## 4 Discussion
The biomass contained about 32-80 kg N ha\({}^{-1}\) at termination. The proportion of biologically fixed N by cowpea was not determined in this study but a previous study indicated that cowpea terminated 50 days after planting derived 55%-60% of total N in biomass from the atmosphere [29]. Thus, although cowpea might have increased the pool of N in soils through biological fixation, the biomass N was not transferred effectively to the following crop of winter wheat. This resulted to poor growth of winter wheat, and canopy CO\({}_{2}\) fluxes remained similar to the unfertilized treatment. Future research should focus on improving biomass and N productivity of summer legumes and management approaches, such as timing of termination to synchronize N mineralization from decomposing green manures and N demand of winter wheat. Increased soil respiration is expected after inclusion of cover crops in a monoculture due to increased inputs of C [30; 31]. However, this was not observed during the current study, as the cowpea treatment did not have higher soil respiration rates than treatments without cover crops. A possible reason for the similar level of soil respiration from cowpea and fertilized treated plots might be due to similar, or lower, annual yields of biomass produced by the cowpea treatment. Winter wheat yields were lower from the cowpea treatment than the 45-N and 90-N treatments in the previous years. Although grain of winter wheat was harvested, residue was retained in the field, and thus, the fertilized treatments (45-N and 90-N) provided greater amounts of winter wheat residues than cowpea treatments. Another possible reason for similar SR fluxes was decomposition of a large proportion of cowpea biomass during the period between termination of cowpea and start of the flux measurements. Cowpea was terminated on the 17 th of August in 2016, to conserve moisture to support germination and early growth of winter wheat, but SR fluxes were measured from the 25 th of October. A total of 140 mm rainfall was received during this period which might have created soil conditions conducive to decomposition of cowpea biomass since soil moisture is one of the key environmental factor to control decomposition of cover crop residues in these study sites [16; 21]. A recent study documented that legume green manures with low C/N ratio (<15) may lose a large proportion of biomass C within a month after termination, when soil moisture and temperature are favorable for biomass decomposition [16]. The cowpea biomass in this study had average C/N ratio of 15 at termination. Thus, although cowpea could have increased SR fluxes immediately after termination in August, long-term increases in soil respiration, an indicator of improved soil health, was not observed. In this study, we did not observe strong responses of N\({}_{2}\)O emissions in response to N fertilizers and cowpea green manures, since emissions from all treatments remained low. The first three months during flux measurements were dry, with only 53 mm precipitation received during October through December 2016. Therefore, soil conditions were not conducive for denitrification, a major microbial pathway for N\({}_{2}\)O production in the soil [32]. Although frequent large precipitation events occurred in February and April, emissions did not increase during that time period. The low emissions during these periods was possibly due to efficient uptake of available inorganic N in soil by winter wheat, which was growing rapidly as evidenced by high rates of GPP flux. Legume green manures generally increase emissions of N\({}_{2}\)O after termination [15; 33], but no significant N\({}_{2}\)O emissions were detected from the cowpea treatment in this study. Since there was a gap between termination of cowpea and inception of measurement, and significant amounts of rainfall were received during the gap, rainfall induced pulses of N\({}_{2}\)O emissions from the cowpea treatment were likely missed.