Soil water, soil nitrogen and productivity of lucerne–wheat sequences on deep silt loams in a summer dominant rainfall environment

The management of water and nutrients in farming systems that incorporate alternating phases of lucerne pasture with annual grain crops posses additional challenges to rotations with annual crops only. Strategies for balancing water and nutrient resources within and across rotations will be governed by local soil, climatic, skill and economic constraints. On the Loess Plateau in China, farmers have been encouraged to grow lucerne (Medicago sativa) to reduce soil erosion and improve soil fertility on cropping lands but with little supporting information on how to incorporate lucerne within subsistence cropping systems. Lucerne–wheat rotation experiments were established at two locations in Gansu Province and examined the yield of lucerne and wheat along with changes in soil water and nitrogen (N). Lucerne proved to be well adapted to the high water holding capacity soils and summer dominant rainfall environment of the region with annual production of around 12 t ha−1 at the higher rainfall site (Qingyang). An old (30 years), sparse stand of lucerne growing in the drier location (Dingxi) was much less productive, being dependent on incident rainfall. Apparent water use efficiency (WUE) of lucerne over individual harvest periods ranged from 4 to 56 kg ha−1 mm−1 at Qingyang. Lucerne was able to dry the soil to the crop lower limit (CLL) to depths of 3 m and there was clear evidence that lucerne roots were extracting water below this depth.Wheat following lucerne is subject to low plant available soil water at sowing, unless substantial rainfall occurs, but climate variability in this region makes this difficult to predict. Rain which falls during short fallow periods after lucerne termination provides opportunity for N fertiliser responses, which may be greater after large rainfall events that lead to N leaching. In drier environments such as Dingxi, deep drainage and leaching appear unlikely under rotations which incorporate lucerne, and here evaporative water loss from the soil surface presents a more significant management challenge. The overall variability in seasonal rainfall at both sites, even within the short period of this study, indicates that an adaptive management strategy may be required, rather than fixed rotations. Systems modelling may shed further light on the most useful strategies to manage crop rotations within this variability.