Limited dependence on soil nitrogen fixation as subtropical forests develop.

Soil nitrogen (N) fixation, driven by microbial reactions, is critical to support the entrance of nitrogen in nutrient poor and pioneer ecosystems. However, how and why N fixation and soil diazotrophs evolve as forests develop remain poorly understood. Here, we used a 60-year forest rewilding chronosequence and found that soil N fixation activity gradually decreased with increasing forest age, experiencing dramatic drops of 64.8% in intermediate stages and 93.0% in the oldest forests. Further analyses revealed loses in diazotrophic diversity and a significant reduction in the abundance of important diazotrophs (e.g., Desulfovibrio and Pseudomonas) as forest develops. This reduction in N fixation, and associated shifts in soil microbes, was driven by acidification and increases in N content during forest succession. Our results provide new insights on the life history of one of the most important groups of soil organisms in terrestrial ecosystems, with consequences for understanding the buildup of nutrients as forest soil develops.

[Dynamics of soil respiration and its influencing factors in urban forests under nitrogen addition]. / æ°®æ·»åŠ ä¸‹åŸŽå¸‚æ£®æž—åœŸå£¤å‘¼å¸åŠ¨æ€å˜åŒ–åŠå ¶å½±å“å› ç´ .

Urban forest is an important carbon pool, soil respiration of which is an important part of terrestrial carbon cycle. To understand the dynamics and influencing factors of soil respiration in urban forest under the background of increasing nitrogen deposition, we conducted dynamic observation on soil respiration rate, temperature, moisture and chemical properties by adding 0 (CK), 50 (LN), 100 (HN) kg N·m-2·a-1 ammonium nitrate to a typical urban forest. The results showed that soil respiration had significant seasonal variation, which was not affected by nitrogen addition. Soil respiration was significantly correlated with soil temperature. The interaction between soil temperature and soil moisture could better explain the variation of soil respiration. Nitrogen addition changed temperature sensitivity of soil respiration, with the order of Q10 values as LN (2.12) > CK (2.10) > HN (2.05). Soil nitrate concentration, soil soluble organic carbon, pH, soil carbon to nitrogen ratio had significant correlation with soil respiration. The positive effect of nitrogen deposition on soil respiration was mainly in the growing season, with slightly inhibitive effect in the non-growing season.