Comparison of carbon footprint and net ecosystem carbon budget under organic materials retention combined with reduced mineral fertilizer

<title>Abstract</title> <bold>Background:</bold> Excessive application of chemical fertilizer has resulted in lower nitrogen uptake and utilization of crops, decreasing soil fertility, increasing greenhouse gas emissions and worse environment pollution and organic fertilizer materials retention is regard as the key to solve these problems. Whether the retention of <italic>Astragalus sinicus </italic>L<italic>. </italic>(Chinese milk vetch) and rice straw would induce lower greenhouse gas emissions and higher carbon sink remains unclear. The objective of this study is to conduct an assessment of carbon budget under <italic>Astragalus sinicus</italic> L. and rice straw retention combined with reduced mineral fertilizer based on the two years field experiment in a paddy field in southern China. Four treatments were designed: control CK (winter follow, 120 kg ha^-1N fertilizerfor each rice season) and three treatments with <italic>Astragalus sinicus</italic> L. and rice straw retention named RA, RB and RC (reduced N fertilizer 15%, 27.5% and 40% in turn during each rice season).<bold>Results: </bold>Treatments RA, RB and RC increased GHG emissions by 9.30%~101.25%, among which CH₄ accounted for more than 60%; Carbon input of crops from Treatments RA, RB and RC increased by 2.25%~12.10% compared with control CK over the two years. Though treatments RA, RB and RC enhanced CO₂ emission, however, treatment RB decreased carbon footprint and became C sink.<bold>Conclusions:</bold> The results of this study reveal that treatment RB is better in reducing chemical fertilizer amount, increasing crop yield and C balance, which is more conductive to sustainable development of agriculture.