Introduction

1. Changes in Belowground Sustainability due to Transition from Continuous Wheat to Switchgrass Based Production System Along a Productivity Gradient in Dryland Pacific Northwest, USA Chatterjee1,2, D.S. Long2, J. Kendall1, F.J. Pierce1 and H.P. Collins3 1. Center for Precision Agricultural Systems, Washington State University, Prosser, WA 2. USDA –ARS, Pendleton, OR 3. USDA-ARS, Prosser, WA Changes in field soil CO2efflux and cumulative flux from switchgrass and wheat soils along different productivity levels during June 23 –August 2nd , 2010 Introduction Ethanol production from cellulosic plant materials will require removal of crop residues and/or the production of dedicated bioenergy crops, both of which could have considerable impacts on below ground soil sustainability. This study at USDA-ARS Research Station, Pendleton, OR was initiated in 2007 to quantify changes in belowground sustainability during the early transition to cellulosic based energy production. • Experimental Methods • Field site is located in intermediate rainfall zone (MAP of 41 cm) and soils are classified as silt-loam, mesicTypicHaploxerolls, derived from loess. • Three net primary productivity (NPP) levels (high, medium and low) were developed with variations of irrigation, fertilizer and seeding rate. Low NPP gets only local rainfall, medium NPP is supplemented to high rainfall (>40 cm) and high NPP receives additional water to replenish full ET. • Main plot factor- NPP levels were placed in strips and sub plot factor- crop, switchgrass (SG) and continuous winter wheat (WW) and sub-sub plot factor -residue (remove/not) were assigned randomly in split-split with four replications. The whole experiment is laid out in strip-split-split design. • Soil samples were collected up to 120 cm depth in fall 2009 to determine soil organic carbon (SOC) using dry combustion auto analyzer method. Another set surface soil sampling (10 cm) was done in spring 2010 to study soil biogeochemical processes. • Field soil CO2 efflux was measured using LICOR 6400 infrared gas analyzer during June 23 to August 2nd, 2010. • For C mineralization rate, soil samples were incubated at 25°C for 105 days. Soil CO2 efflux were measured weekly using LICOR infrared gas analyzer. Using three pool constrained model, total SOC pool was separated into (1) active(Ca), (2) slow (Cs) and (3) nonhydrolyzable or Cr pools (determined by digestion with 6M HCl, fractions). • C mineralization rate = Ca*kae(-ka*days)+ (Csoc-Cr-Ca)*kse(-ks*days) + Cr*kr*e(-kr*day) • Inorganic N mineralization rate was determined by ion exchange resin bag method by incubating resin bag in field condition from 16th March to Aug 2nd, 2010. Soil extracted with KCI was analyzed for inorganic N (NH4+ and NO3-) content. Net N mineralization rate was calculated by subtracting initial inorganic soil N content from final soil and resin inorganic N content. • Soil microbial biomass C (µg C g-1 soil) was determined by chloroform fumigation extraction of soil samples ( 50% WHC) incubated at 25 °C for 7 days. K2SO4 extractable C was determined with DOC analyzer. • only residue removed plots were compared for this presentation. Results Three years after plantation, SG above-ground biomass yield was lower than wheat production system and SG had minimum effect on SOC. SG soil produced higher CO2 efflux than wheat during growing season at medium and low NPP. Introduction of SG increased relative contribution of active C pool and decreased resistant C pool to total SOC pool in comparison to wheat soil only within medium NPP. SG soil showed a rise in N loss due to mineralization in response to medium and high NPP condition. Microbial biomass showed a positive response to SG transition across the NPP levels. Conclusion Knowledge of primary productivity is critical for SG production. In dry environment, increasing net productivity increased the loss of C and N from soils under SG production. Three year cycle is not enough to produce a significant effect on SOC pool but increasing microbial biomass in comparison to wheat soil indicates a favorable environment for building up the SOC pool in long-term. Acknowledgments: Authors are thankful to John McCallum, Ellie Murray, Wayne Polumsky, Bob Correa, Daryl Haasch and Patrick Scharf for their sincere support to lab analyses and field operations. This research was supported in part by a grant from the CSREES NRI Competitive Grants Program 2007-35107- 18279. Project Proposal: 2007-03159. SG root at 90 cm SG root at 20 cm