Multi-site evaluation of stratified and balanced sampling of soil organic carbon stocks in agricultural fields oleh Eric Potash, Kaiyu Guan, Andrew J. Margenot, DoKyoung Lee, Arvid Boe, Michael Douglass, Emily Heaton, Chunhwa Jang, Virginia Jin, Nan Li, Rob Mitchell, Nictor Namoi, Marty Schmer, Sheng Wang, Colleen Zumpf