The dos and don'ts of no-till continuous cropping: Evidence from wheat yield and nitrogen use efficiency

When crop-pasture rotation is converted to a single fallow/soybean or winter crop/soybean annual cropping, wheat grain yield declines progressively as the annual cropping phase lengthens, regardless of the tillage system. This decline can be attributed to i) depletion of the soil nutrient supply capacity and ii) subtle but cumulative degradation of soil physical properties. The objectives of this study were to disentangle and quantify the limitations on wheat yield imposed by these processes, and to identify the cropping sequence that preserves soil quality and enables high wheat yield. Wheat was grown for two years at three nitrogen (N) fertilization rates (0, 80 and 190 kg ha⁻¹) in soils after a 20-years experiment with six cropping systems. The cropping systems are crop-pasture rotations with tillage (ROT_CT) or no-till (ROT_NT), continuous cropping with no-till and high frequency of sorghum and maize (CC_NT_C4) or soybean and sunflower (CC_NT_C3) or winter fallow (CC_NT_WF), and continuous annual cropping under conventional tillage (CC_CT). Soil quality was assessed based on chemical (soil organic carbon, total soil N concentration and potentially mineralizable N) and physical properties (field water infiltration rate and soil aggregate stability). In each system, we estimated the yield gap due to N supply (Y_g^N) limitations and the yield gap due to soil properties other than N supply limitations (Y_g^others), so that the total yield gap (Y_g^T) is the sum of Y_g^N and Y_g^others. Systems that degraded chemical and physical properties had lower yield, grain N concentration and fertilizer N use efficiency (NUE_f, kg of grain kg⁻¹ of N added). Only two systems, ROT_NT and CC_NT_C4, achieved Y_max (7.2 Mg ha^-1). For these two systems Y_g^T = Y_g^N. For the other systems, the percentage of Y_g^T explained by Y_g^others varied between 23 % and 50 %. Rotations that increased the soil N supply (N uptake with no N fertilizer) also increased NUE_f. Wheat under ROT_NT reached the maximum yield obtained under CC_CT with 45 % less N fertilizer (104 vs 190 kg ha⁻¹) and higher NUE_f (50 vs 27 kg kg⁻¹). Comparing ROT_NT and CC_NT_C4 to other continuous no-till cropping systems (CC_NT_WF and CC_NT_C3), the N fertilizer required was increased from 104 and 107 to 152 and 163 kg ha-¹, respectively. In conclusion, rotating annual crops under no-till is not enough to preserve soil productivity. Sustainable intensification under continuous no-till would require either re-balancing crop sequences towards crop-pasture rotations or a shift towards a lower frequency of soybean in favor of higher frequency of maize and sorghum in the summer phase of the rotation.