Wastewater reuse in irrigation: short-term effect on soil carbon and nitrogen stocks in Brazilian semi-arid region

Abstract The main process that opposes the Greenhouse Effect is called “carbon sequestration”, a phenomenon that essentially seeks to establish a dynamic balance between greenhouse gas (GHG) emissions from the immobilization of its elements, especially C and N. This work evaluated the stocks of carbon and nitrogen in soils of the Brazilian semi-arid submitted to irrigation with treated wastewater from domestic sewage. For this purpose, the carbon and nitrogen stocks in the soil of the semi-arid region of Brazil were compared for four different uses: Open Native Caatinga (ONC); Treated Wastewater Effluent Irrigation (TSEI); Surface Water Irrigation (SWI); and Traditional Rainfed Agriculture (TRA). The hypothesis considered in this research was that the application of treated effluents in agriculture, besides being an alternative that can mitigate the problem of water scarcity of the semiarid region, is also capable of influencing the storage capacity of C and N in the soil, due to its organic load. The results indicate that among the different land uses evaluated, soil C and N stocks were highest in ONC, and decreased with the agricultural land use. The greatest accumulation of C and N in Caatinga is due to the presence of forest litter, and the influence of irrigation with treated sewage effluent was not detected in the present study. It can be concluded that the contribution of vegetation residues to the soil surface is the main factor contributing to C and N storage.摘要 与温室效应相对的核心过程被称为碳固存（carbon sequestration），其本质是通过固持温室气体（greenhouse gas）的组分（尤其是碳C和氮N），在温室气体排放与固持之间建立动态平衡。本研究针对巴西半干旱区采用生活污水处理后再生水灌溉的土壤，评估了其碳氮储量。为此，研究对比了巴西半干旱区四种不同土地利用方式下的土壤碳氮储量：开放原生卡廷加植被（Open Native Caatinga, ONC）、再生污水灌溉（Treated Wastewater Effluent Irrigation, TSEI）、地表水灌溉（Surface Water Irrigation, SWI）以及传统雨养农业（Traditional Rainfed Agriculture, TRA）。本研究提出的假说为：农业生产中施用处理后的污水，除可作为缓解半干旱区水资源短缺的替代方案外，还可因其携带的有机负荷，对土壤碳氮固存能力产生影响。研究结果表明，在所评估的四类土地利用方式中，开放原生卡廷加植被区的土壤碳氮储量最高，且随农业用地利用强度升高呈下降趋势。卡廷加植被区的碳氮累积量最高，源于森林凋落物的输入；本研究未检测到再生污水灌溉对土壤碳氮储量的显著影响。综上可得出结论：地表植物残体的输入是影响土壤碳氮固存的核心驱动因素。