Effects of combined straw and organic fertilizer application as partial replacement for chemical fertilizers on water use efficiency and soil nitrate residue

Objectives We studied the effects of combined straw and organic fertilizer application as replacement for partial chemical fertilizer on crop yield, water use efficiency and soil NO₃^––N residue to improve soil fertility and reducing environmental risks in dryland.

Methods A long-term field experiment was conducted at the Luoyang Dry Farming Experimental Station of Chinese Academy of Agricultural Sciences, Henan Province, since 2007. The tested treatments included no fertilizer control (CK), NPK fertilizer (NPK), and replacing 1/3 of NPK input with straw and organic fertilizer (SOR). The yield, water consumption of winter wheat and summer maize were investigated from 2015 to 2020. The soil nutrient content in the 0–60 cm soil layer and nitrate-N content in the 0–380 cm soil profile were analyzed after the harvest of winter wheat in 2020.

Results Compared to 2007, CK treatment significantly decreased 0–20 cm soil nutrient content, except readily available K, while NPK and SOR treatments improved soil nutrient content in 2020. Compared to NPK, SOR treatment increased organic matter content in 0–60 cm soil layer by 10.4%–16.4%, total N in 0–40 cm soil layer by 16.7%–20.0%, available P and readily available K in 0–20 cm soil layer by 12.9% and 15.4%, respectively (P<0.05). Compared to NPK, SOR treatment increased maize yield by 35.3% in drought years (2015 and 2017) and by 10.1% on average of 5-years; enhanced maize water use efficiency by 42.7%, 12.3% and 18.5% in dry years, normal year (2016) and 5-year on average; and increased the annual yield and water use efficiency by 20.5% and 23.5% in the dry years on average. However, SOR treatment did not significantly change the yield of winter wheat across the 5-years, but significantly reduced the water use efficiency by 9.8% in the normal years (2015—2016, 2016—2017, 2019—2020) and by 7.9% on average of 5-years. The NO₃^––N residue in NPK treatment was 732 kg/hm² and 68.2% was retained in 100–230 cm soil profile, while the NO₃^––N residue in SOR treatment was 833 kg/hm² and 74.8% stayed in 80–200 cm soil profile. Compared with NPK, SOR treatment decreased NO₃^––N residue by 54.9% in 200–230 cm, by 21.1% in 260–290 cm, by 25.0% in 320–350 cm, and by 57.9% in 350–380 cm soil layer.

Conclusions Replacing 1/3 of NPK fertilizer by combined straw and organic fertilizer application did not impact the yield of winter wheat, but significantly increased the yield and water use efficiency of summer maize, and thus increased the annual yield and water use efficiencies. Straw and organic fertilizer application together enhanced the soil organic matter and total N in the 0–60 cm layer, available P and readily available K in 0–20 cm layer, significantly decreased the nitrate-N residue in 200–380 cm soil layer.