Pizarro, Carolina (USDA-ARS, 2231 California Street, NW, Apt. 709, WashingtonDC, 20008; Phone: 301-504-7327; Email: CPIZARRO@anri.barc.usda.gov)

Soil Microbial Biomass, Carbon Pool, and Green House Gas Fluxes in a Long Term Tillage Experiment on the North Atlantic Coastal Plain

H. De-Polli, C. Pizarro *, T.R. Coser, G.W. McCarty, J.L. Starr

Soil microbial biomass (MB) is an important component of soil organic matter and also plays an important role on the exchange of green house gases (GHG) within agricultural ecosystems. To assess impact of plow tillage and no tillage management on long term carbon sequestration as well as the short term balance of GHG emissions from agricultural land, measurements of total soil carbon stocks (including correction for bulk density), MB related attributes and GHG emissions were performed on a tillage experiment under 10 years of continuous culture of corn (Zea mays L.). The soils were characterized within depth ranges ( 0-2.5, 2.5-5, 5-12.5 and 12.5-20 cm) and the measured microbial attributes included microbial biomass C (MBC) and N (MBN), respiration, metabolic quotient (qCO₂), %MBC in total C. GHG measurements were performed throughout one year using a static chamber technique. When comparing plow and no tillage treatments, soil density was higher for no tillage at all depths with the greatest difference within the second depth range. The no tillage soil had higher total carbon stocks (0-20 cm) than plow tillage (6% by volume equivalent and 10% by mass equivalent) with the main difference occurring in the top layer of soil (0-2.5 cm). This was also the case for respiration, N, MBC, and MBN with values being much higher in the top layer for no tillage and the difference between the two treatments decreasing with depth. Values for qCO₂ (specific respiration) were similar at 0-2.5 cm depth for both treatments but higher for plow tillage with increasing depth. With respect to net GHG emissions, plow tillage soil had a tendency for net CH₄-C production and no tillage soil had a net consumption of atmospheric CH₄-C throughout the year. This differential in CH₄ dynamics between treatments was greatest in winter and early spring. In general N₂O emissions were higher for the no tillage than for the plow tillage treatment, but at some sampling times the opposite was found. The highest N₂O-N emission occurred during summer; although sampling period related to short drought, measured emissions were as low as the winter period. CO₂-C emissions did not show a significant difference between the two treatments in most cases, and emissions were low during winter and high during summer. The calculation of global warming potential (GWP-100yr) based on these short-term emission measurements of GHG gave slightly higher carbon equivalent (Ceq) value under plow tillage than no tillage with an overall difference of 1.05 mg Ceq m^-2 h^-1.