Presentation on theme: "Evaluation of Biological Nitrogen Fixation and Below Ground N Contribution of Grain Legumes Using 15 N Techniques 1 Kaizzi C.K., 2 Rebecca Hood-Nowotny,"— Presentation transcript:
Evaluation of Biological Nitrogen Fixation and Below Ground N Contribution of Grain Legumes Using 15 N Techniques 1 Kaizzi C.K., 2 Rebecca Hood-Nowotny, and 3 Charles S. Wortmann 1 Kawanda Agricultural Research Institute, P.O. Box 7065 Kampala, Uganda; 2 Soil Science Unit, FAO/IAEA Laboratories, A-2444 Seibersdorf, Austria; 3 University of Nebraska-Lincoln. I ntroduction Nitrogen (N) deficiency often constrains crop productivity in Sub-Saharan Africa.Economic considerations make biological N fixation (BNF) an attractive N source for resource-poor farmers (Giller and Wilson, 1981; Van Cleemput, 1995). Most studies that evaluate BNF of legumes consider the above ground biomass only. Few have measured the fixed N in the macro- roots and in the soil, thereby underestimating the total N derived from the atmosphere (Ndfa). We evaluated Ndfa in four grain legumes and determined the below ground N contribution using the stem injection 15 N technique (Russell and Fillery, 1996a,b). Materials and methods Soybean [nodulating and non-nodulating], field peas, pigeon peas and cowpeas were evaluated in pots with 1 kg of a 1:1 soil-sand mixture in the greenhouse at FAO/IAEA Agricultural and Biotechnology Laboratories, Seibersdorf, Austria. Two trials were conducted. Trial 1. To measure Ndfa, 10 mg N kg -1 soil labelled (NH 4 ) 2 SO 4 at 5% atom excess 15 N was applied to each pot. The non- nodulating soybean was the reference crop. Trial 2. To measure below ground N contribution, 10 mg N kg -1 soil of ordinary (NH 4 ) 2 SO 4 fertilizer was applied to pots. Each plant received 1 ml of 0.035M urea solution of 95% atom excess 15 N using the stem injection technique. Stems were injected with 15 N at 5 weeks after planting and biomass was harvested 23 days after injection. The bulk and rhizosphere soil were collected. The total N and 15 N of the samples were determined by mass spectrometry. The equations below were used to calculate: N derived from the air (Ndfa) and fertiliser (Ndff), and N from exudates in the bulk soil [RdfN(BS)] and rhizosphere soil [RdfN(Rh)]. In both trials, the shoots were cut at ground level and the recoverable roots were manually separated from the soil. Results N derived from the atmosphere for four legumes Soybean and field pea derived over 89% of their total N from the atmosphere. Ndfa is a valuable source in the low external input smallholder agriculture. Cowpea and pigeon pea did not fix nitrogen. Distribution of legume N in plant and soil as determined by the stem injection technique 34 to 69% of legume N was in the above ground biomass (Fig.1). Harvesting for food or fodder of above ground biomass removes much of this N from field. Below ground N varied by species from 31-66% of the total legume N (Table 1). On average 43% of below ground N for soybean and field peas was Ndfa. 13 to 42% of the total legume N was in the bulk and rhizosphere soil from legume exudates and more than the N in the recoverable root fraction. Conclusions: Up to 89% of legume N was Ndfa; Ndfa is potentially a valuable source of N for the low-input agriculture. Below ground legume N was 31 to 66% of total legume N and therefore a significant N source for subsequent crops since much of the above ground legume biomass is either harvested for food or fodder. Therefore, quantification of below ground legume N is important when evaluating cropping systems to maximise the benefits of legume N sources. References: Giller, K.E., Wilson, K.J., Nitrogen Fixation in Tropical Cropping Systems. CAB International, Wallingford, 313p. Russel, C.A., Fillery, I.R.P., 1996a. In situ 15 N labelling of lupin below-ground biomass. Aust. J. Agric. Res. 47, Russel, C.A., Fillery, I.R.P., 1996b, Estimates of lupin below-ground biomass nitrogen, dry matter and nitrogen turnover to wheat. Aust. J. Agric. Res. 47, Van Cleemput, O., Fertiliser, Sustainable Agriculture and Preservation of the Environment. In: IAEA (Ed.) Nuclear Methods in Soil-Plant Aspects of Sustainable Agriculture. Proceedings of an FAO/IAEA Regional Seminar for Asia and Pacific held in Colombo, Sri Lanka, 5-9 April IAEA-TECDOC-785. Vienna, Austria, pp Acknowledgement: Financial support from the International Atomic Energy Agency (IAEA) through TC project UGA/05/025 Pigeon pea Soybean (fixing) Soybean (non fixing) Field peas Cowpea A cotton wick dipping in a vial containing 1ml of 0.035M urea solution at 95% atom excess 15 N and passing through a hole pierced through the stem to evaluate below ground legume N. The legumes used in the evaluation of Ndfa and below ground legume N. Soybean and field pea derived N from the atmosphere as indicated by the dark green leaves.