ABSTRACT
Dust provides ecosystem-sustaining nutrients to landscapes underlain by intensively weathered soils. Here we show that dust may also be crucial in montane forest ecosystems, dominating nutrient budgets despite continuous replacement of depleted soils with fresh bedrock via erosion. Strontium and neodymium isotopes in modern dust show that Asian sources contribute 18-45% of dust deposition across our Sierra Nevada, California study sites. The remaining dust originates regionally from the nearby Central Valley. Measured dust fluxes are greater than or equal to modern erosional outputs from hillslopes to channels, and account for 10-20% of estimated millennial-average inputs of bedrock P. Our results demonstrate that exogenic dust can drive the evolution of nutrient budgets in montane ecosystems, with implications for predicting forest response to changes in climate and land use.
ABSTRACT
The controls on methane (CH4) flux into and out of soils are not well understood. Environmental variables including temperature, precipitation, and nitrogen (N) status can have strong effects on the magnitude and direction (e.g., uptake vs. release) of CH4 flux. To better understand the interactions between CH4-cycling microorganisms and N in the non-wetland soil system, a meta-analysis was performed on published literature comparing CH4 flux in N amended and matched control plots. An appropriate study index was developed for this purpose. It was found that smaller amounts of N tended to stimulate CH4 uptake while larger amounts tended to inhibit uptake by the soil. When all other variables were accounted for, the switch occurred at 100 kg N x ha(-1) x yr(-1). Managed land and land with a longer duration of fertilization showed greater inhibition of CH4 uptake with added N. These results support the hypotheses that large amounts of available N can inhibit methanotrophy, but also that methanotrophs in upland soils can be N limited in their consumption of CH4 from the atmosphere. There were interactions between other variables and N addition on the CH4 flux response: lower temperature and, to a lesser extent, higher precipitation magnified the inhibition of CH4 uptake due to N addition. Several mechanisms that may cause these trends are discussed, but none could be conclusively supported with this approach. Further controlled and in situ study should be undertaken to isolate the correct mechanism(s) responsible and to model upland CH4 flux.
