Using boosted regression trees to explore key factors controlling saturated and near‐saturated hydraulic conductivity

Hydraulic conductivity at and near saturation is difficult to predict. We investigated, for the first time, the potential of boosted regression trees to identify the key factors that determine saturated and near‐saturated hydraulic conductivities in undisturbed soils with a global meta‐database of tension infiltrometer measurements. Our results demonstrate that pedotransfer functions developed from meta‐databases may strongly over‐estimate prediction performance unless they are validated against each individual data source separately. For such a source‐wise cross‐validation, we estimated the hydraulic conductivity at a tension of 10 cm (K₁₀) and the saturated hydraulic conductivity (K_s) with coefficients of determination of 0.36 and 0.15, respectively. The most important predictors for K₁₀ were the average annual precipitation and temperature at the measurement location, which are key variables for pedogenesis and constrain soil management. More research is required for the in‐depth interpretation of their influence on hydraulic conductivity. The soil clay and organic carbon contents were also important predictors of K₁₀, with hydraulic conductivity decreasing as organic carbon contents increased up to 1.5% and as clay contents increased between about 10 and 40%. The direction of the tension‐sequence with which the infiltrometer data were collected was also a significant predictor. Land use and bulk density were the most important predictors for K_s. The direction of the tension‐sequence and the soil texture class were also important, with both coarse and fine‐textured soils generally having larger K_s values than medium‐textured soils.     

Biogeochemical Controls and Feedbacks on Ocean Primary Production

Changes in oceanic primary production, linked to changes in the network of global biogeochemical cycles, have profoundly influenced the geochemistry of Earth for over 3 billion years. In the contemporary ocean, photosynthetic carbon fixation by marine phytoplankton leads to formation of approximately 45 gigatons of organic carbon per annum, of which 16 gigatons are exported to the ocean interior. Changes in the magnitude of total and export production can strongly influence atmospheric CO2 levels (and hence climate) on geological time scales, as well as set upper bounds for sustainable fisheries harvest. The two fluxes are critically dependent on geophysical processes that determine mixed-layer depth, nutrient fluxes to and within the ocean, and food-web structure. Because the average turnover time of phytoplankton carbon in the ocean is on the order of a week or less, total and export production are extremely sensitive to external forcing and consequently are seldom in steady state. Elucidating the biogeochemical controls and feedbacks on primary production is essential to understanding how oceanic biota responded to and affected natural climatic variability in the geological past, and will respond to anthropogenically influenced changes in coming decades. One of the most crucial feedbacks results from changes in radiative forcing on the hydrological cycle, which influences the aeolian iron flux and, in turn, affects nitrogen fixation and primary production in the oceans.