Long-term agricultural management does not alter the evolution of a soybean-rhizobium mutualism

Leguminous crops, like soybeans, often rely on biologically fixed nitrogen via their symbiosis with rhizobia rather than synthetic nitrogen inputs. However, agricultural management practices may influence the effectiveness of biological nitrogen fixation. While the ecological effects of agricultural management on rhizobia have received some attention, the evolutionary effects have been neglected in comparison. Resource mutualism theory predicts that evolutionary effects are likely, however. Both fertilization and tillage are predicted to cause the evolution of rhizobia that provide fewer growth benefits to plant hosts and fix less nitrogen. This study capitalized on an LTER (Long Term Ecological Research) experiment that manipulated agricultural management practices in a corn-soybean-wheat row crop system for 24 years to investigate whether four different management practices (conventional, no-till, low chemical input, and certified organic) cause rhizobia populations to evolve to become more or less cooperative. We found little evidence that 24 years of varying management practices affect the net growth benefits rhizobia provide to soybeans, although soybean plants inoculated with soils collected from conventional treatments tended to have lower biological nitrogen fixation rates than plants inoculated with soils from the no-till, low input, and organic management treatments. These findings suggest that rhizobia will continue to provide adequate growth benefits to leguminous crops in the future, even in intensively managed systems.

豆科作物（如大豆）通常依靠与根瘤菌的共生关系实现生物固氮，而非依赖合成氮肥输入。然而农业管理措施可能会影响生物固氮的效能。尽管农业管理对根瘤菌的生态效应已受到一定关注，但相较之下其进化效应却被显著忽视。资源共生理论预测，这类进化效应大概率存在：施肥与耕作措施均被推测会促使根瘤菌进化出对宿主植物提供更少生长益处、且固氮能力更弱的类群。本研究依托LTER（长期生态学研究，Long Term Ecological Research）长期实验平台，该实验在玉米-大豆-小麦条播大田种植系统中开展了24年的农业管理措施调控，旨在探究四种不同管理措施——常规农作管理、免耕管理、低化学投入管理以及有机认证管理——是否会导致根瘤菌种群进化出更强或更弱的共生合作能力。研究未发现足够证据表明，24年的差异化管理措施会改变根瘤菌为大豆提供的净生长益处；不过接种了常规农作处理土壤的大豆植株，其生物固氮速率往往低于接种免耕、低化学投入及有机认证管理处理土壤的植株。本研究结果表明，即便在集约化管理的农田系统中，根瘤菌未来仍能为豆科作物提供充足的生长益处。