial ecosystems (Batjes, 1996). The SOC pool actively interacts with atmospheric carbon dioxide through photosynthesis and decomposition processes. Thus, any change in these processes can lead to the change in the concentration of CO2 in the atmosphere. One of the major human activities on soil management is N fertilization. In general, N fertilization increases aboveground biomass production and the SOC pool after N application (Alvarez, 2005). However, the effect of N addition on decomposition (loss of SOC) is inconsistent and often contradictory (Mack et al., 2004; Fog, 1988; Hobbie, 2008). Objective Examine the effect of N addition on switchgrass roots and SOC decomposition Hypothesis Increased N availability by N addition prevents the decomposition of less available pools of C, soil organic matter. Methods Soil for incubation (from Jackson, OH) –Soil texture: Silt loam (sand 14%, silt 70%, and clay 16%) –TC 2.22%, TN 0.24% (C:N = 9.21:1) –50 g (oven dry weight basis), 2 mm sieved, field-moist soil Aerobic incubation Incubation periods: 200 days Moisture content : kPa (30-35% MC) CO 2 Measurement: a closed dynamic chamber method (Gas chromatography) Destructive sampling (5 times) after 0, 10, 45, 100, 200 days –Inorganic N (NO 3 -, NH 4 + ) (KCl extraction), –pH (1:2) Treatments 5 sets 3 replicates N source (KNO3) mg N/g soil (kg N/ha) C source: N0 0 (0) NL (25) NH (100) switchgrass root mg root/g soil C0, 0C0N0C0NLC0NH CL, 5CLN0CLNLCLNH CH, 10CHN0CHNLCHNH Conclusions Adding N shortened the MRT of the organic N pool and decreased CO 2 production. These opposite responses to soil organic C and organic N pools by N addition showed that the N-mining theory was not the best explanation in this study. N addition may increase SOC sequestration by reducing SOC decomposition, but the mechanisms need to be studied further. However, the inhibition of SOC decomposition by N addition may not be very influential under the field conditions, since plants will uptake available N quickly. 4. C budget Parameter C0 CL CH N0NLNHN0NLNHN0NLNH Soil CA22.2 Root CB Total CA+BA+B C lossC (mg C/g soil) C loss, %C/(A+B) cccbbbaab Root C loss, mg/g C-C in C Root C loss, % (C-C in C 0 )/B Effects of inorganic N on soil organic carbon (SOC) and root decomposition Ji Young Jung* and Rattan Lal School of Environment and Natural Resources, The Ohio State University, Columbus, OH *Corresponding author: Introduction Statistics Two-way ANOVA with two factors (C, N) Tukey test for the mean comparison (p<.05) 1. CO 2 -C efflux rate N effectC effect 3. pH N effect C effect N effect C effect 2. Inorganic N (NH 4 + and NO 3 - ) MRT (day) N0NL C CL17994 CH A single pool first order exponential model N t = N i + N o (1-e -t/MRT ) N i : the initial inorganic N content N t : quantity of extractable inorganic N at time t MRT: the steady state MRT of the mineralizable organic N pool (N o ) Results Special thanks to: SEEDS Grant for graduate students from Ohio Agricultural Research and Development, Virginie Bouchard, Peter Curtis, Dave Barker, Julie Jastrow, David Ussiri, Klaus Lorenz, Nicola Lorenz, Keunyae Song, Kyungsoo Yoo, Sindhu Jagadamma for their help with this study. N effect on CO2 efflux rate was evident mostly until 45 days, after that the N effect was not obvious (CO2 efflux rate in NL (0.021 mg N/g soil) and N0 (no addition of N) > NH (0.083 mg N/g soil) at 1, 5 – 45, and 140 days). C treatments had significant effects on CO2-C efflux rate through the whole incubation period (CO2 efflux rate in CH (10 mg root/g soil) > C0 (no addition of root). The CH treatment released the highest amount of CO2 in total. On the other hand, the NH treatment tends to produce less CO2 although the N effect was not significant within each C treatment % of root C was lost through the whole incubation. Adding high N also decreased the decomposition of root C as well despite no statistical difference. The decrease in pH was more significant under less C addition among C treatments. Among N treatments, the pH under NH was the lowest. References Alvarez, R Soil Use Manage. 21:38-52Batjes, N.H Eur. J. Soil Sci. 47: Fog, K Biol. Rev. 63: Hobbie, S.E Ecology 89: Mack, M.C., E.A.G. Schuur, M.S. Bret-Harte, G.R. Shaver and F.S. Chapin Nature 431: