Abstract 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_NTC4) or soybean and sunflower (CC_NTC3) or winter fallow (CC_NTWF), 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 o t h e r s ), so that the total yield gap ( Y g T ) is the sum of Y g N and Y g o t h e r s . Systems that degraded chemical and physical properties had lower yield, grain N concentration and fertilizer N use efficiency (NUEf, kg of grain kg−1 of N added). Only two systems, ROT_NT and CC_NTC4, achieved Y m a x (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 o t h e r s varied between 23 % and 50 %. Rotations that increased the soil N supply (N uptake with no N fertilizer) also increased NUEf. 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 NUEf (50 vs 27 kg kg−1). Comparing ROT_NT and CC_NTC4 to other continuous no-till cropping systems (CC_NTWF and CC_NTC3), 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.

中文翻译：

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免耕连作的注意事项：小麦产量和氮肥利用效率的证据

摘要 当作物-牧草轮作转换为单一的休耕/大豆或冬季作物/大豆一年生作物时，无论耕作制度如何，小麦籽粒产量都会随着一年生作物阶段的延长而逐渐下降。这种下降可归因于 i) 土壤养分供应能力的消耗和 ii) 土壤物理特性的微妙但累积退化。本研究的目的是解开和量化这些过程对小麦产量的限制，并确定可以保持土壤质量和实现高小麦产量的种植顺序。经过 20 年的六种种植系统试验后，小麦在土壤中以三种氮 (N) 施肥率（0、80 和 190 kg ha-1）种植两年。种植系统是耕作（ROT_CT）或免耕（ROT_NT）的作物 - 牧场轮作，高粱和玉米 (CC_NTC4) 或大豆和向日葵 (CC_NTC3) 或冬季休耕 (CC_NTWF) 免耕和高频连作，以及常规耕作下的一年生连作 (CC_CT)。根据化学性质（土壤有机碳、土壤总氮浓度和潜在可矿化氮）和物理性质（田间水分渗透率和土壤团聚体稳定性）评估土壤质量。在每个系统中，我们估计了由于氮供应 (Y g N ) 限制引起的产量差距和由除氮供应限制以外的土壤特性 ( Y gothers ) 引起的产量差距，因此总产量差距 ( Y g T ) 是Y g N 和 Y gothers 的总和。降低化学和物理特性的系统具有较低的产量、谷物 N 浓度和肥料 N 使用效率（NUEf，每公斤谷物 kg−1 添加的 N）。只有两个系统 ROT_NT 和 CC_NTC4 达到了 Y 最大值（7.2 Mg ha-1）。对于这两个系统 Y g T = Y g N 。对于其他系统，Y gothers 解释的 Y g T 的百分比在 23% 到 50% 之间变化。增加土壤氮供应（没有施氮肥时吸收氮）的轮作也增加了 NUEf。ROT_NT 下的小麦达到 CC_CT 下获得的最大产量，氮肥减少 45%（104 对 190 kg ha-1）和 NUEf 增加（50 对 27 kg kg-1）。将 ROT_NT 和 CC_NTC4 与其他连续免耕种植系统（CC_NTWF 和 CC_NTC3）进行比较，所需的氮肥分别从 104 和 107 增加到 152 和 163 kg ha-1。总之，在免耕下轮作一年生作物不足以保持土壤生产力。