The effects of maize straw return and N fertilizer application on soil quality and crop yield have been extensively investigated.  However, the effects of different amounts of maize straw returned to the field with different nitrogen application rates on the soil–crop system quality, abundance of functional N cycle microorganisms, N₂O emissions, and crop N nutrition status of crops have not been thoroughly explored.  The objective of this study was to assess the effects of different summer maize straw return rates and N application rates on i) soil quality and crop productivity; ii) the community of N cycle functional microorganisms and N₂O emission; and iii) crop N status.  The results indicated that crop yields increased by 7.62 to 12.69% at 210 kg ha^–1 of N application for full straw return (SN) and half return (1/2SN) compared to the no-return treatment (CK).  No significant difference was noted in the yields between the full straw return reduced by 15% (178.5 kg N ha^–1) of N fertilizer (S-15%N) and SN.  The surface soil layer (0–20 cm) showed significantly higher levels of soil organic matter (SOM), the community of N-cycling functional microorganisms, crop N nutrition status and N uptake efficiency in SN, 1/2SN, and S-15%N as compared to other treatments.  Compared to SN, S-15%N and 1/2SN reduced cumulative N₂O emission fluxes by 19.11 and 5.51%, respectively.  Furthermore, the nitrogen nutrient index (NNI) values of 1/2SN and S-15%N were closer to the critical N requirement than SN.  In summary, schemes for determining the optimal rates of straw return and N application (1/2SN and S-15%N) based on SOM, NNI, cumulative N₂O emission fluxes, and yield can be applied to the annual production of winter wheat and summer maize in China.

Characterizing the N uptake and utilization of different maize hybrids is essential for optimizing N application and increasing the profits from maize production.  Research trials were conducted with controlled-release urea (CRU) as a base fertilizer (TC) and urea split application in one (T1), two (T2), or three (T3) stages to evaluate the effects on N uptake, N use efficiency (NUE), and yield using the ¹⁵N tracer technique and two maize hybrids: DH518 (a mid-early-maturing hybrid) and DH605 (a late-maturing hybrid).  According to the results, compared with urea, CRU as a base fertilizer and urea split applications in two and three stages significantly increased grain yield and NUE while reducing environmental N loss.  Compared with T1, the grain yields of the TC, T2, and T3 treatments were respectively increased by 11.1, 9.8, and 11.7% in DH518 and by 16.4, 15.7, and 22.9% in DH605.  Regression analysis showed that the grain yield of DH518 displayed a bilinear trend with an initial rapid increase and then a slow increase with increases in post-anthesis N accumulation, total N accumulation, N recovery efficiency, and N nutrition index (NNI).  By contrast, DH605 consistently showed a linear regression relationship with a rapid increase.  The crop recovery N efficiency (CRN) values in the T3 treatment for urea applied at the sowing stage and as topdressing at the 9th-leaf stage (V9) in DH518 were 60.0 and 62.4% higher than under topdressing at the tasseling stage (VT) stage, respectively, while the CRN values of urea topdressing at the V9 and VT stages in DH605 were 37.7 and 37.1% higher than when applied at the sowing stage, respectively.  The higher pre-anthesis N demand and shorter growth period of DH518 maintained the N supply–demand balance, resulting in the NNI (NNI≥0.988) falling within the range of a low yield increase under the T2 and TC treatments, while the N status of DH605 plants only reached optimal levels in the T3 treatment.  Therefore, a three-stage split application of urea or applying CRU as a base fertilizer and topdressing with urea in the later growth stages is recommended for obtaining an optimal yield in mid-late-maturing hybrids.  In addition, for mid-early-maturing hybrids, applying CRU or reducing the number of split applications, e.g., a two-stage split application, can ensure an adequate N supply in the later growth stages and increase production, and thus profits.