Greenhouse Gases Emission and Carbon Sequestration in Agro-Ecosystems under Long-Term No-Till: Implications for Global Warming Mitigation Pierre-André Jacinthe 1, Raj K. Shrestha 2, Serdar Bilen 2, Warren A. Dick 2, Rattan Lal 2 1 Earth Sciences, Indiana University Purdue University Indianapolis, IN, 2 School of Natural Resources and Environment, Ohio State University, OH CONTEXT No-till (NT) farming is a method of producing crops without tillage other than the small amount of soil disturbance that occurs at time of planting. No-till (NT) management has gained wide acceptance in US agriculture (Fig. 1), and could contribute to global warming mitigation by offsetting fossil fuel emission. In Ohio for example, NT is used to produce approximately 25% of the corn and 65% of the soybean crop (Fig. 2). While C sequestration in NT systems is fairly well documented, the dynamics of greenhouse gases (GHG) emission is less well understood. However, the literature abounds with viewpoints and assumptions. Due to greater crop residue on NT surface and generally higher soil moisture, it is often assumed that production of carbon dioxide (CO 2 ), nitrous oxide (N 2 O) and methane (CH 4 ) is greater in NT systems compared to conventional tillage (MP). But it is also possible that long-term implementation of NT could increase soil macro-porosity, lead to the evolution of an active population of methanotrophs, and ultimately result in enhanced CH4 uptake. Field data are needed to test the validity of these conflicting assumptions. OBJECTIVES (1)monitor GHG fluxes in a no-till chronosequebce, and examine relationships between gas fluxes and biophysical properties of NT soils; (2)create a greenhouse gas budget for soils under long-term no-till crop production. MATERIAL and METHODS Chronosequence of experimental sites: Field sites selected for this study were at five locations in Ohio (Fig. 3 and Table): Mount Gilead (10 years no-till), Bucyrus (14 years no-till), Centerburg (37 years no-till), Wooster (48 y)and South Charleston (48 years no-till). Field sites were located on soil series with similar drainage characteristics so that duration of NT management is the experimental factor. Contrasting soil drainage characteristics at the oldest NT sites: moderately well-drained (MWD) in Wooster and somewhat poorly-drained (SPD) in South Charleston. At each location, a forested area was included as reference site. Monitoring of GHG fluxes Every other week using static chambers (Fig. 4). Analysis of soil air samples by gas chromatography (Varian 3800 interfaced with CombiPal) autosampler. GHG fluxes computed using change in gas concentration in chamber headspace. Preliminary data are presented for the cropping season. Fig. 5. Mean GHG fluxes at NT sites (see Table 1) during the period August 2009 – October Fluxes measured at corresponding MP sites are shown as red triangles. Error bars represent standard deviations. Fig. 1. No-Till trend in US agriculture (Conservation Technology Information Center ) Fig. 3. Location of the study sites in Ohio Fig. 7. Relative contribution of CO 2 and N 2 O to GWP of no-till agro-ecosystems with NT duration. Fig. 6. Daily fluxes of N 2 O at the PD site under NT for 48 years. SUMMARY  GHG fluxes were affected by NT duration and soil drainage characteristics. At cropland sites under NT for > 15 years (Fig. 5), CH 4 uptake ( mg CH 4 -C m -2 d -1 ) was greater than under MP (-0.014). At the sites under NT for 48 years, CH 4 uptake was significantly (10-12 times ) higher at the MWD than at SPD site.  When data from all sites were pooled, N 2 O fluxes were significantly higher under MP (2.01 mg N 2 O-N m -2 d -1 ) than under NT (0.73), but the trend varied with NT duration. While at recent ( 40 years.  N 2 O fluxes tended to be very strong following rainfall and fertilizer application at the long-term NT plots (> 48 y) established on SPD soils (Fig. 6).  At the older NT sites (Fig. 7), N 2 O emissions accounted for >55 % of the global warming potential (GWP, sum of all GHG expressed as CO 2 equivalents). Since SOC accretion rate (Fig. 8) declines with NT duration, SOC sequestration may not be sufficient to offset the increased N 2 O emission in long-term NT soils. A whole-system evaluation approach (accounting for all inputs) is needed to elucidate this question.  These findings call for the development and implementation of fertilizer-N management practices (timing, amount, type and method of application) that reduce N 2 O emissions in long-term NT croplands in order to harness the environmental benefits of no- till farming. Fig. 4. Static chamber Fig. 2. Plow-till (MP) and no-till (NT) crop production Authors acknowledge the contribution of the TUBITAK Scholarship Program, and the Ohio State University/The Ohio Agriculture Research and Development Center. Funding provided by USDA-NRI grant Fig. 8. (a) SOC stocks (0-30 cm) at the study sites as related to tillage, and (b) differential SOC sequestration rate with NT duration.