Relationships among climate, soil nutrients, and enzyme stoichiometry in low-latitude ecosystems: A pan-tropical analysis

Background/Question/Methods

The relative activities of soil enzymes involved in mineralizing organic carbon (C), nitrogen (N), and phosphorus (P) reveal stoichiometric and energetic constraints on microbial biomass growth. Although tropical forests and grasslands are a major component of the global C cycle, relatively little is known about how C and nutrient availability influence microbial metabolism in these systems.  To explore controls on microbial element limitation in tropical soils, we performed a meta-analysis to examine the activities of β-glucosidase (BG), β-N-acetyl-glucosaminidase (NAG), and acid/alkaline phosphatase (AP) as indicators of microbial demand for C, N, and P, respectively. We examined variation in activities of BG, NAG, AP and their ratios in relation to climate, soil nutrient content, and plant community composition in ecosystems throughout the geographical tropics. Next, we used the results of our analysis to parameterize a biogeochemical equilibrium model that relates microbial growth efficiency to extracellular enzyme activity.  

Results/Conclusions

We found that BG:AP and NAG:AP ratios in tropical soils are significantly lower than those of temperate ecosystems overall, suggesting high microbial demand for P relative to C or N.  The ratio of NAG:AP increased significantly with soil pH. Additionally, correlations between enzyme activities and mean annual temperature (MAT) and precipitation (MAP) suggest some climatic regulation of microbial enzyme allocation in tropical soils, with higher AP activities in wetter ecosystems. Taken together, these results suggest that soil microbes allocate more resources toward P acquisition in old, highly weathered, low-pH, or leached soils that are characteristic of many tropical ecosystems. Furthermore, the biogeochemical equilibrium model predicts low microbial growth efficiencies in the most P-limited soils. Therefore, we suggest that P availability may strongly influence microbial enzyme allocation, biomass growth, and C mineralization in highly weathered tropical soils.