Agriculture in the semi-arid is often challenged by overuse of nitrogen (N), inadequate soil water and heavy carbon emissions thereby threatening sustainability. Field experiments were conducted to investigate the effect of nitrogen fertilization levels (N<sub>0</sub> – 0, N<sub>100</sub> – 100, N<sub>200</sub> – 200, N<sub>300</sub> – 300 kg N/ha) on soil water dynamics, soil respiration (Rs), net ecosystem production (NEP), and biomass yields. Zero nitrogen soils decreased Rs by 23% and 16% compared to N<sub>300</sub> and N<sub>200</sub> soils, respectively. However, biomass yield was greatest under N<sub>300</sub> compared with N<sub>0</sub>, which therefore translated into increased net primary production by 89% and NEP by 101% compared to N<sub>0</sub>. To a lesser extent, N<sub>200</sub> increased net primary production by 69% and net ecosystem production by 79% compared to N<sub>0</sub>. Grain yields were greatest under N<sub>300</sub> compared with N<sub>100</sub> and N<sub>0</sub>, which therefore translated into increased carbon emission efficiency (CEE) by 53, 39 and 3% under N<sub>300</sub> compared to N<sub>0</sub>, N<sub>100</sub> and N<sub>200</sub> treatments, respectively. There appears potential for 200 kg N/ha to be used to improve yield and increase CEE.
Results of laboratory and field experiments of the direct effect of increasing CO2 on net primary production of macroalgal species in brackish-water ecosystems
