Vegetation productivity drives soil mineral associated organic matter formation and the relative contribution of sorption capacity across climates.

Abstract

Mineral associated organic matter (MAOM) plays an important role in soil carbon (C) storage due to its long residence time in soils. Commonly, MAOM formation is assumed to be controlled by landscape scale factors such as ecosystem type and climate. However, within climates and landscapes rates of vegetative productivity and mineral sorption capacity contribute to MAOM formation. To what degree are climate, productivity, or sorption to mineral surfaces responsible for rates of MAOM development in meadow ecosystems with large, dynamic soil C stocks? We combined an in-situ13C pulse labeling experiment in montane meadows to measure MAOM formation with laboratory incubations to assess mineral sorption rates of organic C. We compared 13C label recovery and existing sorption capacity in high and low productivity regions of humid and semiarid montane climates and found that productivity explained the greatest variation among plots. In low productivity plots, the assimilated 13C label was rapidly incorporated into the MAOM fraction but was destabilized over one year of observation in both climates. Conversely, high productivity plots incorporated plant assimilate into MAOM more slowly and retained the labeled C one year later in both climates. The observation of MAOM destabilization in low productivity plots and MAOM retention in high productivity plots may create differences in mineral sorption capacity between regions of high and low productivity. This trend was supported by laboratory incubations where minerals from low productivity plots in either climate were able to sorb ten times as much organic C onto soil minerals as high productivity plots in either climate. Our results suggest that low vegetation productivity may limit the relative contribution of mineral sorption in MAOM formation across climates, making productivity the most important driver in MAOM formation we measured. The interaction of plant productivity and mineral sorption provide mechanisms for previously observed large soil C stocks and soil C dynamics in montane meadows.