Soil organic matter, greenhouse gas emission and global warming: Following one-time high rate compost application with annual cover crop planting in a dryland winter wheat-fallow rotation

In dryland organic winter wheat-fallow rotation systems of U.S. Central High Plains, the usual practice of 10–15 Mg/ha compost application every 3–4 years, has not yielded desired soil organic matter (SOM) improvements. One-time high rate compost application and annual cover crop planting (instead of repetitive tillage) to control weeds have been proposed for carry-over SOM improvement in these systems. In this study, 15, 30 and 45 Mg/ha compost rates and controls (‘no amendment’ and inorganic fertilizer) were considered. One-half of fallow blocks were planted to cover crop mixtures. SOM indices: soil nitrate (NO 3 − ), ammonium (NH 4 + ), dissolved organic C/N (DOC/N), potential mineralizable N (PMN), carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) emissions, and soil moisture were measured bi-weekly across three winter wheat growing seasons. The 45 Mg/ha compost affected 25–35% more NO 3 − in the first growing season; 27–70%; 8–49%; 12–44% more DON, PMN, DOC and 25–57% less CO 2 in the second growing season; but did not worsen global warming potential in any growing season. In the fallow phases, cover crops depleted 10–14% soil moisture in the first two growing seasons, which was recovered by precipitation in succeeding wheat phases. Cover crops utilized 21–46% soil NO 3 − in these growing seasons which reduced N 2 O emission by 26%. The 45 Mg/ha compost improved SOM and ensured environmental quality, but annual cover crop biomass (< 2000 kg/ha) did not add significant SOM. Therefore, integrating larger biomass producing cover crops with 45 Mg/ha compost in wheat-fallow rotations may be a better prospect.