Effects of nitrogen (N) supply on the limiting step of CO2 assimilation rate (A) at 380 µmol mol−1 CO2 concentration (A380) at several leaf temperatures were studied in several crops, since N nutrition alters N allocation between photosynthetic components. Contents of leaf N, ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) and cytochrome f (cyt f) increased with increasing N supply, but the cyt f/Rubisco ratio decreased. Large leaf N content was linked to a high stomatal (gs) and mesophyll conductance (gm), but resulted in a lower intercellular (Ci) and chloroplast CO2 concentration (Cc) because the increase in gs and gm was insufficient to compensate for change in A380. The A-Cc response was used to estimate the maximum rate of RuBP carboxylation (Vcmax) and chloroplast electron transport (Jmax). The Jmax/Vcmax ratio decreased with reductions in leaf N content, which was consistent with the results of the cyt f/Rubisco ratio. Analysis using the C3 photosynthesis model indicated that A380 tended to be limited by RuBP carboxylation in plants grown at low N concentration, whereas it was limited by RuBP regeneration in plants grown at high N concentration. We conclude that the limiting step of A380 depends on leaf N content and is mainly determined by N partitioning between Rubisco and electron transport components.

DOI: http://dx.doi.org/10.1111/j.1365-3040.2011.02280.x

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The accident at the Chernobyl Nuclear Power Plant (CNPP) on April 26, 1986 is the most serious nuclear disaster in human history. Surprisingly, while the area proximal to the CNPP remains substantially contaminated with long-lived radioisotopes including 90Sr and 137Cs, the local ecosystem has been able to adapt. To evaluate plant adaptation, seeds of a local flax (Linum usitatissimum) variety Kyivskyi were sown in radio-contaminated and control fields of the Chernobyl region. A total protein fraction was isolated from mature seeds, and analyzed using 2-dimensional electrophoresis combined with tandem-mass spectrometry. Interestingly, growth of the plants in the radio-contaminated environment had little effect on proteome and only 35 protein spots differed in abundance (p-value of ≤0.05) out of 720 protein spots that were quantified for seeds harvested from both radio-contaminated and control fields. Of the 35 differentially abundant spots, 28 proteins were identified using state-of-the-art MSE method. Based on the observed changes, the proteome of seeds from plants grown in radio-contaminated soil display minor adjustments to multiple signaling pathways.

DOI:  http://dx.doi.org/10.1021/es100895s

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Precision agriculture comprises a set of technologies that combines sensors, information systems, enhanced machinery, and informed management to optimize production by accounting for variability and uncertainties within agricultural systems. Adapting production inputs site-specifically within a field and individually for each animal allows better use of resources to maintain the quality of the environment while improving the sustainability of the food supply. Precision agriculture provides a means to monitor the food production chain and manage both the quantity and quality of agricultural produce.

DOI: http://dx.doi.org/10.1126/science.1183899

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For some parameters, such as nitrogen loading and atmospheric CO2 concentrations, we may have already stepped out of our safety zone and need to back-pedal quickly. For others, such as ocean acidification, we may still have enough time to avoid catastrophic change if we act wisely.But do we understand the Earth system well enough to know the real limits to environmental degradation? And if we can define them, even roughly, would doing so would ultimately help or hinder efforts to protect the planet?

We posed these questions to seven leading experts, who were invited to respond to the ‘planetary boundaries’ proposal. Each author brings specific expertise to evaluating one aspect of the proposed framework. Their responses can be freely accessed at Nature Reports Climate Change. We’ve weighed in with our own thoughts in an editorial in Nature, and with a podcast. All of Nature’s coverage, plus a full length version of the paper by Rockström and colleagues, can be accessed here.

The commentaries are available individually at the following links:

William H. Schlesinger, President of the Cary Institute of Ecosystem Studies in Millbrook, New York comments on the boundary for global nitrogen and phosphorus cycles (html|pdf).

Steve Bass, senior fellow at the International Institute for Environment and Development, UK comments on the boundary for land-use change (html|pdf).

Myles Allen, physicist and climatologist at the University of Oxford, UK comments on the boundary for climate change (html|pdf).

Mario J. Molina, director of the Mario Molina Center for Strategic Studies in Energy and the Environment in Mexico City comments on the boundary for stratospheric ozone depletion (html|pdf).

David Molden, deputy director general for research at the International Water Management Institute in Sri Lanka comments on the boundary for freshwater availability (html|pdf).

Peter Brewer, ocean chemist and Senior Scientist at the Monterey Bay Aquarium Research Institute in Moss Landing, California comments on the boundary for ocean acidification (html|pdf).

Cristián Samper, Director of the Smithsonian National Museum of Natural History in Washington DC comments on the boundary for biodiversity loss (html|pdf).

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To meet the challenge of maintaining the Holocene state, we propose a framework based on ‘planetary boundaries’. These boundaries define the safe operating space for humanity with respect to the Earth system and are associated with the planet’s biophysical subsystems or processes. Although Earth’s complex systems sometimes respond smoothly to changing pressures, it seems that this will prove to be the exception rather than the rule. Many subsystems of Earth react in a nonlinear, often abrupt, way, and are particularly sensitive around threshold levels of certain key variables. If these thresholds are crossed, then important subsystems, such as a monsoon system, could shift into a new state, often with deleterious or potentially even disastrous consequences for humans8, 9.

Most of these thresholds can be defined by a critical value for one or more control variables, such as carbon dioxide concentration. Not all processes or subsystems on Earth have well-defined thresholds, although human actions that undermine the resilience of such processes or subsystems — for example, land and water degradation — can increase the risk that thresholds will also be crossed in other processes, such as the climate system.

We have tried to identify the Earth-system processes and associated thresholds which, if crossed, could generate unacceptable environmental change. We have found nine such processes for which we believe it is necessary to define planetary boundaries: climate change; rate of biodiversity loss (terrestrial and marine); interference with the nitrogen and phosphorus cycles; stratospheric ozone depletion; ocean acidification; global freshwater use; change in land use; chemical pollution; and atmospheric aerosol loading (see Fig. 1 and Table).Figure 1: Beyond the boundary.

doi:http://dx.doi.org/10.1038/461472a

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This article provides a synthesis of literature values to trace the fate of 150 Tg/yr anthropogenic nitrogen applied by humans to the Earth’s land surface. Approximately 9 TgN/yr may be accumulating in the terrestrial biosphere in pools with residence times of ten to several hundred years. Enhanced fluvial transport of nitrogen in rivers and percolation to groundwater accounts for ≈35 and 15 TgN/yr, respectively. Greater denitrification in terrestrial soils and wetlands may account for the loss of ≈17 TgN/yr from the land surface, calculated by a compilation of data on the fraction of N2O emitted to the atmosphere and the current global rise of this gas in the atmosphere. A recent estimate of atmospheric transport of reactive nitrogen from land to sea (NOx and NHx) accounts for 48 TgN/yr. The total of these enhanced sinks, 124 TgN/yr, is less than the human-enhanced inputs to the land surface, indicating areas of needed additional attention to global nitrogen biogeochemistry. Policy makers should focus on increasing nitrogen-use efficiency in fertilization, reducing transport of reactive N to rivers and groundwater, and maximizing denitrification to its N2 endproduct.

On the fate of anthropogenic nitrogen — PNAS

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