Soil phosphorus (P) availability is thought to limit above-ground net primary production in many tropical forests, and recent data suggest that low P content may also constrain soil carbon (C) fluxes. Here we investigated the effects of nutrient availability on soil respiration in three sites occupying a long substrate age gradient (LSAG) in Hawai’i, where the nature of nutrient limitation to above-ground C storage has been well established. Using a full-factorial (nitrogen [N] × P) fertilization experiment in combination with short-term substrate induced growth response (SIGR) and long-term soil incubations, we assessed the nature and extent of nutrient limitation to soil respiration across the gradient, as well as compared soil nutrient controls over above- and below-ground C cycling. We hypothesized that, across the three study sites (Thurston: 300 yr old; Laupahoehoe: 20 kyr old; and Koke’e: 4100 kyr old), soil respiration responses to nutrient additions would be consistent with above-ground responses; namely, N limitation at the youngest site, N/P co-limitation at the intermediate site, and P limitation at the oldest site.
Results/Conclusions

We found that nutrient availability greatly constrained C efflux from all soils and that long-term field fertilizations effectively eliminated this nutrient limitation.  Furthermore, the extent of nutrient limitation—estimated by the relative respiration rates in soils incubated with nutrients vs those without—was higher in soils with lower initial nutrient availability, suggesting that C losses from nutrient-poor soils, such as those found in many tropical forests, may be more responsive to anthropogenic nutrient inputs. Above- and below-ground responses to added nutrients were consistent at some but not all sites, and soil nutrient status helped explain these patterns. Notably, the addition of N and P in combination consistently elicited a large increase in soil respiration rates, showing responses that were not predictable from either above-ground responses nor from soil responses to individual nutrient additions. Together, our data suggest that basic assays of soil nutrient availability provide insight into the nature of nutrient limitation belowground, and that anthropogenic nutrient inputs may result in soil respiration responses that are not predictable from plant responses alone (and vice versa).  Our data also suggest that physiological and stoichiometric differences between plants and soil microbes may help explain observed variations in above- and below-ground relationships to soil nutrients.