Increasing nitrogen (N) rate could accelerate the decomposition of crop residues, and then improve crop yield by increasing N availability of soil and N uptake of crops.  However, it is not clear whether N rate and plant density should be modified after a long period of cotton stubble return with high N rate.  This study seeks to assess the effects of N rate and plant density on cotton yield, N use efficiency, leaf senescence, soil inorganic N, and apparent N balance in cotton stubble return fields in Liaocheng, China, in 2016 and 2017.  Three plant densities 5.25 (D_5.25), 6.75 (D_6.75) and 8.25 (D_8.25) plants m^–2 and five N rates 0 (N₀), 105 (N₁₀₅), 210 (N₂₁₀), 315 (N₃₁₅), and 420 (N₄₂₀) kg ha^–1  were investigated.  Compared to the combination used by local farmers (D_5.25N₃₁₅), a 33.3% N reduction and a 28.6% increase in plant density (D_6.75N₂₁₀) can maintain high cotton yield, while a 66.7% N reduction at 6.75 plants m^–2 (D_6.75N₁₀₅₎ can only achieve high yield in the first year.  Biological yield increased with the increase of N rate and plant density, and the highest yield was obtained under 420 kg N ha^–1 at 8.25 plants m^–2 (D_8.25N₄₂₀) across the two years under investigation.  Compared to D_5.25N₃₁₅, N agronomic efficiency (NAE) and N recovery efficiency (NRE) in D_6.75N₂₁₀ increased by 30.2 and 54.1%, respectively, and NAE and NRE in D_6.75N₂₁₀ increased by 104.8 and 88.1%, respectively.  Soil inorganic N decreased sharply under 105 kg N ha^–1, but no change was found under 210 kg N ha^–1 at 6.75 plants m^–2.  N deficit occurred under 105 kg N ha^–1, but it did not occurr under 210 kg N ha^–1 at 6.75 plants m^–2.  Net photosynthetic rate and N concentration of leaves under N rate ranging from 210 to 420 kg ha^–1 were higher than those under N rate of 0 or 105 kg N ha^–1 at all three densities.  The findings suggest that D_6.75N₂₁₀ is a superior combination in cotton stubble retaining fields in the Yellow River Valley and other areas with similar ecologies.

Improved utilization of rice (Oryza sativa L.) straw and Chinese milk vetch (Astragalus sinicus L., vetch) has positive effects on rice production.  So far, few studies have investigated the productivity of vetch under different residue management practices in double-rice cropping system.  The effects of rice straw on the growth and nutrient accumulation of vetch across seven years (2011–2017) and the subsequent effects of rice straw and vetch on two succeeding rice crops in a vetch–rice–rice cropping system, with the vetch established by relay cropping, were examined.  The seven-year double-rice experiment consisted of the following treatments: (1) 100% chemical fertilizer (F-F100); (2) only vetch without chemical fertilizer (M-Con); (3) 80% chemical fertilizer plus vetch plus a low-cutting height (low-retained stubble) with the removal of straw (M-F80); (4) 80% chemical fertilizer plus vetch plus a low-cutting height with the retention of straw (M-F80-LR); (5) 80% chemical fertilizer plus vetch plus a high-cutting height (high-retained stubble) with the retention of straw (M-F80-HR); and (6) no fertilizer (F-Con).   The yields of the two rice crops after vetch were not affected by either the cutting height of stubble with retention of straw or by the management of straw (retention vs. removal) with low-cutting height of stubble.  The yields of the two rice crops after vetch were significantly higher for M-F80-HR than for M-F80-LR, but the relative contributions of the high-cutting height and straw retention to the higher rice yield could not be determined in this study.  The yield stability of the double-rice grain in M-F80-HR was also increased, as determined by a sustainable yield index.  Significant increases in vetch biomass and nutrient uptake were observed in the fertilized treatments during the rice season compared with the unfertilized treatments.  In M-F80-HR plots, improvements in the growing environment of the vetch by conserving soil water content were associated with the highest vetch biomass, nutrient uptake, and yield stability of vetch biomass.  These increased nutrient inputs partially replaced the demand for chemical fertilizer and stimulated the rice yields.  It can be concluded that retaining higher-cutting stubble residues with straw retention could be the best straw management practice for increasing the vetch biomass and nutrient use efficiency, thereby allowing utilization of high-cutting height with retention of straw and vetch to improve the stability of rice productivity in a double-rice cropping system.

The double-rice cropping system is a very important intensive cropping system for food security in China.  There have been few studies of the sustainability of yield and accumulation of soil organic carbon (SOC) in the double-rice cropping system following a partial substitution of chemical fertilizer by Chinese milk vetch (Mv).  We conducted a 10-year (2008–2017) field experiment in Nan County, South-Central China, to examine the double-rice productivity and SOC accumulation in a paddy soil in response to different fertilization levels and Mv application (22.5 Mg ha^–1).  Fertilizer and Mv were applied both individually and in combination (sole chemical fertilizers, Mv plus 100, 80, 60, 40, and 0% of the recommended dose of chemical fertilizers, labeled as F100, MF100, MF80, MF60, MF40, and MF0, respectively).  It was found that the grain yields of double-rice crop in treatments receiving Mv were reduced when the dose of chemical fertilizer was reduced, while the change in SOC stock displayed a double peak curve.  The MF100 produced the highest double-rice yield and SOC stock, with the value higher by 13.5 and 26.8% than that in the F100.  However, the grain yields increased in the MF80 (by 8.4% compared to the F100), while the SOC stock only increased by 8.4%.  Analogous to the change of grain yield, the sustainable yield index (SYI) of double rice were improved significantly in the MF100 and MF80 compared to the F100, while there was a slight increase in the MF60 and MF40.  After a certain amount of Mv input (22.5 Mg ha^–1), the carbon sequestration rate was affected by the nutrient input due to the stimulation of microbial biomass.  Compared with the MF0, the MF100 and MF40 resulted in a dramatically higher carbon sequestration rate (with the value higher by 71.6 and 70.1%), whereas the MF80 induced a lower carbon sequestration rate with the value lower by 70.1% compared to the MF0.  Based on the above results we suggested that Mv could partially replace chemical fertilizers (e.g., 40–60%) to improve or maintain the productivity and sustainability of the double-rice cropping system in South-Central China.