Quantifying global patterns of terrestrial nitrogen (N) cycling is central to predicting future patterns of primary productivity, carbon sequestration, nutrient fluxes to aquatic systems, and climate forcing. With limited direct measures of soil N cycling at the global scale, syntheses of the 15N:14N ratio of soil organic matter across climate gradients provide key insights into understanding global patterns of N cycling. In synthesizing data from over 6000 soil samples, we show strong global relationships among soil N isotopes, mean annual temperature (MAT), mean annual precipitation (MAP), and the concentrations of organic carbon and clay in soil. In both hot ecosystems and dry ecosystems, soil organic matter was more enriched in 15N than in corresponding cold ecosystems or wet ecosystems. Below a MAT of 9.8°C, soil δ15N was invariant with MAT. At the global scale, soil organic C concentrations also declined with increasing MAT and decreasing MAP. After standardizing for variation among mineral soils in soil C and clay concentrations, soil δ15N showed no consistent trends across global climate and latitudinal gradients. Our analyses could place new constraints on interpretations of patterns of ecosystem N cycling and global budgets of gaseous N loss.

An ‘anomalous' negative flux, in which carbon dioxide (CO2) enters rather than is released from the ground, was studied in a saline/alkaline soil. Soil sterilization disclosed an inorganic process of CO2 dissolution into (during the night) and out of (during the day) the soil solution, driven by variation in soil temperature. Experimental and modeling analysis revealed that pH and soil moisture were the most important determinants of the magnitude of this inorganic CO2 flux. In the extreme cases of air-dried saline/alkaline soils, this inorganic process was predominant. While the diurnal flux measured was zero sum, leaching of the dissolved inorganic carbon in the soil solution could potentially effect net carbon ecosystem exchange. This finding implies that an inorganic module should be incorporated when dealing with the CO2 flux of saline/alkaline land. Neglecting this inorganic flux may induce erroneous or misleading conclusions in interpreting CO2 fluxes of these ecosystems.

Summary1. Many grasslands worldwide are undergoing succession to woody vegetation, causing complex effects on carbon (C) sequestration, nutrient cycling and biodiversity. Land managers are frequently tasked with maximizing ecosystem services and biodiversity. Nonetheless, there are few studies quantifying trade-offs between ecosystem services and biodiversity during early woody succession. 2. We assessed the consequences of woody succession for C stocks, above-and below-ground taxa richness (plants, nematodes, mites, microbes, fungi), and soil ecosystem function at one site with a native tree, Kunzea ericoides, and one site with a non-native tree, Pinus nigra, both establishing in conservation grasslands. 3. Woody succession at both sites was associated with large gains in above-ground C stocks and, under P. nigra, losses from the mineral soil-C pool. 4. Taxa richness responses were complex, nonlinear and incongruent. While some taxa showed initial increases in richness with woody succession (e.g. plants), other taxa had rapid declines (e.g. plant-feeding and plant-associated nematodes, oribatid mites). 5. Below-ground ecosystem functioning shifted towards increased bacterial energy channels with woody succession, despite no change in bacterial or fungal biomass or fungal hyphal lengths. Most other soil measures were consistent with literature expectations (increased C:N ratios, release of recalcitrant phosphorus). 6. Synthesis and applications. Our gradient-based measurements of woody succession effects on ecosystems did not follow expectations based on comparing end-points of grasslands to homogeneous mature forest. The discordance of biodiversity responses across taxonomic groups suggests that managers cannot rely on the indicator-species concept to ensure conservation of cryptic biodiversity. Carbon sequestration and biodiversity followed non-congruent patterns, with significant losses of taxa richness from some functional groups during woody succession. Management to maximize individual ecosystem services such as carbon sequestration may therefore result in significant negative effects on biodiversity of some, but not all, taxa.

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