Responses of soil mineral N contents, enzyme activities and crop yield to different C/N ratio mediated by straw retention and N fertilization

The quality of straw affects N release after straw retention. As straw with high C/N ratio could result in N immobilization, additional N is needed to compensate N demand of crop. More and more N fertilizer is applied to soil for high crop yields in China. Therefore, how to scientifically apply N fertilizer is a key problem after straw retention. Based on C/N ratio, we carried out a field experiment to study the effects of different C/N ratios on soil mineral N content, microbial parameters and crop yields during the period of 20122013 in a winter wheat-summer maize double cropping system. The field experiment consisted of six treatments: 1) straw removal without fertilizer; 2) straw retention without N fertilizer; 3) straw retention with conventional N fertilizer (C/N ratio 10︰1); 4)straw retention with mineral N fertilizer (adjusted C/N ratio of 16︰1); 5) straw retention with mineral N fertilizer (adjusted C/N ratio of 25︰1); 6) straw retention with an adjusted C/N ratio of 25︰1 using organic N fertilizer (cattle manure). The results showed that: 1) there was no difference between the addition of mineral N fertilizer and organic N fertilizer on soil mineral N content under the same C/N ratio (25︰1) input. In the case of straw retention amended with mineral N fertilizer, C/N ratio of straw retention had significant influence on soil mineral N content, and C/N ratio was negatively related to soil mineral N content. 2) Straw retention amendment with N fertilizer increased soil microbial biomass N content, while no difference in soil microbial biomass N content among different N fertilizer inputs treatments. Straw retention amended with N fertilizer with sitting in integrated judgment, the urease activity had no remarkably different. Straw retention amended with N fertilizer increased FDA hydrolyase activity, and there was an increasing tendency with the decreasing of C/N ratio. 3) Straw retention amended with miner N fertilizer significantly increased crop above-ground biomass. Compared to straw retention and with an adjusted C/N ratio of 25︰1, straw retention with an adjusted C/N ratio of 10︰1 and 16︰1 increased above-ground biomass in both seedling and maturing stage. Compared to straw retention, straw retention amendment with organic N fertilizer had no influence on crop above-ground biomass. Straw retention amendment with miner N fertilizer increased crop yield, especially under the C/N ratio 16︰1 input; however, there was a decreased tendency at straw retention amendment with organic N fertilizer under the C/N ratio 25︰1. We concluded that applying mineral N fertilizer and adjusting C/N ration to 16︰1 may be more suitable N fertilizer practices after straw retention.