Ecosystem functional responses, particularly CO2 emissions, are closely linked to soil substrates and microbial activities. In tropical forests, phosphorus (P) is often considered as a limiting factor for plant growth. But it is still not clear whether P is a limiting factor to microbial CO2 emissions from soils. In this study, we incubated soils collected from 10 cm depth at from in the Serra do Mar in Brazil and El Yunque National Forest in Puerto Rico. The soils were incubated at 26 oC, 33 oC and 43 oC with constant soil moisture for 10 days with or without carbon (C), nitrogen (N), phosphorus (P), and coupled CNP additions. Soil CO2 emission was measured every 2-3 hours using a Micro-Oxymax Respirometer System. We fit the measured CO2 with a Gompertz function model (Ct=Cm *exp(-b*exp(-kt)), where Ct is the accumulated soil CO2 at time t, Cm is potential total soil CO2 accumulation, kis the exponent that indicates the rate of change of soil carbon decomposition.
Soil CO2 emission increased gradually and leveled off after a prolonged period of time. The Gompertz model fit all data well. Adding N or P only did not influence or only slightly increased Cm and k, but Cm and k were dramatically enhanced with the addition of C. However, in some cases emissions from CNP additions were less than C alone, which could indicate enhanced microbial growth or enzyme production. Incubation temperature also significantly enhanced Cm, but reduced k for all soil types. This study demonstrated that unlike plants, soil microbial activity might not be strongly limited by P.