Preface to the special section ‘Soil C and N response to climate, land-use and management changes (Soil C, N and CLUMC)’

Rockstrom et al. (2009) proposed ‘Planetary Boundaries’ which is a concept involving Earth system processes that contain environmental boundaries. In the framework, nitrogen (N) cycle, rate of biodiversity loss and climate change have been transgressed by humanity already. In 2015, the United Nations (UN) established the global agenda for sustainable development until 2030 and defined it as Sustainable Development Goals (SDGs), including 17 specified goals with 169 targets. Many goals were related to land use and management changes (LUMC) with carbon (C) and N dynamics among atmospherebiosphere-pedosphere systems with agricultural activities, such as Zero Hunger (Goal 2), Climate Action (Goal 13), Life on Land (Goal 13) and so on (https://www.undp.org/sustainabledevelopment-goals). Soils could be C sinks or sources depending on climate, land use and management changes (CLUMC) in different plant-soil ecosystems. Soil N and its dynamics not only affect plant growth and crop production but also control soil C sequestration. Thus, C and N dynamics affected by CLUMC in terrestrial ecosystems contribute not only to global and local environmental problems, including global warming, destruction of the ozone layer, acid rain, and eutrophication, but also to changes in soil fertility and crop production related to human sustainable development (Cheng 2020). Many of our colleagues around the world are studying the C and N dynamics by CLUMC in different regions and countries with various ecosystems. We need to share our knowledge and learn from each other researches and discuss how to do the next studies for the UN-SDGs. Therefore, we have held the ‘International Symposium on C and N Dynamics by Land Use and Management Changes in East and Southeast Asian Countries’ on September 10–12, 2018 in Tsuruoka campus, Yamagata University, Japan, and published a special section in Soil Science and Plant Nutrition (SSPN) titled as ‘Soil carbon and nitrogen dynamics by land use and management changes in East and Southeast Asian countries (Soil C and N by LUMC).’ There were 20 papers in the special section, including 4 review papers and 16 original articles (Cheng 2020). Many papers in the special section have been cited many times during the last 2 years. Two years late, we planed the second international symposium for similar topics on September 28–30, 2020 in Tsuruoka, Japan again. Since the outbreaks of COVID-19 pandemic around the world from January 2020, we could not invite and receive guests from foreign countries. Therefore, we decided to hold an International Online Symposium (IOS) on Soil C and N Dynamics by Land Use, Management, and Climate Changes. In this virtual symposium, a total 56 of our colleagues in 14 countries around the world participated and presented their newest research results in this IOS (Figure 1). More than 80 young students in many universities have joined the IOS to learn the front knowledge in the world (Figure 2). The IOS also celebrated Professor Kazuyuki INUBUSHI will officially retire from Chiba University in March 2021 since many of his former students around the world joined the IOS together even during the COVID-19 pandemic period. As much of the research presented in the IOS is valuable, original, and has not been published yet, we organized a special section ‘Soil C and N response to climate, land-use and management changes (Soil C, N and CLUMC)’ in SSPN. In addition, we also collected original related papers from colleagues around the world who did not attend the symposium. Finally, we accepted 20 papers, including 2 review papers and 18 original articles, for publishing in this special section of Soil C, N and CLUMC. Greenhouse gas (GHG) emissions from human activities are causing climate change, including global warming. To slow climate change requires reducing these emissions. The government of many countries declared that they would realize carbon-neutrality (net-zero carbon emissions) by around 2050. Net-zero emissions will be achieved when all GHGs (carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), etc.) emissions released by human activities are counterbalanced by removing GHGs from the atmosphere in a process known as carbon removal. Since the forests are a very important sink of atmospheric GHGs, we need to understand the real potentials of forests for mitigating climate change. In this special section, Sase et al. (2022) reviewed the reactive nitrogen (Nr) from the atmosphere by both domestic and transboundary air pollution and its dynamics in center Japan from their long-term investigation. The current atmospheric dissolved inorganic N is still high, at over 10 kg ha year, which implied the C sequestration and CH4 absorption and N2O emission should be affected by the Nr deposition. Xu (2022) reviewed the effect of freeze-thaw disturbance on soil C and N dynamics and GHG fluxes of East Asia forests according to the laboratory and field measurements. His review showed freeze-thaw disturbance mostly increased concentrations and leaching fluxes of