Mean residence time of the mineralizable organic N pool

1. 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: jung.217@buckeyemail.osu.edu 3. pH Introduction Results 1. CO2-C efflux rate 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). N effect C effect C effect N effect • The decrease in pH was more significant under less C addition among C treatments. Among N treatments, the pH under NH was the lowest. 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. 4. C budget • 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). 2. Inorganic N (NH4+ and NO3-) 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 : 33-20 kPa (30-35% MC) • CO2 Measurement: a closed dynamic chamber method (Gas chromatography) • Destructive sampling (5 times) after 0, 10, 45, 100, 200 days • Inorganic N (NO3-, NH4+) (KCl extraction), • pH (1:2) Treatments C effect N effect • 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. • 35-47% 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. Mean residence time of the mineralizable organic N pool Conclusions A single pool first order exponential model Nt = Ni + No(1-e-t/MRT) Ni: the initial inorganic N content Nt: quantity of extractable inorganic N at time t MRT: the steady state MRT of the mineralizable organic N pool (No) • Adding N shortened the MRT of the organic N pool and decreased CO2 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. Statistics Two-way ANOVA with two factors (C, N) Tukey test for the mean comparison (p<.05) • 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. References Alvarez, R. 2005. Soil Use Manage. 21:38-52 Batjes, N.H. 1996. Eur. J. Soil Sci. 47:151-163 Fog, K. 1988. Biol. Rev. 63:433-462. Hobbie, S.E. 2008. Ecology 89:2633-2644. Mack, M.C., E.A.G. Schuur, M.S. Bret-Harte, G.R. Shaver and F.S. Chapin. 2004. Nature 431:440-443. 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.