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{userid:"admin-2003", "refid":"131","repocollections":"","attachment":"","_thumb":"","articletype":"article","sectionheading":"","title":"Title  : Importance of nitrogen fixation in soil crusts of southern African arid ecosystems: acetylene reduction and stable isotope studies","year":"2003","author":"J N Aranibar, I C Anderson, S Ringrose, S A Macko","journal":"Journal of Arid Environments","volume":"54","number":"2","pages":"345-358","month":"","doi":"","pubmed":"","pdflink":"","urllink":"http:\/\/www.sciencedirect.com\/science\/article\/B6WH9-48FJSR0-6\/2\/f96e219f8c3cc82d6e799de72bf36ad9","abstract":"Cyanobacterial soil crusts may be important in arid and semi-arid ecosystems because of their ability to fix atmospheric nitrogen (N2). These crusts are very sensitive to trampling by animals, and their destruction can decrease ecosystem N inputs, affecting the productivity of the region. The objective of this study was to quantify the nitrogen-fixing activity in soil crusts during the wet season in southern African ecosystems using in situ acetylene reduction assays. The average acetylene reduction rates for each site ranged from 88 to 535 nmol m-2 h-1, were highly variable, and were lower than previously reported for other arid areas. All soil samples showed acetylene reduction activity; however, soils with crusts supported higher rates than did 34;non-crusty 34; soils under litter, moss, or sand. High values of 15N natural abundance ([delta]15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification\/denitrification and ammonia volatilization or atmospheric deposition of 15N-enriched nitrate or ammonium may have caused shifts in [delta]15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha-1 year-1, orders of magnitude lower than values estimated in other studies. The anomalous wet conditions experienced during the year of the study may have increased the temporal availability of soil mineral N and decreased N fixation rates. However, the presence of N fixation activity in all crusts analysed and their ability to survive at high temperature and after long dry periods may provide ecosystem resilience, facilitating ecosystem recovery after severe droughts.","note":"","tags":"soil crusts,N fixation,Kalahari,acetylene reduction,isotopes,savanna, soils, ecosystem ecology, October 2012","publisher":"","booktitle":"","editor":"","address":"","school":"","issn":"0140-1963","isi":"","key":"Aranibar2003","howpublished":"http:\/\/www.sciencedirect.com\/science\/article\/B6WH9-48FJSR0-6\/2\/f96e219f8c3cc82d6e799de72bf36ad9"}
,

{userid:"admin-2003", "articletype":"article","pages":"319-325","author":"M H T Hipondoka, J N Aranibar, C Chirara, M Lihavha, S A Macko","year":"2003","title":"Vertical distribution of grass and tree roots in arid ecosystems of Southern Africa : niche differentiation or competition?","month":"","journal":"Journal of Arid Environments","publisher":"","volume":"54","number":"2","note":"","tags":"trees,grasses,roots,savanna,isotopic,carbon,hydrology,plant autecology,ecology","booktitle":"","editor":"","abstract":"A distinguishing, co-dominance feature of trees and grasses in Savannas was prominently explained by Walter. He hypothesized that water is the limiting factor and grasses are superior competitors for water in the upper soil, while trees have exclusive access at deeper layers. However, a new field evidence from the Kalahari, where the water table is too low for the tree roots' tapping, revealed that both plant types invest the bulk of their roots in the surface horizons. Further investigation should consider focusing on determining the co-dominance mechanism of both plant types without the displacement of the less-efficient species.","address":"","school":"","issn":"0140-1963","doi":"DOI: 10.1006\/jare.2002.1093","isi":"","pubmed":"","key":"Hipondoka2003","howpublished":"http:\/\/www.sciencedirect.com\/science\/article\/B6WH9-48FJSR0-4\/2\/77316abe5302646a4162ede2851856ad","urllink":"http:\/\/www.sciencedirect.com\/science\/article\/B6WH9-48FJSR0-4\/2\/77316abe5302646a4162ede2851856ad","refid":33}
,

{userid:"admin-2003", "articletype":"article","pages":"327-343","author":"C J W Feral, H E Epstein, L Otter, J N Aranibar, H H Shugart, S A Macko, J Ramontsho","year":"2003","title":"Carbon and nitrogen in the soil-plant system along rainfall and land-use gradients in southern Africa","month":"","journal":"Journal of Arid Environments","publisher":"","volume":"54","number":"2","note":"","tags":"soil organic carbon,soil organic nitrogen,rainfall gradient,land-use gradient,Kalahari,savanna,Botswana,soil nutrition,rainfall,grazing,grasslands,land use","booktitle":"","editor":"","abstract":"We examined relationships between vegetation and soil nutrients along rainfall and land use gradients in the Kalahari. Soil organic carbon (SOC) was significantly lower where reduced rainfall and grazing lowered litter inputs to the soil. Soil organic nitrogen (SON) was significantly lower at the driest site and was higher than expected at two grazed sites. Ammonium levels at Mongu (wettest) were significantly higher, while nitrate was lower than expected everywhere. Site-averaged concentrations showed no significant differences in SOC and SON beneath plant canopies, but woody plant canopies alone showed elevated SOC at three sites and SON at one site.","address":"","school":"","issn":"0140-1963","doi":"DOI: 10.1006\/jare.2002.1091","isi":"","pubmed":"","key":"Feral2003","howpublished":"http:\/\/www.sciencedirect.com\/science\/article\/B6WH9-48FJSR0-5\/2\/fa69d5ae2c543001c223a3db4c73875e","urllink":"http:\/\/www.sciencedirect.com\/science\/article\/B6WH9-48FJSR0-5\/2\/fa69d5ae2c543001c223a3db4c73875e","refid":14}
,

{userid:"admin-2008", "refid":"201","repocollections":"","attachment":"","_thumb":"","articletype":"article","sectionheading":"","title":"Nitrogen isotope composition of soils, C3 and C4 plants along land use gradients in southern Africa","year":"2008","author":"J N Aranibar, I C Anderson, H E Epstein, C J W Feral, R J Swap, J Ramontsho, S A Macko","journal":"Journal of Arid Environments","volume":"72","number":"4","pages":"326-337","month":"","doi":"","pubmed":"","pdflink":"","urllink":"http:\/\/www.sciencedirect.com\/science\/article\/B6WH9-4PCXXKF-1\/2\/e95a049bab218cf304ba82479f49fd61","abstract":"This paper provides values for nitrogen isotopic abundances of southern African soils and plants along land use gradients of varying aridity. The [delta]15N values of soils and plants were generally higher in sites with greater land use intensity, except in the most arid site where [delta]15N decreased with land use intensity. The enrichment in 15N with land use intensity agrees with the expected effects of grazing and cultivation on N cycling processes, including increased volatilization of ammonium, exports of plant material, and decreased N2 fixation by the destruction of cyanobacterial soil crusts. Gross mineralization and nitrification rates were more affected by local heterogeneity in the soils than by aridity or land use. In general, C3 plants had significantly higher [delta]15N than C4 plants from the same location, suggesting different N use by the two plant types. This study suggests that land use intensity affects N cycling processes that may result in different and opposite changes of ecosystem [delta]15N, as those observed between the most arid, and the other semi arid sites analyzed. In addition to the 15N enrichment caused by the loss of gaseous and plant 14N, changes in tree and grass cover may affect soil [delta]15N by the differential uptake of soil N with different isotopic abundances.","note":"","tags":"Grasses,Grazing,Mineralization,Nitrification,Savanna,Trees, soils, grazing, October 2012","publisher":"","booktitle":"","editor":"","address":"","school":"","issn":"0140-1963","isi":"","key":"Aranibar2008","howpublished":"http:\/\/www.sciencedirect.com\/science\/article\/B6WH9-4PCXXKF-1\/2\/e95a049bab218cf304ba82479f49fd61"}
,

{userid:"o.jahn", "refid":"27","repocollections":"","attachment":"","_thumb":"","articletype":"incollection","sectionheading":"","title":"Western South America: Ecuador, Peru and Bolivia","year":"2006","author":"F. Angulo Pratolongo, R. MacLeod, O. Jahn, C.J. Idrobo-Medina, E. Gallo-Cajiao, V.R. D\u00edaz, R. Williams, L. Cancino, R. Soria, M. Gasta\u00f1aga, M.M. Rios, M.C. Mu\u00f1oz, G. Londo\u00f1o, H. Aranibar-Rojas, J. Barrio, R.P. Clay, D.C. Oren D.M. Brooks","booktitle":"Conserving Cracids: the most Threatened Family of Birds in the Americas","editor":"D.M. Brooks ","pages":"124-126:","organization":"Misc. Publ. Houston Mus. Nat. Sci., No. 6.","address":"","publisher":"Houston Museum of Natural Science, Houston, USA","isbn":"0-9668278-2-1","doi":"","pubmed":"","pdflink":"http:\/\/www.cracids.org\/AP_Engl_ebook.pdf ","urllink":"","abstract":"","note":"Miscellaneous Publications of The Houston Museum of Natural Science, Number 6","tags":""}
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