The annual double-cropping system of winter wheat-summer maize on the North China Plain has caused severe land degradation, particularly soil infertility due to the biannual tillage and straw removal. Two field experiments were conducted from 2012 to 2014. The in situ incubation of wheat and maize straw was measured under four management practices: (i) straw incorporated with application of chemical fertilizer (SF); (ii) straw incorporated with chemical fertilizer and manure (SFM); (iii) straw incorporated with chemical fertilizer and straw-decomposition inoculants (SFD); and (iv) only straw incorporated (CK). Our results showed that SFM and SFD significantly increased the decomposable fraction of straw mass from 64% to 66% in wheat, and 64%–65% in maize straw, as compared to 60% and 53% in wheat and maize respectively, in CKs. SFM and SFD released 77%–78% N and 84%–85% P in wheat straw, while 65%–71% N and 76%–77% P in maize straw in the following crop growing season. A significant increase in straw decomposition and nutrient release induced higher soil urease, phosphatase and invertase activities in SFM and SFD than SF and CK, which resulted higher available N and P in SFM and SFD. Thus, crop yield in the SFM and SFD treatments increased significantly up-to 18–53%. Application of straw incorporated with chemical fertilizer and manure, and their decomposition inoculant is recommended with the advantages of maintaining soil fertility and improving crop production.