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3. Results & Discussion NBPT decreased N2O-N emissions from urea by approximately 74% and 55% in the barley and the maize crop, respectively. An abatement of 67% and 88% was measured for the emissions of NO (Figure 1). This effect was mostly observed when nitrification was expected to be the main pathway in the production of these reactive N compounds (WFPS≤55%; NO/N2O1). In the case of maize, this occurred within the first month after fertilization and it was associated to a slightly controlled irrigation (1 irrigation event and total amount of 9 mm in the 2 weeks following fertilization). In this study, the abating effect of NBPT over N2O fluxes was greater than that previously reported elsewhere (e.g. Zaman et al., 2009). NBPT delayed urea hydrolysis and this may have explained the lower concentration of ammonium (NH4+-N) measured in this soil. This reduction in the size of the NH4+-N pool may have reduced the nitrification rate in such a way that the production of N2O and NO was also affected.
4. Conclusion The mitigating effect of NBPT was seen when nitrification was the dominant process in the production of N2O. Since NO is mostly produced through nitrification under these conditions, using NBPT was also effective in the abatement of this reactive N gas.
References Ding, W.X., Yu, H.Y. and Zucong, C. C. 2011. Impact of urease and nitrification inhibitors on nitrous oxide emissions from fluvo-aquic soil in the North China Plain. Biology and Fertility of Soils 47, 91-99.
Harrison, R. and Webb, J. 2001. A review of the effect of N fertilizer type on gaseous emissions.
Advances in Agronomy 73, 65–108.
Sanchez-Martin, L., Vallejo, A., Dick, J. and Skiba, U.M. 2008. The influence of soluble carbon and fertilizer nitrogen on nitric oxide and nitrous oxide emissions from two contrasting agricultural soils. Soil Biology and Biochemistry 40, 142-151.
Sanz-Cobena., A., Misselbrook, T.H., Arce, A., Mingot, J.I., Diez, J.A. and Vallejo, A. 2008. An inhibitor of urease activity effectively reduces ammonia emissions from soil treated with urea under Mediterranean conditions. Agriculture Ecosystems and Environment 126, 243-249.
Soil Survey Staff, 1992. Keys to Soil Taxonomy, sixth edition. USDA,Washington DC.
Zaman, M., Saggar, S., Blennerhassett, J.D. and Singh, J. 2009. Effect of urease and nitrification inhibitors on N transformation, gaseous emissions of ammonia and nitrous oxide, pasture yield and N uptake in grazed pasture system. Soil Biology and Biochemistry 41, 1270-1280.
Nitrogen Workshop 2012
Can arbuscular mycorrhizal fungi enhance plant nitrogen capture from organic matter added to soil?
Saia, S.a, Ruisi, P.a, García-Garrido, J.M.b, Benítez E.c, Amato, G.a, Giambalvo, D.a a Dipartimento dei Sistemi Agro-Ambientali, Università degli Studi di Palermo, Palermo, Italy b Departamento de Microbiología, Estación Experimental de Zaidín, CSIC, Granada, Spain c Departamento de Protección Ambiental, Estación Experimental de Zaidín, CSIC, Granada, Spain
1. Background & Objectives Several studies have shown that arbuscular mycorrhizal (AM) fungi are involved in plant nitrogen (N) uptake from inorganic sources. In addition, the AM fungi may be important in plant N capture from decomposing organic matter (OM), but their role is still unclear (Hodge et al., 2010). The present work tested the hypothesis that AM symbiosis can affect durum wheat (Triticum durum) N acquisition from OM added to soil, either by directly or indirectly influencing OM decomposition.
2. Materials & Methods A pot experiment was conducted in a climate-controlled glasshouse (25/19°C day/night temperature; 16 h photoperiod). A complete randomized factorial design with four replicates was adopted. Treatments were: i) AM symbiosis, inoculation of soil with Glomus mosseae (+Myc) and uninoculated control (Myc); ii) organic matter (OM), soil amended with 4.6 g 15N-enriched maize leaves (C:N ratio 22.6:1) per kg of soil (+OM) and unamended soil (–OM). Each pot was filled with 600 g of a quartz sand:soil mixture (2:1). Soil properties were: clay 20% and sand 37%; pH 8.1 (soil:water 1:2); 1.04% organic C; 1.05‰ total N. The soil mixture was steam-sterilised. Before starting the experiment, a soil filtrate was inoculated to normalise the microbial community. Three wheat plants (cv Simeto) per pot were grown. During the experiment, each pot received 5 ml of a modified Hoagland’s solution (with no phosphorus and 10% N) once every 5 days. The dry weights of wheat shoots and roots were recorded 9 weeks after the emergence of the crop and both fractions were analyzed for total N and 15N enrichment using an elemental analyzer–isotope ratio mass spectrometer. The activity of two soil proteolytic enzymes was measured: caseinase (a measure of the protein hydrolysis to monopeptides) and BAA-protease (a measure of amino acid deamination).
Wheat roots were stained with 0.05% trypan blue in lactic acid and AM infection was measured using the grid intersect method (Giovannetti and Mosse, 1980). The recovery of the applied 15N in wheat was calculated according to Allen et al. (2004). An analysis of variance was performed according to the experimental design.
3. Results & Discussion No AM root infection was found in the –Myc treatment. The addition of OM to soil markedly decreased both plant growth and total N uptake and, at the same time, increased the caseinase and BAA-protease activities (Table 1), which suggests an increase in soil microbial activity. Because soil microorganisms outcompete plants for nutrients over short timescales, the depressive effect of OM on plant growth and N uptake may have been caused by the higher sequestration of available inorganic N and other nutrients by microorganisms in +OM. On average, mycorrhizal wheat yielded 20% more biomass and 15% more N than non-mycorrhizal control. Several studies have shown that AM symbiosis improves plant growth and nutrient uptake especially when plants are grown under nutrient-limiting conditions (Azcón et al., 2001). Through AM fungi, plants can better scavenge the soil volume, which enhance their ability to absorb the available N. In addition, as suggested by Hodge et al. (2000), AM fungi could enhance N uptake by host plant being more effective than nonmycorrhizal roots in competing with soil microorganisms for inorganic N. The microbial activity
Nitrogen Workshop 2012
(both caseinase and BAA-protease) was significantly higher in +Myc than –Myc in either +OM and –OM treatments. This should have involved an increase in soil N availability from OM. However, the 15N recovery fraction from the added OM was markedly lower in +Myc than –Myc treatment.
Two mechanisms can be invoked to explain such result: firstly, AM fungi could have acquired N from decomposing OM in the form of amino acids and retained this element primarily for their own growth and metabolism (Hodge and Fitter, 2010). Secondly, mycorrhizal plants are more effective than non-mycorrhizal plants to take up inorganic N; this probably limited N availability in soil, thus forcing soil bacteria to rely on organic compounds for satisfying their N demand, which limited the release of N from OM (Schimel and Bennett, 2004).
4. Conclusion Although AM fungi increased soil N mineralization rates and total plant N uptake, they strongly reduced wheat N recovery from OM. This suggests that AM fungi have marked effects on competition between plants and bacteria for the different sources of N in soil.
References Allen S.C., Jose S., Nair P.K.R., Brecke B.J. and Ramsey C.L. 2004. Competition for 15N-labeled fertilizer in a pecan (Carya illinoensis K Koch)-cotton (Gossypium hirsutum L) alley cropping system in the southern United States. Plant and Soil 263, 151-164.
Azcón R., Ruiz-Lozano J.M. and Rodríguez R. 2001. Differential contribution of arbuscular mycorrhizal fungi to plant nitrate uptake (15N) under increasing N supply to the soil. Canadian Journal of Botany 79, 1175-1180.
Giovannetti M. and Mosse B. 1980. An evaluation of techniques for measuring vesicular-arbuscular mycorrhizal infection roots. New Phytologist 84, 489-500.
Hodge A. and Fitter A.H. 2010. Substantial nitrogen acquisition by arbuscular mycorrhizal fungi from organic material has implications for N cycling. Proceedings of the National Academy of Sciences, USA, 107, 13754-13759.
Hodge A., Helgason T. and Fitter A.H. 2010. Nutritional ecology of arbuscular mycorrhizal fungi. Fungal Ecology 3, 267-273.
Hodge A., Robinson D. and Fitter A.H. 2000. Are microorganisms more effective than plants at competing for nitrogen? Trends in Plant Science 5, 304-308.
Schimel J.P. and Bennett J. 2004. Nitrogen mineralization: Challenges of a changing paradigm. Ecology 85, 591-602.
Nitrogen Workshop 2012
Canola response to N fertilization as affected by preceding crop and location Grant, C.A., Gao, X., O’Donovan, J.T., Zebarth, B., Blackshaw, R.E., Harker, K.N., Lafond, G.P., Johnson, E.N., Gan, Y.
Agriculture and Agri-Food Canada Research Centres, Brandon, Lacombe, Fredericton, Lethbridge, Indian Head, Scott and Swift Current, Canada
1. Background & Objectives Excess N application is a major cause of poor N use efficiency (NUE), contributing to negative environmental impacts and reduced economic benefit. Soil nitrate is used in western Canada to predict soil N supply and N fertilizer recommendations, but its effectiveness may have decreased due to changing crop production practices. Higher yielding cultivars, reduced tillage, cropping intensification, and higher fertilizer input over time may have increased the return of high N crop residues to the soil and increased the contribution of in-season N mineralization to the crop N supply (Grant et al., 2002). A more accurate estimate of the total supply of both inorganic and mineralizable N is needed to predict N requirements and avoid over- or under-fertilization.
2. Materials & Methods Field studies were conducted at 6 sites across western Canada to assess effects of preceding crop, soil characteristics and environment on yield response of canola to N fertilization and to evaluate the effectiveness of various soil tests and modelling approaches in predicting optimum N fertilization rate. The study consisted of a two year crop sequence with preceding crops (fababean grown for seed, fababean used as green manure, and pea, lentil, wheat and canola grown for seed) grown in the first year and canola grown in the second year. Nitrogen fertilizer was applied to the canola as urea, banded at the time of seeding at 0, 30, 60, 90 and 120 kg N ha-1. Soil samples were taken after the growth of the preceding crop but before seeding of the canola and analysed for nitrate, ammonium and for mineralizable N using several techniques. A split-plot design with four replicates was used with preceding crop as the main plot and N rates as sub-plots. Crops were harvested at maturity and analyzed for seed and tissue N. The ability of the various soil tests to predict plant-available N and the yield response to N application is currently being assessed.
3. Results & Discussion Total soil nitrate-N in the upper 60 cm was highest after fababean green manure in half of the sites, but soil nitrate-N was not consistently higher after pulse crops than after canola or wheat (Table 1).
Total canola seed yield and the yield increase with N application varied substantially with location and preceding crop (Figure 1). Seed yield was consistently higher after fababean green manure than wheat or canola, regardless of N fertilizer input or effect on soil nitrate, indicating both a nitrate
Nitrogen Workshop 2012
based and a non-nitrate-based benefit. Seed yield of canola and the yield response to N application was related to soil nitrate-N concentration to some extent, but there were discrepancies. For example, soil nitrate-N at Beaverlodge and Lacombe was low compared to that at Brandon, yet the canola seed yield was as high or higher in the unfertilized check and response to fertilizer application lower at these two sites than at Brandon. This may indicate high levels of mineralizable N at the Beaverlodge and Lacombe sites. Several mineralization tests are currently being evaluated for their ability to more accurately predict plant-available N and potential response to N fertilization at these field locations.
Figure 1. Yield response as a function of nitrogen fertilization and preceding crop at six locations in western Canada.
4. Conclusion Fababean green manure provided both nitrogen and non-nitrogen benefits to the following canola crop. Soil nitrate-N provided an approximate indication of plant-available N and yield response to N application, but better prediction is needed to more accurately determine fertilizer N requirements.
References Grant, C.A., Peterson, G.A. and Campbell, C.A., 2002. Nutrient considerations for diversified cropping systems in the Northern Great Plains. Agronomy Journal 94, 186-198.
Nitrogen Workshop 2012 Carbon and nitrogen residual effects after repeated manure applications Bechini, L.a, Cavalli, D.a, Marino, P.a a Department of Plant Production, Università degli Studi di Milano, Milano, Italy
1. Background & Objectives Repeated applications of animal manure to agricultural soils contribute to the short term fertility (Bechini and Marino, 2009), but determine also the residual effect, i.e. higher crop N availability in manured compared to unmanured soils, through the mineralisation of recalcitrant manure components and the re-mineralisation of microbially-immobilised N (Sørensen, 2004). Manure residual effect has been studied in several field experiments (e.g. Schröder et al., 2007). Compared to field experiments, a laboratory incubation permits the measurement of the net N mineralisation of manures, without the confounding effect of other N inputs or outputs. It also enables the fate of added N in different compartments to be traced (e.g. Sørensen, 1998; Van Kessel and Reeves, 2002). The aim of this laboratory experiment was to estimate under constant soil temperature and water content the residual effect of C and N after repeated applications of dairy cow manure to a clay-loam soil.
2. Materials & Methods A liquid dairy cow manure (dry matter 8.2%; organic C 34.9 g C kg-1 ; total N 3.9 g N kg-1; NH4–N