Nitrous oxide emissions from a tropical Oxisol under monocultures and an integrated system in the Southern Amazon – Brazil

ABSTRACT Although agriculture and livestock systems represent important sources of N2O from the soil, they may also aid in emissions mitigation, mainly when integrated systems are taken into account, such as crop-livestock-forest, for food production. This work assessed the soil N2O emissions from a tropical Oxisol under row-crop, livestock, forest monocultures, and an integrated crop-livestock-forest system in the Southern Amazon - Brazil. Soil N2O emissions were measured using static chambers from November 2014 to October 2016 in four soil use systems [row-crop, livestock, forest, and integrated crop-livestock-forest (CLF)], and in a reference area under native forest fragment. For the whole period, the average of soil N2O fluxes was 16.9, 12.2, and 15.4 µg N2O-N m-2 h-1, to row-crop, livestock, and CLF systems, respectively, all with a similar average among them. The lowest fluxes were observed in the forest system and native forest fragment, with average fluxes of 4.0 and 6.3 µg N2O-N m-2 h-1, respectively, both lower than the agricultural systems. The largest soil N2O fluxes were observed throughout the rainy seasons in the row-crop, livestock, and CLF, mostly after N-fertilizer application to the soil surface or in the planted row. As a consequence, the cumulative emissions were greater in row-crop, livestock, and CLF systems, which in the averages of two cycles emitted respectively 1.40, 1.15, and 1.27 kg N2O-N ha-1 yr-1, all different of the forest system and native forest fragment (0.33 and 0.52 kg N2O-N ha-1 yr-1, respectively). Nitrogen fertilization and soil moisture influenced soil N2O emissions of all systems assessed in the Southern Amazon. The N2O emissions took place after both factors were met, corroborating the hole-in-the-pipe model. Even with more soil use intensification, once in the same area there were three cultures in succession during a year and perennial trees, CLF did not lead to greater N2O emissions from the soil than row-crop and livestock. Thus, CLF represents a good option for N2O mitigation for the edaphic and climatic conditions of the Southern Amazon.

摘要
尽管农业与畜牧系统是土壤氧化亚氮（N₂O）的重要排放源，但它们也可助力温室气体减排，尤其是在应用粮-畜-林综合种养系统开展粮食生产的场景下。本研究针对巴西亚马孙南部地区的热带老成土（Oxisol），采用静态箱法于2014年11月至2016年10月期间，对条播作物种植、畜牧养殖、单一林木种植以及粮-畜-林综合种养系统（CLF）共4种土地利用系统，以及一处原生林片段对照区的土壤N₂O排放进行了观测。
整个观测周期内，条播作物种植、畜牧养殖与CLF系统的土壤N₂O平均通量分别为16.9、12.2和15.4 µg N₂O-N·m⁻²·h⁻¹，三者平均通量水平无显著差异。林木单一种植系统与原生林对照区的N₂O通量最低，平均通量分别为4.0和6.3 µg N₂O-N·m⁻²·h⁻¹，均显著低于各类农业利用系统。条播作物、畜牧养殖与CLF系统的土壤N₂O高排放均集中于雨季时段，且多发生于土壤表面追肥或种植行施肥之后。
因此，条播作物种植、畜牧养殖与CLF系统的累积排放量更高：两个轮作周期的年均累积排放量分别为1.40、1.15和1.27 kg N₂O-N·ha⁻¹·yr⁻¹，均显著高于林木单一种植系统与原生林对照区（年均累积排放量分别为0.33和0.52 kg N₂O-N·ha⁻¹·yr⁻¹）。氮肥施用与土壤含水率是影响亚马孙南部地区所有受试土地利用系统土壤N₂O排放的关键因素。当上述两个条件同时满足时，土壤N₂O排放即会发生，这一结果验证了管道漏洞模型（hole-in-the-pipe model）的合理性。
尽管CLF系统的土地利用强度更高——同一块区域一年内可依次开展三种作物种植并搭配多年生林木——但其土壤N₂O排放量并未高于条播作物种植与畜牧养殖系统。综上，针对亚马孙南部地区的土壤与气候条件，CLF系统是缓解土壤N₂O排放的优良方案。