Abstract: The combination of returning agricultural biomass into soil and reducing application dosage of chemical fertilizer can effectively improve soil quality. Considering the speciality and importance of black soil in Northeast China, a splitplot design experiment was set up in the Conservation Tillage Research and Development Station of the Chinese Academy of Sciences (black soil area, Jilin Province), which combined different straw returning modes with nitrogen fertilizer (N) addition levels. We analyzed crop yields, physicochemical properties and biological properties (0-20 cm) of soils. The feasibility of reducing nitrogen fertilizer dosage on the base of straw returning was discussed. There were six treatments of straw returning modes in the main plots: control (CK), full straw mulching without tillage (NT), crushing straw incorporation directly (SD), addition of compost from straw insitu (SC), addition of biochar from straw insitu (BC), and 9/10 straw +1/10 compost (SDC). The subplots, i.e., fertilizer dosages included: conventional NPK chemical fertilizer (N100); 20% reduction of N fertilizer (N80) (N content of straw was close to 20%); 40% reduction of N (N60, with conventional phosphorus, potassium fertilizer). Straw cooperation modes and N levels significantly interacted to affect maize biomass, maize yield, soil physical and chemical properties (pH, organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP)), and enzyme activities (dehydrogenase, α-glucosidase, β-glucosidase, β-1,4-N-acetylglucosidase, acid phosphomonoesterase and alkaline phosphomonoesterase). Straw cooperation had significant main effects (P<0.05), while N dosage had no significant effect on the examined variables. Among treatments with chemical application alone, N reduction treatment decreased maize biomass and yield and enhanced TP content and activities of N-acetylglucosidase and alkaline phosphatase, but had no significant effect on pH, SOC, TN, and activities of dehydrogenase, αglucosidase, βglucosidase, and acid phosphatase. Straw returning in conjunction with conventional NPK fertilizer (N100) increased soil pH, SOC, TN, and TP (P<0.05), and enhanced activities of dehydrogenase, α-glucosidase and acid phosphomonoesterase to a certain extent, with some treatments having no such effects. Straw returning modes combined with N reduction (N80, N60) had no negative impact on the maize biomass and yield, and enhanced soil pH, SOC, TN and TP (P<0.05). NT with N reduction enhanced β-glucosidase and acid phosphatase activities. SC with N reduction significantly increased activities of β-glucosidase, acid phosphatase and alkaline phosphatase. SDC with N reduction significantly enhanced activities of β-glucosidase and alkaline phosphatase. It is concluded that short-term combined treatment of straw returning and nitrogen reduction application stabilized crop yield, slowed soil acidification, and maintained SOC, TN and TP contents. Therefore, straw cooperation in conjunction with N application reduction has great ameliorative effects on soil nutrient storage, supply, and transformation.

Key words: straw returning, reducing N fertilizer application, soil nutrient, soil enzyme activity.