Conservation Tillage Impacts on Soil Aggregation and Carbon Pools in a Sandy Clay Loam Soil of the Indian Himalayas

Soil conservation and C sequestration are critical issues in rainfed farming of the Indian Himalayas. This study, conducted from 2003 through 2009 on a sandy clay loam soil (Typic Haplaquept) near Almora, India, evaluated the effect of seasonal tillage alterations (year-round conventional tillage [CT-CT], year-round no tillage [NT-NT], CT in the Rabi [winter] season and NT in the Kharif [summer/rainy] season (CT-NT) and the reverse [NT-CT] of the latter treatment) on soil organic carbon (SOC) retention, soil aggregation, aggregate-associated C concentrations and particulate organic matter-carbon (POM-C) in the 0- to 15-cm soil layer. Results indicate that the plots under NT-NT, NT-CT, and CT-NT had nearly 16, 12, and 10% higher total SOC content compared with CT-CT (similar to 12 g kg(-1) soil) in the 0- to 5-cm soil layer. However, tillage had no impact on total SOC content in the subsurface (5- to 15-cm) soil layer. Although the labile pools of SOC were positively affected by conservation tillage practices (CT-NT, NT-NT, and NT-CT), the less labile pool was only influenced by the continuous NT and NT-CT in the 0- to 5-cm depth. Plots under NT-NT and NT-CT had about 27 and 19% higher labile SOC pool than CT-CT plots (5.65 g C kg(-1) soil), and NT-NT and NT-CT plots had about 14 and 11% higher less labile SOC pool than CT-CT plots (2.61 g C kg(-1) soil) in that soil layer. Plots under NT-NT and one seasonal tillage (NT-CT and CT-NT) also had higher macroaggregates and macroaggregate-associated SOC after 6 yr of study than CT-CT plots. Greater proportion of microaggregates within macroaggregates in the plots under NT-NT compared with CT-CT was also observed in the surface layer only. The NT-CT plots had similar effects on all studied soil properties to CT-NT plots, apart from the mean weight diameter (MWD) and labile SOC pool in the 0- to 5-cm soil layer were higher under former plots than the latter. Plots under NT-NT had about 10% higher coarse (250-2000 mu m) intra-aggregate particulate organic matter-C (iPOM-C) within >2000 mu m sand free aggregates in the 0- to 5-cm soil layer compared with CT-CT plots. The fine (53-250 mu m) iPOM-C within the 250- to 2000-mu m aggregates was also higher in the continuous NT plots compared with CT within both >2000 and 250 to 2000 mu m sand free aggregate size classes in that soil layer. Thus, the adoption of continuous NT is the best management option for improvement of soil C under a rainfed lentil [Lens esculentus (L.)]-finger millet [Eleusine coracona (L.) Gaertn.] cropping system of the Indian Himalayas, as the management practice has the potential to improve productivity and soil aggregation with greater accumulation of POM-C, and SOC stabilization apart from other known benefits like weed control, less cultivation cost, and higher profits.