Archive for the category: Biological N fixation and cropping systems

Rolf Nieder¹, Zaur Jumshudov¹, Viridiana Alcántara², Axel Don², Reinhard Well²

¹Institute of Geoecology, Technische Universität Braunschweig, Langer Kamp 19c, Braunschweig 38106, 

  Germany, r.nieder@tu-bs.de

²Thünen Institute of Climate-Smart Agriculture, Bundesallee 50, Braunschweig 38116, Germany

Abstract

On average 45 years after the deep ploughing operation, deep ploughed soils contained 24±5% more total N compared to conventionally ploughed reference soils. However, the mean N stock in the new topsoils was still 8% lower compared to the reference soils. This indicates a long-term N accumulation potential lasting more than 4-5 decades. The potential N mineralization and nitrification capacities of loamy deep ploughed soils were higher compared to sandy deep ploughed soils. All sites showed very low N mineralization potentials and nitrification capacities in the buried Ap material compared to surface Ap horizons.

Peter Sørensen¹,Ingrid K. Thomsen¹, Jaap J. Schröder²

¹ Department of Agroecology, Aarhus University, Blichers Allé 20, 8830 Tjele, Denmark, www.au.dk, ps@agro.au.dk

² Plant Research International, Wageningen University and Research Centre, P.O. Box 16, 6700 AP Wageningen, The Netherlands

Abstract

A general model was developed for net mineralisation of pig and cattle slurry N in arable soil under cool moist climate conditions. The model is based on a 3-year field experiment and literature data and describes the cumulative net mineralisation of manure N during the initial 5 years after spring application. The model estimates a faster mineralisation rate for organic N in pig slurry compared to cattle slurry, and the model includes an initial N immobilisation phase for both manure types. The model estimates a cumulated mineralisation of 71% of the organic N in pig slurry and 51% of the organic N in cattle slurry after 5 years. These estimates are in accordance with other mineralisation studies and studies of manure residual N effects.

Deli Chen¹, Shu Kee Lam¹, Arvin R. Mosier¹, Richard Eckard¹, Peter Vitousek²

¹Crop and Soil Science Section, Faculty of Veterinary and Agricultural Sciences, The University of Melbourne, VIC 3010, Australia, Email: delichen@unimelb.edu.au

²Department of Biological Sciences, Stanford University, Stanford, CA 94305-5020, USA

Abstract

Misunderstanding of the intricacies of nitrogen (N) cycling in agricultural soils has led to improper fertiliser N use in important global agroecosystems, ranging from excessive use to unsustainable exploitation (mining) of soil organic matter reserves. This must be addressed to avoid excessive N accumulation and to ensure adequate N reserve. Here we develop a framework for answering the question “Should soil organic N be mined, and if so, for how long?” to maintain sustainable agricultural production in major agroecosystems worldwide. Agricultural systems where external N input exceeds the capacity of the soil to form soil organic matter are prone to leak reactive N to the environment. Excessive additions need to be halted, and where excess reactive N remains in these systems it needs to be mined, at least for some time. In other agroecosystems, external N input is low and current use of the land mines N acquired through the mineralization of soil organic matter. Thus the paradox of mining soil organic N, where on the one hand it can be desirable for agroecosystem health and on the other threatens agroecosystem function. Untangling the paradox of mining soil organic N and revealing the residual effect of fertiliser N will contribute to answering the question of whether N use efficiency is as low as perceived. This has major implications for food security and environmental quality.

Xinhua He^1,2,3, Junkun Lu³, Lihua Kang³, Janet Sprent⁴, Daping Xu³

¹ Centre of Excellence for Soil Biology, College of Resources and Environment, Southwest University, 2 Tiansheng Road, Beibei, Chongqing, China, 400715, http://zihuan.swu.edu.cn/viscms/zihuanidex/jiaoshou4231/20140901/278581.html and https://www.researchgate.net/profile/Xinhua_He3, ²School of Plant Biology, The University of Western Australia, 35 Stirling Highway, Crawley, Perth, WA, Australia, 6009; xinhua.he@uwa.edu.au

³ Research Institute of Tropical Forestry, Guangzhou, Guangdong, China, 510520

⁴ Division of Plant Sciences, University of Dundee at James Hutton Institute, Dundee, UK, DD2 5DA

Abstract

Understanding plant-parasite interactions between root hemiparasite Santalum album and its host trees has theoretical and practical significance in plantations of precious sandalwood as well as tree nutrition or fertilization management. Nutrient translocation from a host plant is vital to the growth and survival of its root parasitic plant, but few studies have investigated whether a parasitic plant is also able to transfer nutrients to its host. The role of N₂-fixation in nitrogen (N) transfer between 7-month-old Dalbergia odorifera T. Chen nodulated with Bradyrhizobium elkanii DG and its hemiparasite Santalum album Linn. was examined by external ¹⁵N labelling in a pot study. Four paired treatments were used, with ¹⁵N given to either host or hemiparasite and the host either nodulated or grown on combined N. N₂-fixation supplied 41–44% of total N in D. odorifera. Biomass, N and ¹⁵N contents were significantly greater in both nodulated D. odorifera and S. album grown with paired nodulated D. odorifera. Significantly higher total plant ¹⁵N recovery was in N-donor D. odorifera (68–72%) than in N-donor S. album (42–44%), regardless of the nodulation status in D. odorifera. Nitrogen transfer to S. album was significantly greater (27.8–67.8 mg plant⁻¹) than to D. odorifera (2.0–8.9 mg plant⁻¹) and 2.4–4.5 times greater in the nodulated pair than in the non-nodulated pair. Irrespective of the nodulation status, S. album was always the N-sink plant. The amount of two-way N transfer was increased by the presence of effective nodules, resulting in a greater net N transfer (22.6 mg plant⁻¹) from host D. odorifera to hemiparasite S. album. Our results may provide better N management strategies for successfully mixed field plantation of S. album with D. odorifera, both are in great market demanding as preciously fragrant timbers, but have been globally over-exploited in the field.

Murray Unkovich¹ and David Layzell²

School Agriculture, Food and Wine, The University of Adelaide, PMB 1, Glen Osmond SA 5064. Australia. murray.unkovich@adelaide.edu.au

Department of Biological Sciences, University of Calgary, 2500 University Drive N.W., Calgary AB Canada T2N 1N4

Abstract

Despite the fact that N₂ gas comprises 78% of the earth’s atmosphere, there is some evidence that its concentration in legume nodules may be less than that needed to saturate the activity of nitrogenase, the bacterial enzyme responsible for fixing N₂ into NH₄⁺ needed for protein and DNA synthesis in biological systems. This paper reviews that evidence. If this hypothesis were true, to meet their demand for fixed N, legumes would need to produce more nodules, bigger nodules or more nitrogenase enzyme activity per nodule. Given the high energy cost of legume nodules, N₂ limited nitrogenase could reduce legume productivity.

We provide an overview of the published literature on ¹⁵N isotope enrichment associated with nitrogenase activity and evidence to suggest that in legumes nitrogenase may not be N₂ saturated. We then consider the biophysical properties of legume nodules, and the biochemical properties of the nitrogenase enzyme to explore a possible explanation for the isotopic data and the stated hypothesis.

Michael Udvardi¹, Evangelia Kouri¹, John Peters², Amaya Garcia Costas², Florence Mus², Jean-Michel Ane³, Kevin Garcia³, Chris Voigt⁴, Min-Hyung Ryu⁴, Giles Oldroyd⁵, Ponraj Paramasivian⁵, Ramakrishnan Karunakaran⁵, Barney Geddes⁶, and Philip Poole⁶.

¹The Samuel Roberts Noble Foundation, USA

²Montana State University, USA

³University of Wisconsin-Madison, USA;

⁴Massachusetts Institute of Technology, USA;

⁵John Innes Center, UK;

⁶University of Oxford, UK. 

Contact: mudvardi@noble.org

Abstract

Too much nitrogen (N)-fertilizer is used in many agricultural systems, at great environmental cost, while too little is used in the poorest systems, jeopardising food security.  As a step towards solving these contrasting N-related problems, we aim to build synthetic nitrogen-fixing symbioses between bacteria and grasses, based on knowledge gained from decades of research on natural nitrogen-fixing symbioses in legumes.  Key steps in this synthetic biology project include engineering of: signal compound production in bacteria and signal recognition in plants; concomitant biosynthesis of a specialized C-source by the plant for use by the bacteria; catabolism of this specialized C-source for energy production, as well as nitrogen fixation, respiratory protection of nitrogenase, and conditional suppression of ammonia assimilation in bacteria; and, finally, ammonium uptake by plant cells.  Chassis’ for the bacterial synthetic biology are natural endophytes or epiphytes of grasses, while the target model and crop species are barley and maize.  Significant progress has been made in each of these areas. Ultimately, substantial synthetic associative nitrogen-fixation in staple food crops could increase yields of resource-poor farmers and decrease the need for industrial N-fertilizers in resource-rich agricultural systems.