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Vegetation carbon use efficiency (CUE) is a key measure of carbon (C) transfer from the atmosphere to terrestrial biomass, and indirectly reflects how much C is released through autotrophic respiration from the vegetation to the atmosphere. Diagnosing the variability of CUE with climate and other environmental factors is fundamental to understand its driving factors, and to further fill the current gaps in knowledge about the environmental controls on CUE. Thus, to study CUE variability and its driving factors, this study established a global database of site-year CUE based on observations from 188 field measurement sites for five ecosystem types - forest, grass, wetland, crop and tundra. The spatial pattern of CUE was predicted from global climate and soil variables using Random Forest, and compared with estimates from Dynamic Global Vegetation Models (DGVMs) from the TRENDY model ensemble. Globally, we found two prominent CUE gradients in ecosystem types and latitude, that is, CUE varied with ecosystem types, being the highest in wetlands and lowest in grassland, and CUE decreased with latitude with the lowest CUE in tropics, and the highest CUE in higher latitude regions. CUE varied greatly between data-derived CUE and TRENDY-CUE, but also among TRENDY models. Both data-derived and TRENDY-CUE challenged the constant value of 0.5 for CUE, independent of environmental controls. However, given the role of CUE in controlling the spatial and temporal variability of the terrestrial biosphere C cycle, these results emphasize the need to better understand the biotic and abiotic controls on CUE to reduce the uncertainties in prognostic land-process model simulations. Finally, this study proposed a new estimate of net primary production based on CUE and gross primary production, offering another benchmark for net primary production comparison for global carbon modelling. [ABSTRACT FROM AUTHOR]

Profiles of soil organic carbon (SOC) are often characterized by a steep increase of 14C age with depth, often leading to subsoil 14C ages of more than 1000 years. These observations have generally been reproduced in SOC models by introducing a SOC pool that decomposes on the time-scale of millennia. The overemphasis of chemical recalcitrance as the major factor for the persistence of SOC was able to provide a mechanistic justification for these very low decomposition rates. The emerging view on SOC persistence, however, stresses that apart from molecular structure a multitude of mechanisms can lead to the long-term persistence of organic carbon in soils. These mechanisms, however, have not been incorporated into most models. Consequently, we developed the SOC profile model COMISSION which simulates vertically resolved SOC concentrations based on representations of microbial interactions, sorption to minerals, and vertical transport. We calibrated COMISSION using published concentrations of SOC, microbial biomass and mineral-associated OC (MOC), and in addition, 14C contents of SOC and MOC of a Haplic Podzol profile in North-Eastern Bavaria, Germany. In order to elucidate the contribution of the implemented processes to the 14C age in different parts of the profile, we performed model-experiments in which we switched off the limitation of SOC decomposition by microbes, sorptive stabilization on soil minerals, and dissolved OC (DOC) transport. By splitting all model pools into directly litter-derived carbon and microbe-derived organic carbon, we investigated the contribution of repeated microbial recycling to 14C ages throughout the profile. The model-experiments for this site lead to the following implications: Without rejuvenation by DOC transport, SOC in the subsoil would be on average 1700 14C years older. Across the profile, SOC from microbial recycling is on average 1400 14C years older than litter-derived SOC. Without microbial limitation of depolymerization, SOC in the subsoil would be on average 610 14C years younger. Sorptive stabilization is responsible for relatively high 14C ages in the topsoil. The model-experiments further indicate that the high SOC concentrations in the Bh horizon are caused by the interplay between sorptive stabilization and microbial dynamics. Overall, the model-experiments demonstrate that the high 14C ages are not solely caused by slow turnover of a single pool, but that the increase of 14C ages along a soil profile up to ages >1000 years is the result of different mechanisms contributing to the overall persistence of SOC. The dominant reasons for the persistence of SOC are stabilization processes, followed by repeated microbial processing of SOC. [ABSTRACT FROM AUTHOR]

The turnover of fine-roots is a crucial component for the input of C to the soil. The amount of root litter input is depending on estimates of turnover times from different techniques. Turnover times from fine-root cameras (minirhizotrons) often yield 75% higher root litter input estimates than turnover times estimated with the bomb-radiocarbon signature of fine roots. We introduce a generic framework for the analysis of fine-root 14C with different survival functions. So far, mostly an exponential function has been used to estimate the turnover time and mean age of fine roots. In the context of the introduced survival function framework we clarify the terms turnover time, mean residence time, mean longevity, and mean age, which are commonly used in studies of root turnover. Using a unique time series of fine-root 14C ( Fröberg 2012), we test if survival functions other than the exponential function are better in accordance with turnover-time estimates commonly found with other methods. A survival function that corresponds to a two-pool model was best in agreement with minirhizotron-based estimates (mean residence time of 1.9 y). We argue that using fine-root 14C and minirhizotron time-to-death data together would give the best constraints on fine-root turnover. At the same time this could allow quantifying systematic biases inherent to both techniques. [ABSTRACT FROM AUTHOR]

The research on soil organic matter dynamics has seen a paradigmatic shift regarding which processes determine the persistence of soil organic matter in soils. The interaction between microbial decomposition and association of organic matter with the soil mineral matrix has been identified as pivotal for understanding the formation of soil organic matter. Yet, the stabilization of organic matter on soil minerals is currently not explicitly considered in the soil modules of earth system models. Using a vertically integrated SOM model (COMISSION, Ahrens et al. (2015), Soil Biology and Biochemistry, 88, 390-402), we investigate the effect of a limited organo-mineral association capacity and its interaction with microbial processes, such as microbial decomposition and microbial necromass production.We defined a maximum organo-mineral association capacity based on quantile regressions between the observed organo-mineral fraction and the clay + silt content. With the COMISSION model we model organo-mineral associations equivalent to a dynamic Langmuir sorption model. We suggest that plant- and microbe-derived dissolved organic matter and dead microbial cell walls can form organo-mineral associations that are protected from microbial decomposition. With this global modelling study, we show that the inclusion of organo-mineral associations provides a mechanism for long residence times of organic matter in soils. Globally, at ESM grid cell level (200 km) COMISSION captures patterns of soil organic carbon and apparent radiocarbon ages in the top- and subsoil. While COMISSION reproduces mean radiocarbon profiles across the most dominant soil orders of the compiled database, missing processes such as pelo- and cryoturbation or mineralogical control in Oxisols and Andisols explain mismatches between modelled and observed radiocarbon profiles for certain soil orders. [ABSTRACT FROM AUTHOR]