Effects of returning the root of green manure on reducing N application in maize within their intercropping system in Hexi oasis irrigation area

Objectives Maize yield and soil fertility were investigated under the continuous return of green manure (GM) roots to the field and reducing fertilizer input in subsequent maize cultivation in a maize-green manure rotation system. We aimed to provide the basis for nutrient management to achieve sustainable and efficient crop production in the Hexi oasis irrigation area.

Methods The long-term experiment was established in Hexi oasis, Gansu Province, in 2011. The two intercropping systems tested were needle leaf pea and hairy vetch with maize. The aboveground parts of the GM crop were cut as forage while the root remained in the soil. The sowing rows of GM and maize were exchanged every other year. Eight fertilizer treatments in maize were set up as no fertilizer application nor GM root control (CK), conventional N fertilizer rate (N 375 kg/hm²) without GM root retained (CF), retained GM root and applying N fertilizer at 0, 90%, and 80% of the conventional N fertilizer rate. At maize harvest in 2020, we analyzed the yield and NPK accumulation in maize straw and grain. Also, we collected 0–20 cm soil samples to determine the organic matter and N storage.

Results Compared with CF, returning GM roots to the soil only (P<0.05) decreased maize yield. The combination of GM root and 80%, 90% conventional fertilizer application rate resulted in a similar or increased maize yield. It also increased the grain N, P, and K accumulation by 31.7%–56.4%, 37.8%–60.0%, and 61.7%–96.8%, respectively. Consequently, the agronomic efficiency of N fertilizer and apparent utilization by maize increased. The improved Nemoro Index Method showed that needle leaf pea with 80% N fertilizer application consistently enhanced comprehensive soil fertility, increasing by 23.0% compared with CF. Partial least squares regression and aggregated boosted trees analyses showed that fertilization treatments changed maize yield by regulating the soil N storage. The total and inorganic N concentrations had the highest contribution, accounting for 36.5% and 26.8% of the yield increase, respectively.

Conclusions Retaining green manure roots continuously on the field could replace 10%–20% of N fertilizer input for subsequent maize. This practice stabilizes the maize yield, increases NPK content, and improves N fertilization efficiency. The practice improves comprehensive soil fertility by increasing N storage. We recommend that when GM roots are retained in soil, 80% of the conventional N fertilization rate should be applied in maize after needle leaf pea and 80%–90% after hairy vetch.