Nitrogen release and nitrogen use efficiency of plant derived nitrogen fertilisers in organic horticultural soils under glasshouse conditions

As a consequence of the BSE-crisis, alternatives for fertilisers derived from animal residues are being sought for use in organic horticulture. Therefore, the N release of grain legumes (milled seeds of pea, yellow lupin, and fababean) and organic fertilisers of industrially-processed plant and microbial residues (Maltaflor®-spezial, Phytoperls®, Agrobiosol®, Rizi-Korn) was investigated. In an incubation study with one sandy soil (Corg: 1.4%) net N mineralisation of the fertilisers (N content: 3.0-8.5%, C/N: 5.0-13.3) was measured. In pot experiments the apparent N utilisation by perennial ryegrass was determined. In order to investigate the effect of soils on fertiliser N release the pot experiments were conducted with a number of soils with different soil textures (sandy loam - silty clay loam) and Corg contents (Corg: 1.2-8.4) that were sampled from organic horticultural farms. In these experiments the N utilisation was calculated from unlabelled or 15N labelled fertilisers. Two fertilisers with a relatively high N release (fababean, Maltaflor®-spezial) were selected to be examined in glasshouse experiments with grafted tomatoes (variety “Voyager”). The effect of the fertilisers on the yield and the apparent N utilisation of shoots (vegetative biomass plus fruits) was examined. In these experiments also the influence of the termination of fertiliser application was investigated by splitting the whole N amount of 20 g N/m2 into two dressings of 10 g N/m2. During the 7 weeks of incubation net N mineralisation of the applied N (40 mg N / 150 g dry soil) ranged from 42-50% (pea, lupin, Phytoperls®) up to 55-61% (fababean, Agrobiosol®, Maltaflor®-spezial) and was highest with Rizi-Korn (72%). For all fertilisers 70% of the total net mineralisation occurred within the first two weeks. With the exception of Phytoperls® and Agrobiosol® net N mineralisation was closely related to the N content (r2=0.97***) of the fertilisers, and to a lesser degree, to their C/N ratio (r2=0.79***). The relation between fertiliser N content and N release was confirmed by pot experiments with ryegrass. For example, the apparent N utilisation after a 13 weeks growth period was correlated to the fertiliser N content with r2=0.60*** and to the C/N ratio with r2=0.47***, excluding Phytoperls® and Agrobiosol®. It was concluded that the N content of the fertilisers is a good indicator for the N release of most of the tested fertilisers. Besides the N content, soil affected the N utilisation of the fertilisers, but the soil effect was less pronounced compared to the fertiliser N content. Three pot experiments with ryegrass revealed different soil parameters that may affect fertiliser N utilisation. In the first pot experiment with four soils the apparent N utilisation of fertilisers with a low N content (pea) was significantly lower in the fine textured soils compared to the coarse textured soils, whereas fertilisers with higher N contents showed no differences between soils. A second pot experiment conducted with a wider range of seven different soils demonstrated that the apparent N utilisation of two fertilisers with a medium N content (fababean, Maltaflor®-spezial) may differ between soils as well. This effect was neither related to soil texture nor to Corg content. Moreover, the soil dependent differences in the apparent N utilisation were not related to the soil N release potential, determined as N mineralised from the unfertilised soil, although the soils showed a wide range in this characteristic. In the third pot experiment conducted with four soils and 15N labelled fertilisers (1.5-4.6% N) the apparent N utilisation was not related to the N use efficiency (N derived from the fertilisers related to the total N apply). However, a higher N use efficiency from all tested fertilisers was observed on soils with a higher mineralisation of soil-N. From the results obtained it is concluded, that more than one soil parameter may account for variations in the N utilisation of the fertilisers in different soils. However, to provide a sufficient N supply for the plants, it is recommended to use fertilisers with a high N content especially for fine textured soils and for soils with a high N release and to fertilise all soils independently of their N release potential. Fababean and Maltaflor®-spezial were tested for tomato production under glasshouse conditions in two vegetation periods in comparison to horn. In both years and for both fertilisers tomato yield and total shoot N uptake was comparable to horn. The apparent N utilisation was lower in 2003 (fababean: 19%, Maltaflor®-spezial: 33%, horn: 32%) than in 2004 (fababean: 30%, Maltaflor®-spezial: 57%, horn: 38%). The lower level of the apparent N utilisation in 2003 was due to the higher soil N release and higher nitrate input with the irrigation water. Residual NO3--N in both years increased the level of apparent N recovery (plant N uptake plus NO3--N remaining in soil; 2003 / 2004: 34% / 37% for fababean, 59% / 68% for Maltaflor®-spezial, 37% / 57% for horn) as compared to the apparent N utilisation. When the N release of the soil was high (2003), the splitting of the total fertiliser N amount into two applications had no affect on tomato yield and total shoot N uptake. But when the N release of the soil was low (2004), N application only six weeks after planting decreased the N uptake. A split application of a readily available organic fertiliser like Maltaflor®-spezial could be promising and an additional N mineralisation could be achieved by regular tilling of formerly fertilised plots. Results with plant-derived and industrially-processed organic N fertilisers obtained from the incubation experiment were transferable to those from pot experiments. A transfer of these results, which were determined under climatic conditions that are usual in glasshouses, to vegetable production in glasshouse was also possible. However, the present results will not offhand be transferable to field conditions due to the impact of temperature and water supply on N mineralisation and due to losses of N mainly by leaching. Hence, the effectiveness of plant-derived and industrially-processed organic N fertilisers to supply field-grown vegetables has to be investigated under field conditions at different sites and growing seasons. From the results obtained here it can be resumed that grain legumes and organic, industrially-processed fertilisers from plant and microbial residues provide a sufficient N supply for organically grown horticultural crops with long growing periods under glasshouse conditions and they are therefore suited to substitute animal residue-derived fertilisers.