Can variability in snow cover affect nitrogen transfer between cover crops and the subsequent grain crop?

Background/Question/Methods

: Cover crops can potentially improve the sustainability of agroecosystems by increasing nitrogen (N) retention over the winter. However, the success of cover crops in reducing N losses is contingent on the timing of N release from decomposing cover crop residues being synchronized with N uptake by the cash crop. The efficiency of this N transfer may be influenced by variation in winter soil temperature, which can affect the survival of cover crops and the timing of the decomposition of their residues. Variability in snow cover, which can decouple soil temperature from air temperature, may thus affect N transfer and optimal management practices. In late summer, we planted both winter-killed and winter-hardy species of both grass and clover cover crops in 1 m2 plots, and planted corn the subsequent spring. We used 15N tracer to examine the extent to which the cover crops increased retention over fall and winter, and the subsequent transfer of N to the corn. A snow removal treatment also was added to assess how these N responses would be affected by variability in frost exposure over winter. We predicted snow removal would disrupt N transfer to the corn, particularly in the species traditionally classified as winter-hardy.

Results/Conclusions

: While snow removal plots were only marginally colder on average at 5 cm soil depth compared to the ambient snow plots (-0.05 °C versus 0.36 °C), there were four times as many soil freeze thaw cycles in the snow removal plots. Corn yield increased in response to three of the four cover crops, and it was reduced by snow removal, but there was not a significant interaction between these two factors. 15N recovery in the corn kernels was lowest in the control treatment (3.8%), and cover crops nearly doubled kernel 15N recovery (7.4%). 15N recovery in the kernels was reduced by snow removal; however, there was no interaction between cover crop treatment and snow removal. Our data reveal that while snow removal reduced corn yield and impeded 15N recovery, this effect was not more severe in winter hardy crops than in winter-killed crops, nor did it differ between grasses and clovers.