Short and long term environmental effects on N-mineralization rates in arid ecosystems of Biosphere 2

Background/Question/Methods

           In arid ecosystems, the lack of rainfall limits soil microbial activity and in turn nutrient cycling. Soil microclimate and organic content are principal factors in determining rates of in situ N turnover. N-mineralization is the transformation of organic forms of N into mineral forms that can be taken up by plants and microbes, or leached into lower soil layers or ground water. Rainfall increases soil microbial activity and nutrient mineralization and therefore it is important for scientists to further understand the impacts of varying external environmental conditions, such as soil temperature and moisture, and edaphic factors on N-mineralization rates.   

            This study was conducted using soil samples taken from two savannas within Biosphere 2, a manmade structure located in Oracle, Arizona designed to replicate the different biomes of Earth.  The two biomes ("upper and lower" savannah) differ in plant composition and productivity, as well as the imposed climate regime. We used a combination of in situ, reciprocal soil transplants, and lab incubations to tease apart short and long-term effects on N cycling. The relative difference in rates between the reciprocal soil incubations would allow us to tease apart whether N-mineralization rates are controlled more by short term environmental conditions(temperature and moisture) than long term effects (soil properties and plant cover).

Results/Conclusions

           The upper savannah had higher ambient temperatures, soil organic matter content, and supported more microbial biomass and activity than the lower savannah. Soils in the upper savannah had twice the rate of potential N-mineralization, and 10 times greater potential nitrification rate as the lower savannah. In situ N-mineralization rates were elevated in the upper savannah as well. Lab and field incubations demonstrated that soil moisture and temperature affects the N-mineralization and nitrification rates of soil samples and that soils samples containing higher moisture contents will have higher net N-mineralization rates.  Reciprocal transplants showed that both short and long-term controls are important in regulating N-mineralization rates in semi-arid savannah ecosystems.