Fewer but more intense rainfall events expected in the future may affect plant-soil nitrogen (N) cycling, possibly including decoupling soil N availability from plant N uptake. Agroecosystems are already hotspots of harmful N losses, in part due to highly-simplified plant communities and long periods without roots. More diverse crop rotations and inclusion of cover crops, i.e. higher crop rotational complexity, improve soil organic matter (SOM) and alter microbial communities and may elicit plant-soil feedbacks that buffer plant-soil N cycling against more extreme soil moisture regimes. We test this hypothesis using the Biodiversity Gradient Experiment at the Kellogg Biological Station LTER, which has compared rotations with 1–5 cash/cover crops since 2000.
Soil from four of these rotations (monoculture to five crops) was collected for a greenhouse experiment with maize grown under two water regimes: water every 2–3 days (i.e. ambient), and half as much water as ambient every seven days (i.e. future). We used 15N-labeled legume residue to track soil N transformations and plant N uptake. We measured plant and soil responses after six weeks of maize growth, including biomass and 15N uptake, leaf gas exchange, soil microbial community structure and activity, and partitioning of 15N into SOM fractions.
The future water regime reduced total maize biomass by 27% across all soils, with maize biomass lowest in monoculture soil, and similar in other rotation soils. The root:shoot ratio increased from 0.26 to 0.32 in ambient vs. future watering, reflecting reduced shoot biomass. Maize grown in the five-crop soil developed more slowly, but continued growth during the most intense period of water stress. Leaf photosynthetic rates and intrinsic water use efficiency were highest in maize grown in the five-crop soil, including under the future water regime, likely reflecting higher plant N uptake and possibly greater abundance of arbuscular mycorrhizae.
Higher crop rotational complexity increased carbon (C) and N cycling enzyme activities, especially in soils with a history of cover cropping, but the altered water regime did not affect enzyme activities. More microbial-available C in the more complex rotations may have accelerated mineralization and recycling of the 15N cover crop residue, which could contribute to greater 15N accumulation in more stable SOM fractions. By increasing the quantity of bioavailable SOM and abundance of beneficial microbes, more complex rotations can enhance soil biological activity and in turn reduce the impact of more variable soil moisture on plant growth and plant-soil N cycling.