Nitrogen contributions of alley cropped Trifolium pratense may sustain short rotation woody crop yields on marginal lands

Abstract

Incorporating nitrogen-fixing cover crops into short rotation woody crop (SWRC) plantations may increase soil and tree nitrogen while sustaining, or even improving, yields. This study examined the impacts of Trifolium pratense L. at different seeding rates on soil and poplar N and poplar growth when alley cropped in a one-year old plantation for 2 years compared to natural vegetation and a bare-soil control. These vegetation types were applied in the field under a wide (3.05 m) and narrow (1.82 m) tree alley width. Results showed two of four variations including red clover had 151% and 222% higher soil NO3− levels compared to controls in the second year (wide spacing/full seed rate and narrow spacing/half seed rate, respectively). This correlated with a 26% and 23% increase, respectively, in poplar leaf N concentrations compared to controls. At the wide spacing and full clover seed rate, poplar foliar P was 80% lower compared to controls, implying significant competition. However, results also showed the potential to mitigate this effect with wider tree spacing and lower cover crop seeding rates, without compromising nitrogen benefits. Although higher foliar nitrogen levels did not correlate with increased poplar productivity within 2 years, the results of this study suggest nitrogen-fixing cover crops can improve soil and poplar nitrogen and may sustain SWRC nitrogen requirements over one or multiple rotations.