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«International 17 Workshop th Nitrogen The was jointly organised by Teagasc and AFBI Printed by Print Depot Suggested citation Authors, 2012. Title ...»

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2. Materials & Methods Repacked soil cores (Typic Orthic Allophanic Soil) were compacted at pressures of 0, 220 kPa & 400 kPa and treated with or without 15N-labeled synthetic urine applied at 600 kg N ha-1 (enrichment of 50 atom%). Soil cores were then subjected to three successive, 12-day saturation/drainage cycles (from 0 to -10 kPa tension). Daily gas fluxes of N2O, N2 and CO2 were quantified using mini-headspace chambers placed over the cores. Soils sampled prior to commencing the 2nd and 3rd cycles and on completion of the experiment were analysed for inorganic-N, dissolved organic-C (DOC) and pH.

3. Results & Discussion The ratio of N2O to N2 emitted during denitrification depends on factors such as soil pH, soil water status, NO3-N concentration and C supply (Clough et al., 2004). During the 1st drainage cycle, nitrification was limited by a lack of O2 due to low gas diffusion through the core at high water contents (mean WFPS over cycle 1 of 82, 84 & 86% for 0, 220 & 400 kPa compaction respectively) and the high microbial O2 consumption stimulated by high DOC levels (485, 530 and 695 mg kg-1 for 0, 220 and 400 kPa compaction respectively). Hence, the supply of NO3- for denitrification was low ( 25 µg NO3-N g-1); at low NO3- concentrations, N2O was rapidly reduced to N2 (Figure 1, cycle 1). In the 2nd cycle, soil NO3-N concentration began to increase (Figure 2, day 12). N2 was still the predominant product (Figure 1, cycle 2) probably due to complete denitrification when NO3- supply is limited, high soil pH (pH 6.5) and high water filled pore space (WFPS) (ranging from 92 to 74%) that restricted diffusion of N2O from the site of denitrification allowing further reduction to N2. By the 3rd cycle NO3-N concentrations had further increased (Figure 2, day 24) and N2O was the predominant emission product from the 0 and 220 kPa compaction treatments, while N2O and N2 were emitted at similar rates from the 400 kPa treatment (Figure 1, cycle 3).

Higher WFPS at any given tension, higher pH and lower NO3- concentration in the 400 kPa treatment would have favoured N2 emissions compared to the 0 and 220 kPa treatments.

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References Clough, T.J., Kelliher, F.M., Sherlock, R.R. and Ford, C.D. 2004. Lime and soil moisture effects on nitrous oxide emissions from a urine patch. Soil Science Society of America Journal 68, 1600-1609.

de Klein, C.A.M., Barton, L., Sherlock, R.R., Li, Z. and Littlejohn, R.P. 2003. Estimating a nitrous oxide emission factor for animal urine from some New Zealand pastoral soils. Soil Research 41, 381-399.

Nitrogen Workshop 2012

Influence of tree canopies on nitrogen dynamics in Montado - a Portuguese Cork Oak Savanna Fangueiro, D.a, Caldeira, M.C.b, Lecomte, X b., Coutinho, J.c a UIQA, Instituto Superior de Agronomia – Tecnical University of Lisbon, 1349-017 Lisbon, Portugal b CEF, Instituto Superior de Agronomia – Tecnical University of Lisbon, 1349-017 Lisbon, Portugal c Centro de Química, Universidade de Trás-os Montes e Alto-Douro, 5001-801 Vila Real, Portugal

1. Background and Objectives Montado, Portuguese Cork Oak Savanna, is composed of a sparse tree canopy (30-70 trees/ha) and a grassland. The sustainability of such system depends on soil nitrogen (N) availability, particularly on N mineralization. This process is affected by several soil characteristics such as water or carbon content but also by the vegetation. Two distinct types of plant litter, herbaceous litter and more recalcitrant woody plant litter can be found in Montado. The main objective of the present work was to assess the differences in N turnover and N availability in soils under tree canopies compared to open grassland soils. To achieve this goal, the spatial and temporal variability of nitrogen dynamics (mineralization) and soil microbial biomass due to the tree-grassland component of Montado were evaluated.

2. Materials & Methods The study site is a Montado located in Southern Portugal close to Lisbon. At this site 8 plots were

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Nitrogen Workshop 2012 Interactions between Free-living Soil Nematodes and Ryegrass: Effects on Nitrogen Mineralization Gebremikael, M.T.a, Buchan, D.a, De Neve, S.a, a Department of Soil Management, Faculty of Bioscience Engineering, Gent University, Gent, Belgium

1. Background & Objectives Free-living nematodes have been estimated to contribute 8-19% to total N mineralization in soil (Neher and Power, 2005). These results are based on theoretical food web calculations (Hunt et al.,

1987) or very simplified experiments including only a few selected species, often on sterilized media (Ferris et al., 1998). However, N mineralization is controlled by biological interactions between microbes, fauna and plants. To address this issue we conducted an incubation experiment with and without plants, by re-inoculating entire nematode populations into soil cores that had been defaunated using low-dose gamma irradiation which selectively kills fauna while minimally disturbing the microbial population. The objective of this experiment was to investigate the effect of interactions among different feeding groups of free-living soil nematodes, microbes and plants on nitrogen mineralization.

2. Materials & Methods Part of the fresh soil samples collected were gamma irradiated at a 5 kGy dose in order to kill nematodes and other soil fauna. Entire populations of free-living nematodes were extracted from bulk soil using an automated zonal centrifugal machine (Hendrickx, G. 1995) and re-inoculated into cores that had been filled with defaunated soil. Three treatments, each with four replicates, were compared on soil either left bare or planted with Lolium perenne: (i) not irradiated and not inoculated (control) which mainly used for comparing nematode population and dynamics, (ii) defaunated and reinoculated (+Nem), and (iii) defaunated but not re-inoculated (-Nem). The moisture content was adjusted to 50% of the water filled pore space and kept constant by adding distilled water every day. Dynamics of mineral N in soil, plant N uptake, microbial biomass carbon (MBC), and nematode population were determined destructively after 7, 30, 45, 65, and 86 days of incubation in a growth chamber (17oC and 16/8 light/dark hours). Due to the influence of plant uptake on N dynamics in planted microcosms, total mineral N was considered as the sum of mineral N that was found in the soil and taken up by the grass shoots and roots. Two way ANOVA, with two fixed factors: time versus treatment; and palnting versus treatment were separately run to analyze all the parameters and palnt-nematode interactions respectively. Whenever there was significant mean differences (P0.05), Games-Howel post hoc analysis was used in SPSS version 19.





3. Results & Discussion The nematodes population after reinoculation was compared to the control in order to check the efficiency of re-inoculation. At the beginning of incubation the efficiency was found to be 67.4% and 49.5% in bare and planted microcosms respectively. But after 65 days of incubation, the population was found to be higher (P0.05) in +Nem samples than the control in planted microcosms. Total mineral N in bare microcosms was found to be significantly (P0.05) higher in +Nem samples as compared to –Nem samples (Figure 1). Similarly NO3-N concentration was found to be significantly (P0.05) higher in +Nem samples towards the end of the incubation period. Xiao et al. (2010) reported that bacterial feeding nematodes increased ammonia-oxidizing bacterial community which could explain the increased nitrate concentrations. In contrast to bare

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microcosms, no significant difference (p0.05) in total mineral N was found between +Nem samples and –Nem samples in planted microcosms (Figure 1). Plant versus treatment interactions were found statistically significant (p0.05) for –Nem samples.

Figure 1. Dynamics of total mineral N over the incubation period.

The error bars are standard error of the mean (n=4).

Previous investigations reporting increased plant N uptake used only few species of nematodes under sterilized conditions (Ingham et al., 1985). Here, inoculating the entire nematode population instead of few species, which normally consist of plant parasitic nematodes, might have affected N uptake in plants. Data on the composition of the microbial and nematode populations and enzymatic activities is currently being processed.

4. Conclusion Free-living soil nematodes communities can increase nitrification and N mineralization in bare microcosms. The results show that the presence of the entire free living nematodes did not significantly affect total mineral N in the planted microcosms. Data on the functional feeding groups of these nematodes is required to possibly explain the mechanism responsible for the effects of nematodes on N mineralization and plant uptake.

References Ferris, H. et al. 1998. Nitrogen mineralization by bacterial-feeding nematodes: verification and measurement. Plant Soil 203, 159-171 Hunt, H. W. et al. 1987. The detrital food web in a shortgrass prairie, Biol. Fertil. Soils 3, 57-68 Ingham, R. E., Trofymow, J. A., Ingham, E. R. and Coleman, D. C. 1985. Interactions of bacteria, fungi and their nematode grazers: Effects on nutrient cycling and plant growth. Ecological Monographs 55, 119-140 Neher, D.A. and Powers, T.O. 2004. Nematodes. In: Hillel, D., Rosenzweig, C., Powlson, D., Scow, K., Singer, M. and Sparks, D. (editors) Encyclopedia of Soils in the Environment, Vol. 3, pp. 1-5, Academic Press, New York.

Xiao, H., Griffiths, B., Chen, X., Liu, M., Jiao, J., Hu, F. and Li, H. 2010. Influence of bacterial-feeding nematodes on nitrification and the ammonia-oxidizing bacteria (AOB) community composition, Applied Soil Ecology 45, 131-137 Hendrickx, G. (1995). An automated apparatus for extracting free-living nematode stages from soil. Nematologica 41, 308.

Nitrogen Workshop 2012

Ley management effects on N2 fixation, crop N dynamics and residual N Dahlin, A.S.a, Stenberg, M.b, Marstorp, H.a a Swedish University of Agricultural Sciences, Dept. of soil and environment, Uppsala, Sweden b Swedish University of Agricultural Sciences, Dept. of soil and environment, Skara, Sweden

1. Background & Objectives Biological fixation of atmospheric nitrogen (N2) of various species of clover is an important N input into different types of cropping systems, e.g. through fodder production and green manure leys. In order to assess how efficient and environmentally friendly such systems are, it is important not only to determine the amount of fixed N2 but also to understand how N flows and distribution of fixed N2 in the soil/plant system are affected by management practices.

The aim of this study was to: 1) quantify the effect of cutting strategies on N2 fixation and distribution of fixed N2 in the soil-plant system of pure clover leys and mixed clover grass leys, and 2) quantify N supply to a cereal crop.

2. Materials & Methods We tested the effect of cutting regime (harvested, mulched, intact) on the symbiotic N2 fixation and the distribution of the fixed N2 in the soil/plant system in a field experiment with pure red clover and mixed red clover-perennial ryegrass green manure leys. Stands of pure perennial ryegrass were also included for reference. The experiment was carried out in southwestern Sweden and repeated during three consecutive years. Symbiotic N2 fixation in clover was determined with 15N isotope dilution technique (Dahlin and Stenberg, 2010a), belowground N and shoot litter N was determined through labeling of clover leaves with 15N-urea (Dahlin and Stenberg, 2010a), and transfer of N from clover to grass estimated by 15N isotope dilution technique and through labeling of clover leaves with 15N-urea (Dahlin and Stenberg, 2010b). Total amount of fixed N2 in the soil-plant system was calculated on the basis of determinations of N in the different plant and soil fractions. Uptake of N from mulch was determined using 15N labeled mulch material (Dahlin et al., 2011). Nitrogen supply to a following oat crop was determined during the year immediately following incorporation of the leys by determination of soil mineral N and N in crop grain and straw.

3.Results & Discussion The total amount of fixed N2 was higher in the harvested and mulched treatments (average

45.3 g N m-2) than in the intact treatment (mean 31.8 g N m-2). Recycling of N to the ley in the mulched treatment was 21% of the N in the mulch and contributed 13.7% (pure clover) and 2.2% (mixed clover-grass) of clover plant N uptake during re-growth. Uptake of N from mulch did not significantly decrease the amount of fixed N2 in the mulched treatment compared to the harvested treatment but instead contributed to greater total biomass. This is contradictory to the findings of Heuwinkel et al. (2005) who found a reduction of N2 fixation by mulching. However, the quantity of mulch used in their study was approx. 3 times larger than in our study where the mulch corresponded to the standing biomass before cutting. Large amounts of fixed N2 was found in the below ground fractions corresponding to 53%, 46%, and 60% of total fixed N2 in intact, harvested and mulched treatments, respectively. In the harvested treatments most of the remaining fixed N2 had been exported from the field with the harvested shoot. In the mulched treatment 7% of the fixed N2 was lost, presumably via gaseous losses. Although the total N2 fixation did not differ between the harvested and mulch treatments, it was thus a noticeable difference in where the N was located.

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the intact treatments was larger compared to the cut treatment when the transfer via wilting leaves in intact stands and shoots left lying on the ground in the mulched stands was included in the estimates. This suggests that N transfer is not affected by cutting strategies as long as shoot biomass is not left in the field and the cutting frequency is high enough to minimize falling leaf litter. The N transfer contributed strongly to the N budget of the companion ryegrass, especially in the stands where leaf fall contributed to the transfer. The uptake of clover-derived N by a companion crop may have implications for the composition and feeding value of fodder leys as well as for the efficiency catch crops.



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