Soil Organic Carbon Changes to Increasing Cropping Intensity and No-Till in a Semiarid Climate

Soil organic C (SOC) in the semiarid Northern Great Plains (NGP) can benefit from increasing cropping intensity. We evaluated the effect of annual cropping on stocks of SOC adjusted for equivalent mass (SOCEM; 0-30 cm) and the change in SOC adjusted for equivalent mass (Delta SOCEM) over 10-yr at a field site (45 degrees 40'N, 111 degrees 09' W) near Bozeman, MT. The experiment consisted of two fallow-wheat (Triticum aestivum L.; F-W) rotations under till and no-till (NT), and five annual cropping systems under NT factored with two N levels (moderate and high), and alfalfa-perennial grass system (Conservation Reserve Program, CRP). After 10-yr, we found SOCEM of five NT annual cropping (X = 37.4 Mg ha(-1)) was significantly (P < 0.01) greater than NT F-W (35.1 Mg ha(-1)), and till F-W (33.7 Mg ha(-1)). The greatest SOCEM was found in CRP (39.9 Mg ha(-1)). Accretion of SOCEM was observed in three systems with the largest gains occurring in the CRP (0.24 Mg ha(-1) yr(-1)), followed by NT continuous wheat (0.13 Mg ha(-1) yr(-1)) and NT pea (Pisum sativum L.)/oil seed-wheat (0.09 Mg ha(-1) yr(-1)). Soil organic C loss was observed in all other systems with the largest loss in the till F-W (-0.29 Mg ha(-1) yr(-1)). Among the cropping systems, Delta SOCEM was directly related to net primary productivity (NPP; r(2) = 0.73) and total C (TC; shoot + root + rhizodeposit) inputs (r(2) = 0.86). We found SOCEM was maintained at 7.0 Mg ha(-1) yr(-1) of net primary productivity (NPP) and 2.6 Mg ha(-1) yr(-1) of TC inputs with accretion and loss occurring above and below these thresholds.