Henning Høgh-Jensen

Rhizodeposition of nitrogen by red clover, white clover and ryegrass leys

Correct assessment of the rhizodeposition of N in grassland is essential for the evaluation of biological N2-fixation of legumes, for the total N balance of agro-ecosystems, and for the pre-cropping value of grasslands. Using a leaf-feeding technique by which plants were 15 N labelled while growing in mezotrons in the field, the rhizodeposition of N by unfertilised red clover, white clover and perennial ryegrass growing in pure stands was shown to amount to 64, 71 and 9 g N m 22 , respectively, over two complete growing seasons. The corresponding values for red clover and white clover growing in mixtures with ryegrass were 89 and 32 g N m 22 , respectively. The rhizodeposited N compounds, including fine roots, constituted more than 80% of the total plant-derived N in the soil, and in all cases exceeded the amount of N present in stubble. In the mixtures of red clover‐ryegrass and white clover‐ryegrass and the pure stands of red clover, white clover and ryegrass, respectively, the rhizodeposition constituted a 1.05, 1.52, 1.26, 2.21 and 2.77 fold increase over the total N in the shoots harvested during the two production years. In pure stands and mixtures of clover, 84 and 92%, respectively, of this N derived from biological N2 fixation. It is concluded that rhizodeposition provides a very substantial input of N to the legume-based grassland systems with great consequences for ecosystem N balance and turnover. Furthermore, the amount of atmospheric-derived N in the rhizodeposits may exceed that in the harvested shoots. q 2001 Elsevier Science Ltd. All rights reserved.

Interactions between white clover and ryegrass under contrasting nitrogen availability: N2 fixation, N fertilizer recovery, N transfer and water use efficiency

Seasonal variation in N2 fixation, N transfer from clover to ryegrass, and soil N absorption in white clover–ryegrass swards were investigated under field conditions over three consecutive years. The plots were established with different seeding ratios of clover and ryegrass and contrasting fertilizer N ranging from 3 to 72 kg ha-1 year-1.An initially poor clover population needed at least one growing season to reach the same yield output as an initially well established clover population. The clover content of the sward decreased by the annual application of 72 kg N ha-1 but not by smaller N dressings.The total amount of atmospherically derived N in clover growing in mixture with ryegrass was, on average over the three years equal to 83, 71, 68 and 60 kg N ha-1 for the treatments of 3, 24, 48 and 72 kg N ha-1, respectively. The proportion of atmospherically derived N declined with increasing N application, but never became smaller than 80% of total clover N. The proportion of atmospherically derived N in a pure stand white clover amounted to 60–80% of the total N content, equivalent to 109, 110, 103 and 90 kg N ha-1 for the treatments of 3, 24, 48 and 72 kg N ha-1, respectively.Only small amounts of atmospherically derived N was transferred to the associated ryegrass during the first production year, while in each of the following years up to 21 kg ha-1 was transferred. The average amount of N transferred from clover to ryegrass was equivalent to 3, 16 and 31% of the N accumulated in ryegrass in the first, second and third production year, respectively. Expressed relative to the total amount of fixed N2 in the clover–ryegrass mixture, the transfer amounted to 3, 17 and 22% in the first, second and third production year, respectively. Thus transfer of atmospherically derived N from clover contributed significantly to the N economy of the associated ryegrass.The clover–ryegrass mixture absorbed constantly higher amount of soil derived N than the pure stands of the two species. Only 11% of the total accumulated fertilizer N and soil derived N in the mixture was contained within the clover component. Lower water use efficiencies for the plants grown in mixture compared to pure stands were mainly related to the increased N uptake in the mixture, with the subsequent increase in growth compared to the pure stands.It is concluded that positive interactions between clover and ryegrass growing in mixture ensure a more efficient fixation of atmospheric N2 and absorption of fertilizer N and soil derived N than pure stands of the same species.

Below-ground nitrogen transfer between different grassland species : Direct quantification by 15N leaf feeding compared with indirect dilution of soil 15N

Nitrogen (N) transfer from one species to another is important for the N cycling in low-input grassland. In the present work, estimates obtained by an indirect 15N dilution technique were compared with estimates obtained by a direct 15N leaf feeding technique over two complete growing seasons in red clover-ryegrass and white clover-ryegrass mixtures under field conditions.The direct technique confirmed that N transfer between clovers and ryegrass is a bi-directional process. The transfer of N from both clovers to ryegrass occurred within 25 days upon the first labelling event. A very high N transfer occurred from white clover to the associated ryegrass, 4.5 and 7.5 g m−2 in the 1st and 2nd production year, respectively. The corresponding values for transfer from red clover to the associated ryegrass were 1.7 and 3.6 g m−2. Quantified relatively to the total above-ground N content of white clover- ryegrass and red clover-ryegrass mixtures, the N transfer exceeded 50% and 10%, respectively, in three out of seven harvests. The N transfer from 15N labelled grass to associated clovers constituted a relatively constant proportion of approx. 8% of the above-ground N content of the mixtures.Estimates based on the soil 15N dilution technique generally underestimated the net N transfer by more than 50% compared to the direct 15N labelling technique. Furthermore, the indirect 15N dilution technique estimated only marginal differences between red and white clover in the quantities of N transferred, whereas the direct 15N labelling technique showed the N transfer from white clover to the associated ryegrass to be significantly higher than that involving red clover. It is concluded that N transfer is a much more dynamic and quantitatively important process in grassland than previously recognised.

Measurement of biological dinitrogen fixation in grassland: Comparison of the enriched 15N dilution and the natural 15N abundance methods at different nitrogen application rates and defoliation frequencies

A plant mixture of white clover (Trifolium repens L.), red clover (Trifolium pratense L.), and ryegrass (Lolium perenne L.) was established in the spring of 1991 under a cover-crop of barley. Treatments were two levels of nitrogen (400 and 20 kg N ha-1) and two cutting intensities (3 and 6 cuts per season). Fixation of atmospheric derived nitrogen was estimated by two 15N dilution methods, one based on application of 15N to the soil, the other utilising small differences in natural abundance of 15N.Both methods showed that application of 400 kg N ha-1 significantly reduced dinitrogen fixation, while cutting frequency had no effect. Atmospheric derived nitrogen constituted between 50 and 64% of harvested clover nitrogen in the high-N treatment, while between 73% and 96% of the harvested clover nitrogen was derived from the atmosphere in the low-N treatment. The amounts of fixed dinitrogen varied between 31–72 kg N ha-1 and 118–161 kg N ha-1 in the high-N and low-N treatment, respectively. The highest values for biological dinitrogen fixation were estimated by the enriched 15N dilution method.Estimates of transfer of atmospheric derived nitrogen from clover to grass obtained by the natural 15N abundance method were consistently higher than those obtained by the enriched 15N dilution method. Neither mineral nitrogen application nor defoliation frequency affected transfer of atmospheric derived nitrogen from clover to grass.Isotopic fractionation of 14N and 15N (B value) was estimated by comparing results for nitrogen fixation obtained by the enriched 15N dilution and the natural 15N abundance method, respectively. B was on average +1.20, which was in agreement with a B value determined by growing white clover in a nitrogen free media.

Nitrogen leaching from conventional versus organic farming systems — a systems modelling approach

Abstract The level of nitrogen leaching from organic compared to conventional farming was evaluated by using a systems modelling approach. Two different methods were used for estimating and evaluating nitrate leaching. A simple function was used in which nitrate leaching is dependent on percolation, soil clay content, average nitrogen input and crop sequence. A nitrogen balance model was used to estimate the long-term potential for nitrate leaching. These methods were applied to models of both current conventional farming systems in Denmark in 1996 and of well-managed organic farming systems. On average, the total estimated nitrogen input to the organic systems was lower (104–216 kg N ha −1 year −1 ) than to the conventional farming systems (146–311 kg N ha −1 year −1 ). The N-balances in the organic fields showed a surplus of nitrogen (net input of nitrogen) in to the root zone of 60–143 kg N ha −1 year −1 . In the conventional systems the surplus varied from 25 to 155 kg N ha −1 year −1 . The modelled nitrogen leaching from the organic systems varied from 19 to 30 kg N ha −1 year −1 on loamy soils to 36–65 kg N ha −1 year −1 on sandy soils. The modelled nitrogen leaching from the organic systems was always lower than from the comparable conventional agricultural systems due to: (I) the lower total input of nitrogen to the organic systems; and (II) the composition of the organic crop rotations including extensive use of catch crops. However, the modelling of nitrogen leaching has many uncertainties, principally due to difficulties in predicting the nitrogen leaching from different types of grass fields. Comparison of the results from two methods: (i) modelling of nitrogen leaching; and (ii) N-balances for the root zones, showed that organic arable crop production and dairy/beef farming on sandy soils are farming systems with a clear potential for lower nitrogen leaching than from the selected conventional systems. It is still uncertain whether the nitrogen leaching is lower or higher from organic arable crop production systems on loamy soil and organic pig production on loamy and sandy soil than from the same conventional systems in Denmark. The results point to the need for future research in the following areas: (i) the ability to build up soil organic nitrogen in organic farming systems and the consequences for both the level of crop production and nitrogen leaching in the long term; (ii) the effects of catch crops in organic crop rotations; and (iii) a better operational understanding of nitrogen leaching from different types of organically managed grass and grass-clover fields.

Carbon sequestration potential of organic agriculture in northern Europe – a modelling approach

Decline in carbon content in agricultural soils contributes both to climate change and to soil fertility problems. The CENTURY element dynamics simulation model was tested and adapted for Northern European agricultural conditions using long-term datasets from Askov experimental farm in southern Denmark. The part of the model dealing with decomposition was tested in isolation using a bare fallow experiment and it could predict soil organic matter levels with high accuracy. In the cropping experiments predictions were less accurate. The crop production was not accurately predicted. Predictions were more accurate on loamy than on sandy soils. The model was used to predict the effect of conversion to organic agriculture on carbon sequestration as soil organic matter. It predicted an increase in soil organic matter during the first 50 years of about 10–40 g C m−2 y−1, and a stable level after about 100 years. The use of grass-clovers in the rotation and as cover crops was particularly important for the increase in organic matter.

A Simple Model for Estimation of Atmospherically-Derived Nitrogen in Grass-Clover Systems

ABSTRACT Atmospherically-derived nitrogen can be a major source of nitrogen in organic or other low-input farming systems. It is, however, in practice difficult to quantify the contribution of nitrogen from the atmosphere especially when dealing with grass-clover leys in the crop rotation. In this study a simple model based on knowledge about the content of clover in herbage and the time after establishment is presented and calibrated to a range of production conditions in Denmark. A statistical analysis of three experiments showed that the content of clover made a satisfactory description of the variation in atmospherically-derived nitrogen above stubble height (harvested fixed-N2). The variation in harvested fixed-N2 was considerable (38–208 kg N ha−1 year−1) due to experimental manipulation by rate of N-fertilizer, cutting frequency, soil type and irrigation. It was, however, also found that the time after establishment of the grass-clover leys has a significant effect in that way, that at a given cont...

The Nitrogen Transfer Between Plants: An Important but Difficult Flux to Quantify

The exchange of nutrients, nitrogen in particular, between closely associated plants has attracted considerable interest due to its importance in agroecosystems under low external nutrient-input management. The intuitive observation of farmers that grasses benefit from near associations with clovers has not been easy to quantify, mainly because (i) the net effect is measured against large background fluxes, and (ii) excluding one species from one agroecosystem change the system fundamentally. The study of Moyer-Henry et al. (pp. 7–20 in this issue) approaches this problem elegantly by choosing a soil with a relatively low background mineralisation of nitrogen, while maintaining the same species in the system, although in one case as a non-nodulating variety. Their study confirms that substantial inter-plant Nitrogen transfer occurs. The study does, however, also raise the old question of direct and/or indirect transfer pathways (Virtanen et al., 1937; Wilson and Wyss, 1937). The N-natural-abundance technique may not be accurate enough to give reliable data on N fluxes, because it is even more sensitive than N-isotope dilution techniques to inhomogeneous distributions of tracers and roots (Luxhøj et al., 2003). The use of dual or triple tracers have been used in other studies (Kuzyakov, 2001) but it would be appropriate to apply direct plant labelling techniques using dual or triple tracers to obtain accurate information on this important process under field or semifield conditions. Nitrogen-transfer studies in light of high fluxes of nitrogen in the growth media

Variations in the natural abundance of 15N in ryegrass/white clover shoot material as influenced by cattle grazing

Biological N2 fixation in clover is an important source of N in low external-N input farming systems. Using the natural 15N-abundance method, variations in N2 fixation were investigated in grazed and mowed plots of a ryegrass/white clover field. Ryegrass δ15N varied considerably, from 0.2 to 5.6‰ under mowed conditions and from –3.3 to 11.6‰ under grazed conditions. Variations in δ15N white clover were lower than in ryegrass, especially in the mowed plots (SE = 0.05‰, n = 20). The variations in the percentage of nitrogen derived from the atmosphere (%Ndfa) in white clover were highest in the grazed plots where it ranged from 12 to 96% (mean = 64%) compared with the mowed plots where it ranged from 64 to 92% (mean = 79%). Thus, the N2 fixation per unit white clover DM in the grazed ley was lower and more variable than under mowing conditions.Urine from dairy cows equivalent to 0, 200, 400 and 800 kg N ha-1 was applied to a ryegrass/white clover plot 6, 4 or 2 weeks before harvest. Without urine application δ15N of ryegrass was positive. By increasing urine application (δ15N = –1‰) two weeks before sampling, the δ15N of ryegrass decreased strongly to about –7‰ (P < 0.001). However, this effect was only observed when urine was applied two weeks before sampling. When applying 800 kg N four and six weeks before sampling, δ15N in ryegrass was not significantly different from the treatment without urine application. White clover δ15N was unaffected by whatever changes occurred in δ15N of the plant-available soil N pool (reflected in δ15N of ryegrass). This indicates that within the time span of this experiment, N2 fixation per unit DM was not affected by urine. Therefore, newly deposited urine may not be the main contributing factor to the variation in %Ndfa found in the grazed fields. This experiment suggested that the natural abundance method can be applied for estimating %Ndfa without disturbance in natural animal-grazed systems.

Residual nitrogen effect of clover-ryegrass swards on yield and N uptake of a subsequent winter wheat crop as studied by use of 15N methodology and mathematical modelling

Abstract The residual effect of 2-year-old swards of clover-ryegrass mixture and ryegrass in monoculture on yield and N uptake in a subsequent winter wheat crop was investigated by use of the 15 N dilution method and by mathematical modelling. The amount of N in the wheat crop, derived from clover-ryegrass residues was 25–43% greater than that derived from residues of ryegrass which had been growing in monoculture. Expressed in absolute values, the N uptake in the subsequent winter wheat crop was 23–28 kg N ha −1 greater after clover-ryegrass mixture than after ryegrass in monoculture. Up to about 54 kg N ha −1 of the N mineralised from the clover-ryegrass crop was calculated to be leached, whereas only 11 kg N ha −1 was leached following ryegrass in monoculture.