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Incubation experiment, nutrient use efficiency and total N recovery in ryegrass and white clover systems

Nutrient fluxes in soil plant systems, by BrynerS, HattF, WeberV

Nitrogen is often the most limiting nutrient in organic cropping systems where no synthetic mineral N fertilizers are applied and where total N inputs are usually lower than in conventional systems (Oberson et al. 2007). There are several possibilities to improve N supply in organic systems such as the use of farmyard manure and green manures. However, farmyard manure is linked to animal production and not always available in sufficient amounts. Growing of green manures on the other hand competes for land with seasonal crops (Venkatesveralu et al., 2007). Therefore, inclusion of N2 fixing crops in rotations represents an important option to improve N supply and to maintain soil fertility in organic farming systems (Oberson et al. 2007). The farming system and associated fertilization practice does not only influence microbial activity and processes of nutrient cycling in the soil (Mäder et al., 2002) but also the extent of symbiotic N2 fixation by legume crops (Oberson et al., 2007).
To assess the N use efficiency as well as the fate of fertilizer N not taken up by the plants a pot experiment was carried out in the spring semester 2008. The objective of the study was to compare the N use efficiency of mineral and organic fertilizers by the non-fixing Italian ryegrass (Lolium multiflorum) and the N2 fixing white clover (Trifolium repens). The fate of fertilizer N not taken up by the plant was investigated and differentiated into N recovered from the soil and losses from the system. Symbiotic N2 fixation of white clover was estimated by the natural abundance method. In addition, selected soil analyses were performed in order to understand the effect of the fertilizer treatment on the nutrient dynamics in the soil. The following hypotheses were to be tested with the present pot experiment:

  1. The N use efficiency, assessed by the recovery of N in the plant biomass, is greater for the water soluble mineral fertilizer than the Organic N fertilizer.
  2. The N recovery in the soil is greater for the organic than the Mineral N fertilizer because of lower uptake by the plant and lower gaseous losses.
  3. Total N recovery (i.e. the sum of recovery in plant biomass and soil) is higher for the organic than mineral fertilizer because of lower losses from organic fertilizer (in pots were no leaching occurs).
  4. Ryegrass will use fertilizer N more efficiently than the clover because of larger production during the experimental period, larger root system and because clover will additionally fix N2 from the atmosphere.

Overall discussion and conclusion
Application of organic fertilizer has often been shown to improve long-term microbial activity in the soil (Langmeier et al 2002). Our incubation experiment revealed that organic fertilization increased microbial biomass and activity. However, this effect was mainly observed during the first incubation period. During the second incubation period on the other hand the impact of the fertilization treatment was either reduced as in the case of respiration rate or even not obvious as in the case of mineralization rate. Microbial biomass and enzyme activities were only measured once seven days after incubation. The assessments for fertilizer use efficiency, N2 fixation and total N recovery were made at the end of the experiment and it is therefore difficult to compare these results with microbial biomass and enzyme activities. It would have been interesting to determine the effect of microbial activity during the first incubation period on the above mentioned plant traits.
Fertilizer type, application rate and method may greatly influence N fluxes and availability in the soil. In our experiment, one third of the mineral N applied with the mineral fertilizer was immediately adsorbed to soil particles and removed from the available N pool. This reflects one problem of applying mineral fertilizer in agriculture because the fertilizer will not stay uniformly plant available in space and time. In the organic fertilization treatment, incorporation of hay into the soil led to a net immobilization of N from the soil in the beginning. Mineral N was therefore made unavailable to the plant. However, there was no clear response in total N uptake of ryegrass in the OrgN treatment. Even though ryegrass reached a smaller dry matter yield after organic fertilization than after no fertilization, this effect was compensated with a slightly higher N concentration in the tissue in the OrgN treatment. It is possible that immobilization of organic fertilizer by soil microorganisms had an impact on plant growth in the beginning of the experiment, but this effect was neutralized later on with a net mobilization in the organically treated soil. However, the mineral N availability stayed lower in the OrgN treatment compared to the 0N treatment over the whole 6 weeks. Furthermore, the 0N treatment was more affected by the K limitation than the OrgN treatment which might have had a negative effect on the yield in the 0N.
In conclusion, the OrgN treatment showed no or a slight negative effect concerning yield in the tissue. When taking into account biological activity, the OrgN treatment seems to be beneficial compared to the other treatments. To gain a better insight into effects of organic fertilization on biological activity, yield, N uptake and losses, the experiment should be done on long-term to allow the organic fertilizer to be decomposed to a large extent.
N2 fixation was highest in the 0N treatment followed by OrgN and MinN treatment. In contrast, the mineral N availability was lowest in the OrgN followed by 0N and MinN. N2 fixation was therefore not fully correlated with the amount of available mineral N. However, OrgN and 0N were very similar concerning N2 fixation indicating that the clover was indifferent for these two fertilizer treatments. In the MinN treatment, a clear inhibition of N2 fixation can be seen.
Identification of significant factors influencing fertilization treatments requires that only one is varying in each treatment. Unfortunately, we had several restraints in our experiment which complicated interpretation of the data. Ryegrass was N limited in all treatments due to different reasons such as low N availability, high biomass production or a limitation in the other major nutrients P and K. The applied organic fertilizer contained some additional P and K but it is not clear how readily plant available those nutrients were. Clover was also limited in P and K. P deficiency is known to reduce the N2 fixation. However, this can not be evaluated in our experiment because all treatments were P limited. It is difficult to determine how these limitations and their interactions influenced growing behaviour and yield. The obtained results confirm most hypotheses or show a tendency in favour of them.
Hypothesis 1 stated that the N use efficiency (N recovery) was greater for the mineral fertilizer treatment than the organic fertilizer treatment. This hypothesis could be confirmed for both the clover and the ryegrass.
The organic fertilizer treatment was assumed to have a higher N recovery in the soil because the plants can take up less fertilizer N and because the volatilizing losses are minimized (Hypothesis 2). Hypothesis 2 was confirmed. On the one hand, the N recovery in the plant was lower in the organic than in the mineral fertilizer treatment. Furthermore, no N losses were observed with the organic fertilizer treatment whereas in the mineral fertilizer treatment 18% of applied fertilizer was lost due to denitrification.
In hypothesis 3, the total N recovery is stated to be higher for the organic than in the mineral fertilizer treatment due to lower losses. Hypothesis 3 is verified with regard to lower losses (hypothesis 2).
Hypothesis 4 assumed that ryegrass will use fertilizer N more efficiently than the clover because of larger dry matter production (above- and below-ground) and because clover will fix additional N2. This hypothesis can be seen in two ways. First, it can be interpreted on the plant level considering Ndff and N recovery in the plants. When looking at Ndff, which is a relative measurement, the hypothesis was confirmed. Taking into account N recovery, the hypothesis could not be confirmed because higher biomass production of the ryegrass, higher N concentration and N2 fixation of the clover caused similar N recovery for both species. Secondly, when looking at the whole soil-plant system, the hypothesis can neither be confirmed. The N recovery was identical for both species depending on the fertilizer treatment. From this point of view, it can not be said that ryegrass systems use fertilizer N more efficiently than clover systems.


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June 30th, 2008
Topic: Crop Science, Plant nutrition Tags: None

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