New techniques have identified a wide range of organisms with the capacity to carry out biological nitrogen fixation (BNF)—greatly expanding our appreciation of the diversity and ubiquity of N fixers—but our understanding of the rates and controls of BNF at ecosystem and global scales has not advanced at the same pace. Nevertheless, determining rates and controls of BNF is crucial to placing anthropogenic changes to the N cycle in context, and to understanding, predicting and managing many aspects of global environmental change. Here, we estimate terrestrial BNF for a pre-industrial world by combining information on N fluxes with 15 N relative abundance data for terrestrial ecosystems. Our estimate is that pre-industrial N fixation was 58 (range of 40–100) Tg N fixed yr −1 ; adding conservative assumptions for geological N reduces our best estimate to 44 Tg N yr −1 . This approach yields substantially lower estimates than most recent calculations; it suggests that the magnitude of human alternation of the N cycle is substantially larger than has been assumed.

Nitrogen (N) availability is thought to frequently limit terrestrial ecosystem processes, and explicit consideration of N biogeochemistry, including biological N 2 fixation, is central to understanding ecosystem responses to environmental change. Yet, the importance of free-living N 2 fixation-a process that occurs on a wide variety of substrates, is nearly ubiquitous in terrestrial ecosystems, and may often represent the dominant pathway for acquiring newly available N-is often underappreciated. Here, we draw from studies that investigate free-living N 2 fixation from functional, physiological, genetic, and ecological perspectives. We show that recent research and analytical advances have generated a wealth of new information that provides novel insight into the ecology of N 2 fixation as well as raises new questions and priorities for future work. These priorities include a need to better integrate free-living N 2 fixation into conceptual and analytical evaluations of the N cycle's role in a variety of global change scenarios. 489 Annu. Rev. Ecol. Evol. Syst. 2011.42:489-512. Downloaded from www.annualreviews.org by NORTH CAROLINA STATE UNIVERSITY on 10/13/12. For personal use only.

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Vertebrate corpse decomposition provides an important stage in nutrient cycling in most terrestrial habitats, yet microbially mediated processes are poorly understood. Here we combine deep microbial community characterization, community-level metabolic reconstruction, and soil biogeochemical assessment to understand the principles governing microbial community assembly during decomposition of mouse and human corpses on different soil substrates. We find a suite of bacterial and fungal groups that contribute to nitrogen cycling and a reproducible network of decomposers that emerge on predictable time scales. Our results show that this decomposer community is derived primarily from bulk soil, but key decomposers are ubiquitous in low abundance. Soil type was not a dominant factor driving community development, and the process of decomposition is sufficiently reproducible to offer new opportunities for forensic investigations.T he process of decay and decomposition in mammalian and other vertebrate taxa is a key step in biological nutrient cycling. Without the action of vertebrate and invertebrate scavengers, bacteria, archaea, fungi, and protists, chemical decomposition of animal waste would proceed extremely slowly and lead to reservoirs of biochemical waste (1). The coevolution of microbial decomposers with the availability of vertebrate corpses over the past 400 million years is expected to result in conservation of key biochemical metabolic pathways and cross-kingdom ecological interactions for efficient recycling of nutrient reserves. Although mammalian corpses likely represent a relatively small component of the detritus pool (2, 3) in most ecosystems, their role in nutrient cycling and community dynamics may be disproportionately large relative to input size, owing to the high nutrient content of corpses (3, 4) and their rapid rates of decomposition [e.g., up to three orders of magnitude faster than plant litter (2)]. These qualities make corpses a distinct and potentially critical driver of terrestrial function (5, 6).When a mammalian body is decomposing, microbial and biochemical activity results in a series of decomposition stages (5) that are associated with a reproducible microbial succession across mice (7), swine (8), and human corpses (9). Yet the microbial metabolism and successional ecology underpinning decomposition are still poorly understood. At present, we do not fully comprehend (i) whether microbial taxa that drive decomposition are ubiquitous across environment, season, and host phylogeny; (ii) whether microbes that drive decomposition derive primarily from the host or from the environment; and (iii) whether the metabolic succession of microbial decomposition is conserved across the physicochemical context of decay and host phylogeny.Several questions arise: Are microbial decomposer communities ubiquitous? What is the origin of the microbial decomposer community? How does mammalian decomposition affect the metabolic capacity of microbial communities? To answer these questions, we used mouse...

Nitrogen (N) and phosphorus (P) availability regulate plant productivity throughout the terrestrial biosphere, influencing the patterns and magnitude of net primary production (NPP) by land plants both now and into the future. These nutrients enter ecosystems via geologic and atmospheric pathways and are recycled to varying degrees through the plant-soil-microbe system via organic matter decay processes. However, the proportion of global NPP that can be attributed to new nutrient inputs versus recycled nutrients is unresolved, as are the large-scale patterns of variation across terrestrial ecosystems. Here, we combined satellite imagery, biogeochemical modeling, and empirical observations to identify previously unrecognized patterns of new versus recycled nutrient (N and P) productivity on land. Our analysis points to tropical forests as a hotspot of new NPP fueled by new N (accounting for 45% of total new NPP globally), much higher than previous estimates from temperate and high-latitude regions. The large fraction of tropical forest NPP resulting from new N is driven by the high capacity for N fixation, although this varies considerably within this diverse biome; N deposition explains a much smaller proportion of new NPP. By contrast, the contribution of new N to primary productivity is lower outside the tropics, and worldwide, new P inputs are uniformly low relative to plant demands. These results imply that new N inputs have the greatest capacity to fuel additional NPP by terrestrial plants, whereas low P availability may ultimately constrain NPP across much of the terrestrial biosphere.carbon cycle | nutrient cycling | stoichiometry R ates of net primary productivity (NPP) vary widely across the terrestrial biosphere, with tropical forests accounting for more than one-third of total global annual NPP, and nearly 40% of NPP in natural ecosystems (1, 2). At the global scale, latitudinal variations in climate help explain broad patterns of NPP observed across the land surface, and ample rainfall and sunlight, warm temperatures, and long growing seasons near the equator fuel high rates of NPP in tropical forests (1). Mineral nutrientsespecially nitrogen (N) and phosphorus (P)-also influence the patterns and magnitude of NPP, mainly via strong regulatory effects on plant growth and photosynthesis (3). Multiple lines of evidence suggest that N, P, or N + P colimitation are nearly ubiquitous in the terrestrial biosphere (4-8), yet the extent to which nutrient availability might constrain future plant productivity-an important pathway toward higher net global C storage-remains contentious but potentially profound (9-11). For example, model forecasts that consider nutrient limitations of NPP suggest modest (0.18-0.3°C) to up to 3°C of additional warming by 2100 compared with carbon-climate simulations (12, 13). These differences hinge largely on N fixation responses to elevated CO 2 and climate (12).In the 1970s, the widely recognized importance of new nutrient inputs in sustaining algal productivity, ecosystem func...