What controls microbial growth in tropical soils? The role of carbon and phosphorus.

Tropical forest ecosystems are important components of global carbon (C) and nutrient cycles. Many tropical rainforests grow on old and highly weathered soils depleted in phosphorus (P) and other rock-derived nutrients. While plants in such forests are usually P limited, it remains unclear if heterotrophic microbial communities are also limited by P or rather by C or energy. Elemental limitations of microorganisms in soil are often approached by measurements of changes in respiration rates or microbial biomass in response to additions of nutrients or carbon. However, it has been argued lately, that microbial growth rather than respiration or biomass should be used to assess microbial limitations.

 

In this study we asked the question whether the growth of heterotrophic microbial communities in tropical soil is limited by available P or by C. We sampled soils along a topographic gradient (plateau, slope, bottom) differing in soil texture and total and available P concentrations from a highly weathered site in French Guiana. We incubated these soils in the laboratory with cellulose as a C source, phosphate (pH adjusted) and with a combination of both. We determined microbial growth by measuring the incorporation of ¹⁸O from labelled water into microbial DNA.

 

In general, plateau soils were higher in microbial C, while bottom soils were higher in microbial P, leading to increased microbial C:P ratios in plateau soils compared to bottom soils. Microbial C, N and P did not respond to the addition of cellulose. Microbial P on the other hand was significantly increased by P additions, with no interactive effect between cellulose and P. Although microbial C was significantly higher in plateau soils, respiration rates were similar to those of bottom soils. This led to similar mass specific respiration rates in plateau and slope soils, with bottom soils being significantly higher. Moreover, we found that C and P addition increased mass specific respiration rates and both nutrient additions showed a positive interactive effect. Gross microbial growth rates were stimulated by P additions but were unresponsive to C additions alone. However, the addition of carbon further stimulated the effect of P on growth.

 

The observed interactive effect of C and P additions on gross microbial growth rates suggests a co-limitation of microorganisms by C and P in highly weathered soils. We argue that co-limitation bears significant ecological advantages for microorganisms as it minimizes the investments in acquiring nutrients for growth.We further conclude that microorganisms in tropical soils are highly efficient in taking up and storing P from the environment. In our experiment, microbial P almost doubled in the six days after P addition, while microbial C was not enhanced. This also means that the microbes were not homeostatic with regard to their C:P ratios. Finally, our study demonstrates the importance of investigating gross microbial growth rates, rather than respiration or biomass, for inferring nutrient limitations.