Nematode abundance in cold seep sediments of the Darwin mud volcano

During the JC-10 cruise (2007), we sampled the Darwin mud volcano (MV) for meiofaunal community and trophic structure in relation of pore-water geochemistry along a 10 m transect from a seep site on the rim of the crater towards the MV slope. Sediment samples were retrieved by the ROV Isis using push cores. On board and after the pore water extraction, the top 10 cm of the cores were sliced into 1 cm sections and fixed them in 4% formaldehyde for meiofaunal community analysis. In the home laboratory, the formaldehyde-fixed samples were washed over a 32 µm mesh sieve and extracted the meiofauna from the sediment by Ludox centrifugation (Heip et al. 1985). Meiofauna was then sorted, enumerated and identified at coarse taxonomic level. From each slice, ca. 100 nematodes were identified to genus level. Afterwards, abundance of Nematoda were depth integrated over the top 5 cm to gain individual abundances per 10 cm**2. Overall, total nematode biomass in the top 5 cm of the seep sediment core was ~10x higher than that in the core taken 1100 m away. Nematode genus composition varied little among cores and was mainly dominated by Sabatieria.

Global assessment of soil phosphorus retention potential

Limited availability of P in soils to crops may be due to deficiency and/or severe P retention. Earlier studies that drew on large soil profile databases have indicated that it is not (yet) feasible to present meaningful values for "plant-available" soil P, obtained according to comparable analytical methods, that may be linked to soil geographical databases derived from 1:5 million scale FAO Digital Soil Map of the World, such as the 5 x 5 arc-minute version of the ISRIC-WISE database. Therefore, an alternative solution for studying possible crop responses to fertilizer-P applied to soils, at a broad scale, was sought. The approach described in this report considers the inherent capacity of soils to retain phosphorus (P retention), in various forms. Main controlling factors of P retention processes, at the broad scale under consideration, are considered to be pH, soil mineralogy, and clay content. First, derived values for these properties were used to rate the inferred capacity for P retention of the component soil units of each map unit (or grid cell) using four classes (i.e., Low, Moderate, High, and Very High). Subsequently, the overall soil phosphorus retention potential was assessed for each mapping unit, taking into account the P-ratings and relative proportion of each component soil unit. Each P retention class has been assigned to a likely fertilizer P recovery fraction, derived from the literature, thereby permitting spatially more detailed, integrated model-based studies of environmental sustainability and agricultural production at the global and continental level (< 1:5 million). Nonetheless, uncertainties remain high; the present analysis provides an approximation of world soil phosphorus retention potential.