A field study of cover crops to improve soil biochemical properties in bulk and rhizosphere soils of lettuce (Lactuca sativa L.)

Hairy vetch (Vicia villosa Roth; HV) and rye (Secale cereale L.) cover crops have been reported to increase soil nitrogen and microbial biomass and diversity. However, quantitative analyses based on field experiments have been limited. A field trial with HV and rye cover crop plots was compared with control plots that received 2.5 g N m^‑2 synthetic fertilizer. Lettuce (Lactuca sativa L.) was rotated after the cover crops were incorporated and harvested at 33 days, when bulk soil samples were collected and tested for nitrate, ammonium, and microbial biomass nitrogen (MBN). DNA from soils that attach lettuce roots (i.e., rhizosphere soil) was targeted for bacterial (16S rRNA) community determination through the next generation sequencing (NGS) approach. Lettuce yields in HV (0.20 kg m^‑2) and control (0.21 kg m^‑2) treatments were similar, whereas that in rye (0.15 kg m^‑2) was reduced because of lower nitrate and ammonium concentrations in rye (7.56 and 8.06 mg kg^‑1 soil) compared with HV (16.8 mg kg^‑1 soil) and control (29.9 mg kg^‑1 soil). MBN in HV and rye treatments was 2.5 and 3 times lower than that in control in bulk soil, indicating smaller microbial biomass under the cover crop treatments. Bacterial diversity, determined as the Shannon index at family level, was enhanced in HV (3.98), compared with that in control (3.93) and rye (3.80) in lettuce rhizosphere soil. Specifically, bacteria in the families Bacillaceae, Bacillales incertae sedis, and Sphingomonadaceae, which include plant growth promotors, were relatively more abundant in HV (3.16, 0.38 and 4.46%, respectively) than in either control (1.79, 0.12 and 3.91%, respectively) or rye (2.24, 0.12 and 2.11%, respectively). Moreover, those families were positively correlated with the ability of lettuce to take up N under HV amendments, based on the results of a redundancy analysis (RDA). In summary, HV cover crops improved chemical and reduced biological properties in bulk soil but increased microbial properties in rhizosphere soil, which corresponded to lettuce yields.