In the wake of the decarbonisation of the entire economy, the use of peat in horticulture is under increasing pressure. In Germany, wood fibre is by far the most important substitute in growing media for professional horticulture. However, due to readily microbial degradability and a wide C:N ratio, nitrogen immobilization is a major problem of wood fibre products and considerably limits their use. Concurrently, stability of wood fibre is assessed by incubation experiments in which the change of mineral nitrogen is measured. However, these experiments have three shortcomings: First, they are quite labour and time-consuming as incubation period is up to 21 days. Second, if long-term fertilizers – especially urea-aldehyde condensation products – are applied by the manufacturer, the experiments only provide information about the stability of the N balance, but not about the stability of the material itself. Third, comparability and repeatability of the incubation experiments are rather poor. To overcome these shortcomings new approaches to evaluate stability of wood fibre were tested.

The setup of the first approach is similar to the currently used incubation experiments. However, the incubation period is only five days and additionally to change in mineral nitrogen from the beginning to the end of the experiment, carbon mineralization is analysed continuously by Oxitop®-C measuring heads. The second approach focusses on the characterization of readily decomposable nitrogen and carbon fractions in the wood fibre. On the one hand, hydrolysable nitrogen and carbon is measured and on the other hand, a stepwise thermal fractionation of carbon under pyrolytic conditions is done.

The results of the two approaches are compared to nitrogen immobilization measured in common incubation experiments and further validated by nitrogen balances deduced form short-term pot experiments with Chinese cabbage.

Green roofs play a crucial role in climate change mitigation strategies. On the one hand, they should reduce the risk of urban floods after heavy rainfalls. On the other hand, they should cool the city by evapotranspiration and avoid the formation of urban heat islands. However, concurrently most green roofs in Germany are extensive ones. They are designed as dry sites, with shallow and well-drained vegetation layers, no additional irrigation and greened with highly drought-adapted plants - mainly sedum. Indeed, during the last years a new kind of extensive green roofs – still with shallow vegetation layers – but equipped with sub-surface irrigation and voluminous retention elements as temporary water storage beneath the vegetation layer as well as greened with plants with high transpiration, were developed. But also these new kind of green roofs are facing a conflict of objectives between storm water retention and cooling performance.

To solve this conflict of objectives an adaptive and sensor based water management system was developed. It consists of two main components: Irrigation management of the plants and management of storm water runoff, especially the control of the retention element. For the irrigation management a decision matrix was developed which adapts the irrigation strategy, depending if cooling performance or water retention has higher priority. Prioritization is based on thermal discomfort and rain forecast. The decision matrix uses environmental data, data of the weather forecast and information about water resources (e.g. grey water, water supply in the retention element). For management of storm water, run-off is actively controlled and the vegetation layer used as temporary water storage. In combination with a retention element, this can quadruple water retention capacity of green roofs. The amount of water stored in the green roof system – especially the retention element – is mainly controlled on basis of real-time, radar-based precipitation data.

Co-composted biochar is suspected to be an effective sorbent for nitrate and thus might reduce nitrate leaching from agricultural soils. However, the underlying mechanism is unknown now and the effect on nitrogen availability is under discussion. In the current study, a pot trial with spinach was conducted to evaluate the influence of two co-composted biochars on nitrogen availability. Two wood-based biochars were co-composted with grass clippings and woody scrap material from landscaping activities. Additionally, a compost without biochar was prepared. Composts were mixed to an arable soil at rates of 30, 60 and 120 t ha^-1 and nitrate was applied on basis of 100 kg N per hectare. Afterwards, mixtures were filled in balcony boxes, moisturized and placed in a greenhouse for 40 days. As control the soil without compost was treated in the same way. Furthermore, additional treatments without and with the highest application rate, but without nitrogen fertilization, were prepared. After the 40 days these balcony boxes were also fertilized with 100 kg ha^-1 of nitrogen and spinach – pre-cultivated in press pots – was planted in all balcony boxes.

Irrespectively of application rates and – in case of the highest application rate – of date of nitrogen fertilization, no negative effect of co-composted biochar on N uptake was observed. In addition, the nitrogen balance for treatments with co-composted biochar was not significantly different from the control. However, for treatments with compost without biochar an increasing balance shortfall with increasing amounts of compost was observed. This might due to a nitrogen immobilization of woody residues. In view of the fact, that N uptake by plants was comparable and significant amounts of easily extractable nitrate were found in the soil at the end of the experiment, we assume that the co-composted biochars used in this experiment had no adsorption capacity for nitrate.

Due to depletion of global phosphorus resources, there is an increasing interest in phosphorus recycling products as fertilizer for growing media. However, in contrast to currently used fertilizers, which are almost completely based on calcium phosphates, at least in sewage-sludge based recycling products a remarkably amounts of phosphorus can be bound to iron or aluminum. In previous studies, the CaCl₂/DTPA (CAT) and CAL method partly underestimated plant availability of such sewage-sludge based recycling products, which might be due to a low solubility of iron- and aluminium-bound phosphorus in these two extractants. Thus, a pot trial with marigold was done: plants were fertilized with mono-calcium phosphate (MCaP), di-calcium phosphate (DCaP), tri-calcium phosphate (TCaP), aluminium phosphate (AlP) or iron phosphate (FeP) at growing media pH of 5.5 and 6.5, respectively. No effect on plant growth was observed for the three calcium phosphates irrespectively of pH. However, plant fresh weight was reduced if P was applied as FeP and AlP, whereby no effect of pH was found for AlP but for FeP. Against expectations, plant availability was poorer at the higher pH. In contrary to fresh weight, P uptake of plants decreases from mono- to tri-calcium phosphate and with increasing pH. For aluminium and iron phosphates results for P uptake was comparable to those reported for fresh weight. Whereas plant availability of the three calcium phosphates was well reflected by CAT and overestimated by CAL, P uptake from iron phosphate was strongly underestimated by CAT but matches CAL soluble P. Availability of aluminum phosphate was strongly underestimated by both extractants as well as by the combination of CAT and CAL soluble phosphorus. This supports previous findings for various sewage-sludge based products and raises the question of the need of other extractants.

Extensive green roofs are a key component of urban water management in the future. On the one hand, they should mitigate urban heat islands, for which evapotranspiration has to be maximized, and on the other hand, they are supposed to reduce the risk of urban floods after heavy precipitation events. To achieve these goals, an exact measurement of the water supply status is necessary. In arable soils as well as in organic growing media, dielectric sensors are widely common. However, there is only little knowledge about the suitability of this kind of sensors for mineral and coarse-textured substrates used for extensive green roofs. In the current research four dielectric sensors (EC-5, 10 HS, SMT 100 and Aquaflex TR) were tested using five different green roof substrates. The five substrates were filled in plastic boxes of 80 x 60 cm. Substrate height was 15 cm and the four sensors were placed at half height. Afterwards, white lupine was sown and cultivated up to a height of about 30 cm. For testing the sensors, the substrates were saturated with water and then they have been left to dry out until plants show severe signs of wilt. During dry out, the sensor signals as well as the weight of the boxes were recorded automatically every five minutes. For each substrate six consecutive drying cycles were done. With exception of the Aquaflex TR, for all sensors output signals were closely linear correlated to the weight loss and signal ranges were comparable for the six drying cycles. However, significant differences in output signals of the sensors between the five tested substrates were observed. This indicates a need for substrate specific calibrations. The relation between the output signal of the Aquaflex TR and the weight loss was not linear, but also reproducible and suitable to assess water supply status of plants.

Traditionally in Germany, chopped hop bines are returned to hop gardens after the harvest season in late autumn. However, due to more stringent legal regulations addressing the application of manure, this common practice is banned. Furthermore, the return of hop residues bears phytopathological risks, such as the spreading of Verticillium spores. Aerobic composting of hop bines might solve both problems. Nitrogen is conserved over winter, and pathogens are killed due to high temperatures during composting. However, nitrogen release from mature composts is relatively poor. Thus, in a pot trial, the nitrogen fertilizing effect of aerobically composted hop bines was compared to that of fresh hop bines (air-dried directly after harvest), aged hop bines (stored on a heap for four weeks according to common practice) as well as mature green waste compost and green manure (Phacelia). Lamb's lettuce was cultivated in balcony boxes filled with topsoil used for arable crops. The plant density was 116 plants m^-2, and nitrogen was applied on a basis of 130 kg ha^‑1. Nitrogen from hop bines, green waste compost, and green manure was estimated to become plant available at rates of 100, 75, 50, and 25%, respectively. The control was fertilized with ammonium nitrate. With the exceptions of composted hop bines and green manure, plant growth in organic fertilized treatments was significantly less compared to the control, even if only 25% of the nitrogen was estimated to become plant available. The fertilizing effect of composted hop bines was comparable to that of fresh hop bines and higher than aged ones. However, nitrogen release of all kinds of hop bines was significantly less compared to Phacelia. The results indicate that nitrogen in hop bines can be conserved over winter by aerobic composting, but due to remarkable gaseous N losses during the composting process, the overall efficiency is relatively poor.

Due to high nitrate loads in ground water, the German government tightened up legal regulations addressing the application of green manure crops during the last years. Among others, the current practice of putting back chopped hop bines to hop gardens after harvest was banned. However, there is a lack of knowledge about the nitrogen dynamics of chopped hop bines and their contribution to nitrate leaching. Indeed, pot trials revealed a fertilization effect of chopped hop bines, but in incubation experiments, no increase of mineral nitrogen was found. This might be due to the heterogeneity of hop bines, which consist of N rich, “green” parts (leaves, small branches) as well as the “woody” main stem. To test this hypothesis a pot trial with Italian ryegrass and an incubation experiment were conducted using “green” and “woody” hop material at various ratios. Therefore, entire hop bines were cut off and after removal of the cones divided into leaves, petioles, residues of cones and small side shoots (“green”) as well as into the lignified parts of the stem (“woody”). The two fractions were applied to an arable soil with ratios of 100:0, 70:30, 50:50, 30:70, 10:90 and 0:100 on a mass base, respectively. N uptake of ryegrass as well as net mineralization in unplanted Mitscherlich vessels were measured. Results confirmed the hypothesis: N uptake by plants was significantly higher than net mineralization in unplanted pots. For the woody material a nitrogen immobilization potential of about 4 to 5 g kg^‑1 dry matter was found, whereas for green material a mineral fertilizer equivalent of 18% was calculated. Overall, the results indicate that application of chopped hop bines is not part of the nitrate problem but might be - in combination with catch crops - a good strategy to preserve the bounded nitrogen available for the next growing season.

Due to coarse textured, fast-draining growing media with poor buffer capacity, nitrogen leaching after heavy rainfall is a major concern in container nurseries. The amendment of growing media with Chabazite – a natural zeolite with high ammonium (NH₄⁺) exchange capacity – in combination with NH₄⁺ fertilization might reduce nitrogen losses significantly. This was tested in a pot trial with Photinia fraseri. Growing media amended with Chabazite were compared with a pumice containing growing medium as control. Chabazite was used on the one hand untreated and on the other hand it was charged with ammonium sulfate ((NH₄)₂SO₄) with loads of 1 and 5 g ammonium nitrogen (NH₄-N) pot^‑1_. During the first part of the trial, pots with untreated Chabazite were repeatedly fertilized with NH₄⁺ and leached after each fertilizer application. Pots with charged Chabazite were also leached on the same dates. In the second part, pots with untreated Chabazite received an excessive NH₄⁺ supply and were leached several times afterwards. In both parts of the trial, NH₄⁺ was adsorbed by Chabazite after fertilization. Higher amounts of NH₄⁺ were leached from growing media with pumice than from those with untreated Chabazite. In leachates from growing media with NH₄⁺-charged Chabazite, higher amounts of nitrate (NO₃^-) were only found in the case of charging with 5 g N pot^‑1. Whereas after excessive NH₄⁺ fertilization leaching of NH₄⁺ remained low in presence of Chabazite, summing up to 5% of the applied N, in the control leached NH₄⁺ was 28% of fertilized NH₄⁺. Thus, it can be concluded that fertilized NH₄⁺ was adsorbed to Chabazite and thus protected from direct leaching as well as from nitrification and subsequent leaching as NO₃^-. This protection decreased with increasing NH₄⁺ charging of Chabazite. Hence, the combination of Chabazite with repeated NH₄⁺ fertilization is a promising approach to reduce nitrogen leaching from container nurseries.

Amendment of growing media with Chabazite, a natural zeolite with a high ammonium sorption capacity, in combination with ammonium fertilization might be a promising approach to reduce nitrogen leaching in container nurseries. However, it is unclear if ammonium, which is adsorbed to Chabazite is still plant-available. Thus, a pot trial with Photinia fraseri was conducted with growing media containing 25 vol % of either pumice or Chabazite. Chabazite firstly was used as received and secondly was charged with two levels of ammonium resulting in nitrogen loads of 1 and 5 g N pot^‑1, respectively. The untreated Chabazite was fertilized with 1 g N pot^‑1 as ammonium sulfate divided in repeated applications in accordance with the control (pumice). In these two treatments plant growth and nitrogen uptake was more or less the same. Plants in the growing media with NH₄-charged Chabazite receiving also 1 g N pot^‑1 produced a significantly lower fresh mass compared to the untreated Chabazite and pumice and suffered from slight nitrogen deficiency. If 5 g N pot^‑1 were applied with charged Chabazite, plants grew better and took up more nitrogen. The results indicate that Chabazite did not influence negatively plant availability of fertilized ammonium. However, ammonium from charged Chabazite is only partly plant-available. With increasing NH₄ charging of Chabazite, ammonium and nitrate accumulated in the growing media.

Background

Using secondary phosphorus (P) sources recovered from waste streams for base fertilization of soilless growing media may help conserve the global phosphate resources. However, little is known regarding the plant availability of P from secondary sources for soilless plant production.

Aim

The objective of this study was to investigate the plant availability of P from secondary sources using a peat-based growing medium and standard extraction methods.

Methods

Seventeen secondary P sources differing in terms of feedstock and processing were tested, and their fertilization efficacy was compared with that of water-soluble monocalcium phosphate (MCP-P) in a pot experiment using marigolds. The correlation of plant P uptake with calcium chloride + diethylenetriaminepentaacetate (CAT) and/or calcium–acetate–lactate (CAL) extractable P amount in the growing medium (PCAT and PCAL, respectively) was examined.

Results

Only three of the 17 tested secondary P sources (all struvite forms) exhibited comparable fertilization efficacy to MCP-P. The remaining sources showed P uptake varying between 4% and 91% of MCP-P. For precipitates from liquid waste streams and untreated organic waste, except for sewage sludge and compost, the P uptake ranged from 71% to 100%. The highest variability (4%–86%) was observed for thermally and thermochemically treated secondary P sources. Plant P uptake was closely correlated with PCAT (R2 = 0.76) and PCAL (R2 = 0.78), and this correlation became even stronger when PCAT and PCAL were combined (R2 = 0.89).

Conclusions

Most struvite forms meet the requirements of base P fertilization for soilless plant production and can substitute water-soluble fertilizers obtained from primary P sources. The combination of PCAT and PCAL may be a promising approach to characterize the plant availability of P from secondary sources in soilless growing media and warrants further consideration.

Due to the depletion of global phosphorus deposits and stricter legal regulations regarding phosphorus loads in wastewater, the recovery of phosphorus from wastewater is gaining more and more interest. There are plenty of techniques to recover phosphorus from wastewater and sewage sludge. Depending on the recovery technique and the subsequent processing, the availability of phosphorus to plants can differ widely. However, beside the chemical characteristics also grain size of the fertilizer, as is well known from rock phosphates, might be essential for plant availability. Thus, the plant availability of phosphorus in three wastewater-based fertilizers - sewage sludge ash (SSA), Mephrec and struvite - in relation to their grain size was evaluated in a pot trial with marigold. Recycled P fertilizers were applied to a peat based growing medium on basis of 30 mg P per pot (1) as received and (2) after grinding to a fine powder with a disc mill. For comparison, rock phosphate (Hyperphos) was treated in the same way and water soluble mono calcium phosphate (CaP) was used as benchmark. Marigold seedlings were pricked in 10-cm pots and cultivated for seven weeks according to common horticultural practice in a glass-sheltered greenhouse. With exception for finely ground struvite, P uptake of plants from the recycled fertilizers was significantly less than from CaP. However, P uptake from coarse struvite and powdered SSA was at least 50% of P uptake from CaP, whereas it was less than 5% for the remaining treatments. For all three recycled fertilizers as well as for the used rock phosphate grinding significantly increased P uptake, but only for SSA the effect was of practical relevance. The results reveal that grinding can improve the plant availability of phosphorus in recycled fertilizers, but the effect strongly depends on the chemical characteristic of the product.