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Purpose: Perennial crops have been suggested as a more sustainable alternative to the currently most common cropping systems. Compared with annual plants, perennial plants produce more biomass and have deeper roots, and are expected to lead to higher soil organic carbon (SOC). This hypothesis, however, has not been well tested for grain crops. Methods: Using perennial intermediate wheatgrass (IWG, Thinopyrum intermedium) and annual winter wheat (Triticum aestivum) as focal species, and native grassland as reference, we quantified the SOC accumulation via a process-based model, describing water and heat exchanges and carbon-nitrogen cycling in the canopy and soil to a depth of 2 m. The model includes C fixation via photosynthesis, plant biomass growth and litter production, physical protection of SOC, depolymerisation, C mineralisation, nitrification, denitrification, microbial growth, and necromass turnover in the soil. While of general applicability, we considered a sandy loam under warm-summer humid continental climate. Results: Following a conversion from native grassland, IWG reduced SOC losses by at least 38%, especially in the particulate organic carbon (POC) pool, within the top 2 m of soil, compared with annual wheat. Soil microbial biomass and soil respiration were higher in IWG than annual wheat. Shifting from annual wheat to high photosynthetic capacity IWG increased SOC by about 33 g C m−2 y−1 (averaged over a 4-year continuous IWG cropping), with a large fraction of SOC gain stemming from restoring POC. Conclusion: Compared with annual grains, perennial grains can increase soil carbon sequestration and maintain SOC at levels nearer to that of native grasslands. [ABSTRACT FROM AUTHOR]

Biological nitrogen fixation represents the largest natural flux of new nitrogen (N) into terrestrial ecosystems, providing a critical N source to support net primary productivity of both natural and agricultural systems. When they are common, symbiotic associations between plants and bacteria can add more than 100 kg N ha−1 y−1 to ecosystems. Yet, these associations are uncommon in many terrestrial ecosystems. In most cases, N inputs derive from more cryptic sources, including mutualistic and/or free-living microorganisms in soil, plant litter, decomposing roots and wood, lichens, insects, and mosses, among others. Unfortunately, large gaps remain in the understanding of cryptic N fixation. We conducted a literature review to explore rates, patterns, and controls of cryptic N fixation in both unmanaged and agricultural ecosystems. Our analysis indicates that, as is common with N fixation, rates are highly variable across most cryptic niches, with N inputs in any particular cryptic niche ranging from near zero to more than 20 kg ha−1 y−1. Such large variation underscores the need for more comprehensive measurements of N fixation by organisms not in symbiotic relationships with vascular plants in terrestrial ecosystems, as well as identifying the factors that govern cryptic N fixation rates. We highlight several challenges, opportunities, and priorities in this important research area, and we propose a conceptual model that posits an interacting hierarchy of biophysical and biogeochemical controls over N fixation that should generate valuable new hypotheses and research. [ABSTRACT FROM AUTHOR]

We measured nitrogenase activity (acetylene reduction) of asymbiotic, heterotrophic, nitrogen-fixing bacteria on leaf litter from the tree Metrosideros polymorpha collected from six sites on the Hawaiian archipelago. At all sites M. polymorpha was the dominant tree, and its litter was the most abundant on the forest floor. The sites spanned a soil chronosequence of 300 to 4.1 million y. We estimated potential nitrogen fixation associated with this leaf litter to be highest at the youngest site (1.25 kg ha-1 y-1), declining to between 0.05 and 0.22 kg ha-1 y-1 at the oldest four sites on the chronosequence. To investigate how the availability of weathered elements influences N fixation rates at different stages of soil development, we sampled M. polymorpha leaf litter from complete, factorial fertilization experiments located at the 300-y, 20,000-y and 4.1 million–y sites. At the youngest and oldest sites, nitrogenase activity on leaf litter increased significantly in the plots fertilized with phosphorus and “total” (all nutrients except N and P); no significant increases in nitrogenase activity were measured in leaf litter from treatments at the middle-aged site. The results suggest that the highest rates of N fixation are sustained during the “building” or early phase of ecosystem development when N is accumulating and inputs of geologically cycled (lithophilic) nutrients from weathering are substantial. [ABSTRACT FROM AUTHOR]

We used a chronosequence comprised of 10 y, 52 y and 142 y old 'a'a lava flows on Mauna Loa, Hawaii, to determine the accumulation of organic matter and nitrogen and rates of nitrogen fixation through time.The mass of organic matter (live and dead biomass and soil organic matter) on the 1984, 1942 and 1852 lava flows was 0.6, 2.2 and 7.6 kg m^-2, respectively, while total N was 4.8, 10.9 and 85.7 g m^-2. We estimated the total rates of nitrogen fixation for the three differentaged ecosystems using an acetylene reduction assay calibrated with ^15N incubations. While mean rates of total N fixation remained largely constant across the three sites - between 2.0 and 3.1 kg ha^-1 y^-1- the most important sources of N fixation changed. On the 10 y flow, the most important fixer was the pioneering cyanolichen, Stereocaulon vulcani. After 52 years of ecosystem development, the most important N fixer was a cyanoalga, while after 142 years, the predominant N fixers were heterotrophic bacteria associated with leaf litter, twigsand detritus. The total amount of N accumulated after 52 years of ecosystem development was equivalent to cumulative inputs through biological N fixation. After 142 years, however, cumulative inputs from N fixation could only account for between 27-59% of the total nitrogen accrued. We used fertilizer additions of all essential nutrients other than N to test whether the availability of lithophilic nutrients regulated rates of N fixation in early ecosystem development. Rates of nitrogen fixation by the lichen, S. vulcani, approximately doubled when fertilized on the 1984 and 1942 flows. Rates of N-fixation by heterotrophic nitrogen fixing bacteria on leaf litter of Metrosideros polymorpha also increased significantly when fertilized with lithophilicnutrients. These findings suggest that weathering rates of lava in part regulate rates of nitrogen fixation in these young ecosystems. [ABSTRACT FROM AUTHOR]

Nitrogen is often a limiting factor to net primary productivity (NPP) and other processes in terrestrial ecosystems. In most temperate freshwater ecosystems, when nitrogen becomes limiting to NPP, populations of N-fixing cyanobacteria experience a competitive advantage, and begin to grow and fix nitrogen until the next most limiting resource is encountered; typically phosphorus or light. Why is it that N-fixing plants do not generally function to overcome N limitation in terrestrial ecosystems in the same way that cyanobacteria function in aquatic ecosystems? To address this question in a particular ecosystem, one must first know whether the flora includes a potential set of nitrogen fixers. I suggest that the presence or absence of N-fixing plant symbioses is foremost an evolutionary consideration, determined to a large extent by constraints on the geographical radiation of woody members of the family Fabaceae. Ecological factors such as competition,nutrient deficiencies, grazing and fire are useful to explain the success of N-fixing plants only when considered against the geographical distribution of potential N-fixers. [ABSTRACT FROM AUTHOR]