Plant growth is usually constrained by the availability of nutrients, water, or temperature, rather than photosynthetic carbon (C) fixation. Under these conditions leaf growth is curtailed more than C fixation, and the surplus photosynthates are exported from the leaf. In plants limited by nitrogen (N) or phosphorus (P), photosynthates are converted into sugars and secondary metabolites. Some surplus C is translocated to roots and released as root exudates or transferred to root-associated microorganisms. Surplus C is also produced under low moisture availability, low temperature, and high atmospheric CO₂ concentrations, with similar below-ground effects. Many interactions among above- and below-ground ecosystem components can be parsimoniously explained by the production, distribution, and release of surplus C under conditions that limit plant growth.

植物的生长通常受养分，水或温度的限制，而不是受光合作用碳（C）固定的约束。在这些条件下，叶片生长的抑制作用比C固定作用要大得多，多余的光合产物从叶片中输出。在受氮（N）或磷（P）限制的植物中，光合产物被转化为糖和次级代谢产物。一些过剩的C易位到根，并以根分泌物的形式释放或转移到与根相关的微生物中。多余的C也会在水分含量低，温度低和大气CO ₂含量高的情况下产生浓度，具有类似的地下效应。在限制植物生长的条件下，过量C的产生，分布和释放可以简单地解释地下和地下生态系统各组成部分之间的许多相互作用。