Root plasticity of maize intercropped with a legume green manure contributes to the maintenance of grain yield under reduced nitrogen inputs

Objectives

Intercropping offers multiple benefits in agroecosystem services and functions. However, the mechanisms by which intercrop roots contribute to the enhanced benefits are poorly understood. This study determined the root contribution of intercropped legume green manure to intercropped maize yields under reduced nitrogen fertilization.

Methods

A three-year field experiment (2019–2021) was conducted in an oasis irrigation area of northwestern China using a split-plot design with three replicates. The main plots compared maize-common vetch intercropping (IM) with maize monoculture (M), while subplots included a 25 % nitrogen reduction (N1: 270 kg ha⁻¹) and a conventional nitrogen rate (N2: 360 kg ha⁻¹).

Results

Root weight density (RWD), root length density (RLD), and root surface area density (RSAD) in monoculture maize were symmetrically distributed under both nitrogen levels. In contrast, intercropped maize exhibited an asymmetric root spatial distribution during co-growth with common vetch, with significant suppression of RWD, RLD, and RSAD in soil zones adjacent to the common vetch, particularly beneath its rows. This suppression was also evident in the 0–60 cm soil layer near the common vetch row. Moreover, RSAD in the 60–120 cm layer remained consistently lower beneath the common vetch rows than in the maize inter-row zones, regardless of nitrogen input level. RWD and RLD at these depths, however, were less affected. Following the clipping of leguminous green manure, intercropped maize roots expanded laterally into previously occupied common vetch zones, increasing root proliferation throughout the 0–120 cm profile. Reduced nitrogen input enhanced vertical root penetration, promoting deeper rooting in underutilized subsoil layers. In monoculture maize, nitrogen reduction resulted in declines of 7 %-11 % in grain number per spike (GNS) and 9 %-20 % in thousand kernel weight (TKW), while spike number per unit area (SN) remained unaffected. However, these reductions were mitigated in the intercropping system. Mantel test analysis revealed that total RWD during the reproductive stages was positively associated with GNS, while total RLD during the dough and maturity stages contributed to TKW.

Conclusion

Intercropping leguminous green manure with maize mitigated the adverse impacts of nitrogen reduction on maize yield by enhancing maize root plasticity.

Significance

Selectively choosing intercrops with high root plasticity can improve crop productivity and system resilience by enhancing intercrops’ adaptability to climate change in irrigated areas.