Effects of pasture-sugarcane conversion and intensification on soil microbiota

Brazil is a global leader in the production of sugarcane and ethanol with an area of approximately 8.7 million hectares dedicated to sugarcane harvest. Despite the constant increase in sugarcane production, to prevent its expansion over native Brazilian biomes, the conversion of underutilized pasture to sugarcane is one approach to be considered. Soil microbial community involved in nitrogen cycling in pasture and sugarcane fields, fertilized or not, may provide information about nitrogen transformation in soils. To evaluate the effects of pasture-to-sugarcane conversion as well as the intensification of both systems, by nitrogen fertilization, soil properties, ammonium and nitrate concentration in soil, abundance, diversity and taxonomy of target genes, nifH, amoA, nirK, nirS and nosZ clade I and clade II, and microbial groups associated with nitrogen cycling were measured. The abundance and composition of soil microbial communities, including nitrification- and denitrification-associated genes, were influenced by the conversion from pasture to sugarcane but remained unaffected by fertilizer application. The amoA genes from both archaea and bacteria were more abundant in sugarcane soils, while nirS and nosZ (clade I) genes were more dominant in pasture soils. Land-use change also led to an increase in archaeal populations, whereas fungal abundance showed no response to either fertilization or land use. These findings indicate that nitrogen-cycling soil communities are more sensitive to land-use change than to fertilizer application.

巴西是全球甘蔗与乙醇生产的领军国家，其甘蔗种植总面积约达870万公顷。尽管甘蔗产量持续攀升，但为避免种植规模扩张至巴西原生生物群系，将未充分利用的牧场改造为甘蔗田是一项值得考量的解决方案。针对牧场与甘蔗田（无论是否施加氮肥）中参与氮循环的土壤微生物群落，其相关特征可为解析土壤氮转化过程提供依据。为评估牧场转甘蔗田的土地利用变化效应，以及两类系统（牧场与甘蔗田）通过氮肥施用产生的集约化影响，本研究测定了多项指标：土壤理化性质、土壤铵态氮与硝态氮浓度、目标功能基因（固氮酶铁蛋白亚基基因(nifH)、氨单加氧酶亚基A基因(amoA)、亚硝酸还原酶K基因(nirK)、亚硝酸还原酶S基因(nirS)以及氧化亚氮还原酶Z基因(nosZ)进化支I与进化支II）的丰度、多样性与分类学特征，以及与氮循环相关的微生物类群。研究结果显示，土壤微生物群落的丰度与组成（包括与硝化、反硝化相关的功能基因）受牧场转甘蔗田的土地利用变化显著影响，但不受氮肥施用的干扰。古菌与细菌来源的amoA基因在甘蔗田土壤中丰度更高，而nirS与nosZ（进化支I）基因在牧场土壤中占主导地位。土地利用变化同时使古菌种群数量增加，而真菌丰度既不受氮肥施用影响，也不对土地利用变化产生响应。综上，土壤氮循环微生物群落对土地利用变化的敏感性高于其对氮肥施用的响应。