Reconsidering the phosphorus limitation of soil microbial activity in tropical forests

1. It has long been believed that soil microbial activity in tropical forest ecosystems is limited by phosphorus (P) rather than nitrogen (N) availability. In this study, we reviewed the methods used to determine the limiting nutrients and evaluated the validity of the widespread P-limitation hypothesis in tropical forest soils.
2. The most commonly used analysis method entails testing whether fertilization increased microbial biomass or soil respiration. Fertilization using microbial biomass as an indicator was not a satisfactory method because standing microbial biomass does not always signal microbial activity. An increase in soil respiration after fertilization was also an insufficient indicator because the negative response shown by organic matter decomposition to nutrient addition can also be driven by nutrient shortage (nutrient mining). Nutrient amendment can also shift microbial communities toward more copiotrophic organisms, which may exhibit lower microbial respiration rates.
3. We suggest that P addition may accelerate soil organic matter decomposition compared with N, which is independent of the nutrient limitation of soil microbial activity. The negative response of organic matter decomposition to N addition through nutrient mining is driven by N shortage, which is less likely to occur with P. N addition inhibits microbial activity via chemical reactions, whereas P addition may stimulate activity through replacement by C bound to sorption sites in the soil, improving C availability. Thus, the P-limitation hypothesis must be reconsidered because of the contrasting effects of exogenous N and P addition on soil microbial activity, which could lead to misdetection of P limitation on soil microbial activity in tropical forest ecosystems.
4. We recommend caution in applying the statement that soil microbial activity in tropical forest ecosystems is limited by P until a novel method is established to accurately determine the nutrients limiting soil microbial activity at the ecosystem level. We propose alternative ways to describe nutrient shortage for soil microbes.