Partial replacement of chemical fertilizer by manure affects maize root traits in North China Plain

In order to achieve high yields and environmental friendliness simultaneously, the replacement of chemical fertilizers by manure has become a research hotspot in recent years. Roots absorb nitrogen, participate in its assimilation and contribute to the cereal's dry matter accumulation. A 5-year filed experiment in the North China Plain was initiated to assess the response of root morphology and distribution of summer maize (Zea mays L.) to fertilizer application and contribution to crop yield. The treatments included CK (unfertilized control), NPK (inorganic nitrogen/phosphorus/potassium fertilizer) and NPKM (manure + 70% NPK). We determined the root biomass, root diameter, root length density (RLD) of three diameters (>0.8 mm, 1stRLD; 0.2-0.8 mm, 2ndRLD; <0.2 mm, 3rdRLD) and the soil chemical properties at 60 cm with 10 cm increments. At 40-60 cm, NPKM significantly decreased the root diameter than NPK. Fertilization showed no effect on total RLD, 1stRLD, 2ndRLD and 3rdRLD for a 60-cm soil profile. At 40-50 cm, the NPKM increased the RLD compared to NPK, mainly by increasing the 2ndRLD and 3rdRLD. Under CK and NPK, root lengths of 0-20 cm made up 62% and 57% of the total root length, respectively. Under NPKM, root lengths of 40-60 cm made up 46% of the total root length. Our results indicate that maize yield was preserved after replacing 30% of N fertilizer with manure, presumably depending on the change of root vertical distribution pattern and increase of the fine root length in deeper soil.