Nachhaltige Intensivierung der landwirtschaftlichen Produktion: ein Überblick vier verschiedener Bodenzusätze

Lime is used as a soil amendment to counteract acidification, to increase permeability, and to modify aggregate stability. Lime causes clay flocculation and thus improvement of soil structure and hydraulic conductivity. It has beneficial effects on earthworm activity and macroporosity. This has consequences for vegetation cover as well as C and N in soils: because of better crop growth, more crop residues could remain in the soil and enhance soil organic carbon (SOC).

Permeability to allow for water and air flows is an important feature of soils. If soil permeability is impaired, crusts are likely to develop at the soil surface, leading to soil capping, hindering water transport, and indirectly favoring erosion. To improve the permeability of the soils, polyvalent ions such as Ca²⁺, including quicklime/burnt lime (CaO) and finely ground limestone powder (CaCO₃), can be supplied as soil additives (Becher, 2001). How effectively liming material acts is largely determined by how fast the respective amendment can be solubilized (Holland et al., 2018). Besides the material’s reactivity, its particle size, soil temperature, and moisture are important. Quicklime and hydrated lime (Ca(OH)₂) react much faster than limestone because the solubility product is three orders of magnitude larger for CaO and Ca(OH)₂ than for CaCO₃ (Haynes, 2013). While this is advantageous from the ecological point of view, there is an economic drawback to using quicklime and hydrated lime: both are more expensive additives than limestone powder.

Soil acidification can pose a serious threat for agricultural production. Besides wet and dry deposition, ammonium-based fertilizers, urea, and the presence of elemental sulfur contribute substantially to soil acidification. To a lesser extent, root exudates, legume growth, and the mineralization of organic matter can acidify soils (Goulding et al., 2016). Acidification leads to loss of exchangeable cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺), accumulation of H⁺ and Al³⁺, and dissolution of first Mn and later also Fe minerals. As Al³⁺ concentrations rise, the element’s toxicity impairs crop yields. Low pH also hampers phosphorus availability to plants. Lime amendments increase the pH and are widely used to mitigate the effects of soil acidification, diminishing Al³⁺ toxicity and ameliorating phosphorus uptake through increasing mineralization (Holland et al., 2018). Soil structure and organic matter content determine how much of the amendment is needed to mitigate acidification: while soils with high clay content necessitate the application of high amounts of lime, soils with high organic matter content require less lime to counteract acidification. The most common amendments are limestone and dolomite, which not only resupplies Ca²⁺ but also Mg²⁺; less frequently, quicklime, hydrated lime, and natural shell sands are also being applied (Holland et al., 2018).

The short-term effect of liming is suggested to increase soil biological activity, and hence labile C forms, that can be used by microbes (Biasi et al., 2008), or indirectly by changing the community structure (through pH), to groups with lower carbon use efficiency. These respire more CO₂ per unit carbon that they take up (Keiblinger et al., 2010), resulting in higher CO₂ emission rates (Fornara et al., 2011). However, regarding higher soil biological activity upon lime application, it should be noted that especially quicklime can also be used for disinfection because of the rapid pH increase that it induces. This practice is commonly applied, for example, in the treatment of wastewater and biosolids (Smith et al., 1998).

In order to investigate the short- and medium-term effects of lime additives, a greenhouse pot experiment was carried out. While application of 2,000 kg/ha of CaO significantly and instantaneously increased soil aggregate stability, the addition of CaCO₃ failed to affect soil aggregates (Keiblinger et al., 2016). This effect of quicklime was more efficient for soils with high clay content and cation exchange capacity (CEC), making quicklime application a valid option to improve the aggregate stability of fine-textured agricultural soils (Keiblinger et al., 2016). The effects of lime on physical characteristics of soil such as soil aggregate stability, clay dispersion, infiltration rates, water retention, and hydraulic conductivity are discussed controversially in literature. Reported detrimental effects seem to be related to application rates and time: while low application rates initially affected these parameters adversely within the first months after application, positive effects on soil physical properties were observed after half a year (Andry et al., 2009). High application rates were more effective in improving soil physical properties, without initial negative effects (Andry et al., 2009).

The pH increase upon lime application affects soil N transformation rates. In general, liming promotes N fixation by the soil microbial community (Holland et al., 2018). The effects on N mineralization are less clear-cut. N mineralization has been reported to increase, decrease, or remain unchanged (Bailey et al 1995; Kemmitt et al., 2006; Wachendorf et al., 2015) but probably ameliorates with repeated lime applications (Holland et al., 2018); however, if augmented, also availability for plant uptake increases and crop yields can benefit correspondingly, but N losses via nitrate leaching into groundwater can be spurred, especially in the absence of corresponding plant uptake and upon excessive rainfall (Holland et al., 2018). Liming was shown to reduce N₂O emissions, but increased NH₃ and CO₂ emissions counteract this positive effect and render liming questionable as a GHG emission reduction strategy (Sommer and Ersbøll, 1996; Kunhikrishnan et al., 2016; Holland et al., 2018). Lime application can also induce nitrification and thus reduce lime’s buffering effect (Meiwes, 1995); as a consequence, soil nitrate concentrations increase with lime application rates and time (Fuentes et al., 2006). For an overview of the effect of lime amendment to soils, refer to Table 1.

Zeolites chemically belong to the group of alumosilicates and are of natural volcanic origin. They belong to the most common minerals in sedimentary rocks and have a microporous structure. Their internal surface can span several hundred square meters per gram of zeolite (Ramesh and Reddy, 2011). The micropores of zeolites function as a molecular sieve—it can retain molecules that are small enough to enter the pores—and zeolites can act as cation exchangers (Table 1). Zeolites are widely used to purify water, as catalysts and sorbents in the chemical industry, and as soil amendments in agriculture.

The major building blocks of zeolites are crystalline tetrahedrons of [SiO₄]⁵⁻ and [AlO₄]⁴⁻ (Jha and Singh, 2016). The ratio of Al/Si and the resulting charge imbalance in the structure of the respective zeolite determine the ion-exchange capacity and potential acidic sites (Ramesh and Reddy, 2011). Zeolites are classified according to morphological characteristics, chemical composition, crystal structure, effective pore diameter, or natural occurrence. Natural zeolites are often contaminated by other minerals, metals, or quartz, which limits their use to applications that tolerate reduced purity, for example, agricultural use. While synthetic zeolites are of higher purity than natural ones, their elevated price can render their use economically challenging for farmers. Currently, one research focus lies on developing cost-effective and ecologically friendly amendments that can increase crop yield at the same time as reduce nutrient leaching.

Both natural as well as synthetic zeolites are used as soil amendments and can improve soil quality immensely (Jakkular and Wani 2018). The high CEC of zeolites is particularly useful for improving nutrient supply to plants: crop yields should respond positively and nutrient efficiency should be promoted. In addition, zeolites can be used to adsorb toxic metals or pesticides (Eroglu et al., 2017). Zeolites show a two- to threefold higher CEC than other soil minerals. The high CEC of zeolites is crucial for potential reduction of N losses from agricultural fields, because one of the principal N-containing molecules formed after fertilizer hydrolysis in the soil is the positively charged ammonium ion (NH₄⁺). Retarding or preventing the biotical or abiotical transformation of NH₄⁺ into more mobile anionic molecules such as NO₂⁻ and NO₃⁻ or into harmful gaseous species (NH₃, NO_x, and N₂O) is essential for reducing N losses and increasing fertilizer N-use efficiency (Ferretti et al., 2017). The mitigation potential of zeolites for soil NH₃ emissions is known to be very high because NH₃ volatilization is a physical process that can be reduced if NH₄⁺ ions are “physically protected” (e.g., within a mineral lattice). Natural clinoptilolite zeolite as amendment to soils has been described to reduce respiratory activity probably because of the adsorption capacity of zeolites for CO₂ (Muhlbachova and Simon, 2003).

Zeolites do not only optimize the effective nutrient availability or the reduction of nutrient losses but can also improve water use efficiency (Ferretti et al., 2018) in the soil as well as soil ventilation. Zeolites are being developed as fertilizers to amend N forms such as NH₄⁺ or slurry that has previously been incorporated into the zeolite. These fertilizers are suggested to slowly and continuously release N to the plants, thus reducing fertilization costs, with environmental benefits as mentioned above. These charged zeolites have also been reported to decrease soil CO₂ and N₂O emissions but to release more NH₃ and NO_x into the atmosphere than unamended zeolites or soils (Ferretti et al., 2017). If soil was amended with pure zeolite, the emissions of all these gaseous species were reduced. However, considering the N amount applied, there was even a reduction in GHG emissions for the charged zeolite compared to control soil; hence, zeolite can play a valuable role in GHG mitigation and fertilization efficiency (Ferretti et al., 2017). Charged zeolites (that are treated with pig slurry/manure) contain large quantities of N that is readily available to be immobilized into the microbial biomass (Ferretti et al., 2018). In addition, charged-zeolite amended soils show high NO₃⁻ concentrations after a few days of incubation, which indicates the formation via accelerated nitrification. Eventually, oxidative processes are caused by the increased soil porosity and aeration as well as available substrate foster nitrification (Table 1). However, high NO₃⁻ concentrations in soils together with high soil moisture conditions caused by heavy rainfall events or intensive irrigation facilitate N losses from the ecosystem. These losses either occur via NO₃⁻ leaching or as N₂O losses via denitrification under more reductive conditions (Ferretti et al., 2018). In agricultural use, zeolites have been shown to ameliorate water use efficiency as well as water-holding capacity. Infiltration can be improved as the porous structure of zeolites and the resulting capillary suction act as wetting agents. Through decreasing run-off, there is a positive effect on erosion. As water-holding capacity increases, so does the tolerance to desiccation stress (Mumpton, 1985).

The high sorption efficiencies of zeolites also make them potent carriers for herbicides, pesticides, and fungicides (Ramesh and Reddy, 2011). Slow-release synthetic zeolite-bound Zn and Cu fertilizers have been shown to reduce Cd accumulation in crops, highlighting the potential of zeolites to reduce heavy metal contamination (Puschenreiter et al., 2003).

Natural zeolite has been shown to decrease the nitrification due to high sorption of NH₄⁺ (Nguyen and Tanner, 1998, Torma et al., 2014); however, a charged zeolite with high loads of NH₄⁺ has been shown to increase NO₃⁻ concentrations (Ferretti et al., 2018), indicating that the desired nitrification slowing effect of zeolites needs to be carefully calibrated in accordance with slow-release fertilization.

Recently, BC has become another organic amendment of increasing interest also for temperate soils (see Table 1). BC is charcoal that has been produced through pyrolysis from biomass but not from other sources such as landfill wastes or other nonbiological materials. In order to be sustainable, BC production needs to focus on wastes or by-products of other processes such as crop residues, wood chips, cooking wastes, and other similar sources. Feedstocks for BC production should be locally available.

A number of field and pot trials have shown that the unique properties of BCs can enhance the productivity of various crops (Asai et al., 2009; Major et al., 2010; Vaccari et al., 2011) by increasing fertility of the soil. However, the benefits of BC regarding fertilization must be differentiated according to climatic and soil chemical conditions: a meta-study by Jeffery and colleagues has found BC to increase crop yields in tropical soils, on an average, by 25%, whereas there was no such effect in soils in temperate climates (Jeffery et al., 2017a). The study suggests to use BC for fertilization purposes in tropical climates, where soil pH and input use are generally lower than in temperate latitudes, and to concentrate on other positive effects of BC, such as GHG emission reduction and optimizing fertilizer cost, in temperate soils with moderate pH and higher fertilizer abundance.

BC enhances soil fertility by raising soil pH, CEC, and buffer capacity (Lehmann et al., 2003). In addition, BC improves soil physical structure (Chan et al., 2007) and increases soil microbial biomass and nutrient availability (Steinbeiss et al., 2009). BC increases water-holding capacity and water availability (Lehmann et al., 2011) and functions as a microbial habitat (Ameloot et al., 2013), which can further add to amelioration of soil upon BC application. The double face of biochar as potential nutrient supplier, on the one hand, and as competitor for nutrient binding at the biochar surface, on the other hand, make its use challenging. It is necessary to find a balance between binding nutrients to reduce leaching and releasing nutrients slowly to ensure constant fertilization. In addition, BC can bind metals at its surface. This is especially interesting if heavy metal contaminants have to be bound for detoxification purposes (Soja et al., 2018).

BC can sequester carbon and is, therefore, under consideration to mitigate climate change (Lehmann et al., 2011). In addition, several hypotheses could help to investigate and evaluate the potential of BC to reduce GHGs: (i) BC has a porous structure. On the one hand, this could be beneficial. BC could be mixed with fertilizer to develop slow-release fertilizer mixtures, ensuring constant fertilization with less input. Thus resource use efficiency could be increased and input requirements decreased, which would be beneficial from an agronomic as well as from an ecological point of view. Of course this only holds true if the BC is produced locally, whenever possible as a by-product, for example, of cooking (for an article on BC-producing stoves, see Whitman et al., 2009), and if BC production does not compete with other resource uses. Also nitrogen compounds are able to enter the pore. This can decrease the availability of nitrogen for nitrification and denitrification processes by microbial organisms in soil (Van Zwieten et al., 2014). Yield increases upon BC amendment have been reported to derive from a moderate fertilization effect of BC but especially from a reduction in N immobilization (Jeffery et al., 2017b). In addition, the pores provide a habitat for microbial growth and activity that can immobilize available nutrients; (ii) BC can increase the pH of acidic soils and this allows denitrifying microorganisms (Wang et al., 2013; Harter et al., 2014; Van Zwieten et al., 2014) to promote complete denitrification (Cayuela et al., 2013, 2014). The reduction of N₂O losses with BC application has been attributed to the pathway of total denitrification to N₂ (Cayuela et al., 2013). While complete denitrification to N₂ would reduce N₂O emissions, this reduction is beneficial from an ecological perspective, but the still occurring N₂ loss would be detrimental from the economic perspective. In order to be sustainable, BC amendments and use would have to be optimized to balance these effects.

An increased soil pH might also lead to more ammonia release into the atmosphere, thereby contributing to agricultural air pollution, (iii) depending on soil texture, BC can reduce the soil density and improve its porosity (Quin et al., 2014), reduce the anaerobic microsites for denitrification processes and improve oxidative processes such as microbial CH ₄ consumption (Schimmelpfennig et al., 2014). BC can mitigate methane emissions from agricultural activity; however, this effect depends on the management scheme and the pH of the respective soil. While BC application reduces methane emissions of acidic soils and flooded or periodically flooded land such as rice paddies, no such effect could be found for land that exhibits more neutral or alkaline conditions and that is not flooded during crop cultivation (Jeffery et al., 2016). Under these conditions, a reduction in methane oxidation potential was described for arable upland soils. (iv) GHGs such as N₂O and CH₄ were shown to be able to bind to the surface of BCs (Van Zwieten et al., 2015).

While lime, zeolites, and BC influence physical as well as chemical characteristics of soil, the last amendment that shall be discussed in the present article works exclusively on a chemical/enzymatic basis. NIs affect the biological reactions that regulate the conversion of ammonium to nitrate. Several natural and synthetic NIs exist, but only a small number of biological NIs has been identified and characterized in detail. Biological NIs are secreted by plants in the rhizosphere to improve N use efficiency (Subbarao et al., 2015). The best-known synthetic NIs on the market are dicyandiamide (DCD) and 3,4-dimethyl-1H-pyrazole phosphate (DMPP). Nitrapyrin is less common than DCD or DMPP. A major drawback of NIs is that they are biodegradable and thus need to be reapplied in regular intervals. Additionally, their action is limited to the sites of nitrification, further adding to their cost ineffectiveness. However, N losses via nitrification are not only of agronomic value, but they also contribute to environmental pollution. The inhibition of N losses in form of N₂O emissions as well as of NO₃⁻ leaching is influenced by several parameters, such as soil texture, temperature, and water abundance by precipitation or irrigation (Misslebrooket al., 2014) and this complexity keeps efficient NI use from being straightforward. Limited availability and potential negative effects on beneficial soil microbes further add to complexity in NIs application.

DMPP inhibits the conversion of NH₄⁺ to NO₃⁻ by microbial ammonium monooxygenases by competing with the substrate for binding to the enzymes’ active sites (Marsden et al., 2016). DCD, on the other hand, is believed to act as copper chelator of the enzymes active in ammonia oxidation or impairs the uptake or use of ammonia (Chen et al., 2015). Their different modes of action are likely responsible for the discriminate efficiencies of inhibition. DMPP shows a similar or even better inhibitory effect at lower application rates (with about 1/25 to 1/2 the amount compared to DCD, Weiske et al., 2001). While a recent meta-study of both inhibitors shows that DMPP is in some cases not as efficient as DCD, as indicated by higher NH₄⁺ leaching, methane emissions, and crop yields (Yang et al., 2016), it is worthwhile to note that DCD as NIs has been withdrawn from the market in New Zealand after traces were found in milk (The Australian Dairy Farmer, 2013). These contaminations may be due to DCDs’ high water solubility and mobility in soil and subsequent uptake by plants and propagation within the food chain (Zerulla et al., 2001). While the concentrations that were found were considerably lower than the recommended acceptable daily intake that European regulations allow and can be deemed safe, such findings surely hamper consumer acceptance.

A study conducted in the United Kingdom found only a reduction in N₂O levels, but no yield increase or reduction in NO₃⁻ leaching (Misslebrook et al., 2014). Corroborating, a review of sites in Germany could not find significantly higher yields with NIs (Hu et al., 2014), except for one site with higher precipitation: here yields rose and nitrate leaching decreased upon NIs application. Similar to other amendments, the action of NIs also appears to depend on environmental conditions. Hu and colleagues further reported that NIs application could help to maintain high yields while reducing fertilizer. In addition, it allows for a larger flexibility of fertilizer application without N losses. This can be especially advantageous as climate change will be accompanied by more frequent droughts and heavy precipitation events (Hu et al., 2014).

There is potential for NIs application in combination with fertilizers and also with other soil amendments such as BC. While BC can bind gases such as N₂O and CH₄, also higher N₂O emissions were found after the application of BC to soils. This is suggested to result from biotic processes of ammonia oxidation and nitrifier denitrification (Sánchez-García et al., 2014). Higher gross nitrification rates in BC-amended agricultural soils (Prommer et al., 2014) as well as higher abundances of ammonia-oxidizing communities support the nitrification-based increase in N₂O emission rates. The liming effect of BC can also provide favorable condition for soil nitrifiers (Prosser and Nicol, 2012). The use of NIs can overcome the problem of elevated N₂O emissions by BC application with the beneficial side effect of reducing NO₃⁻ leaching potential.

BCs sorption capacity and, hence, the potential efficiency of NIs resulted in greater sorption of DMPP for BC pyrolyzed at lower temperature. This is likely to be related to hydrophobic interactions, as hydrophobicity generally decreases with increasing pyrolysis temperature (Zornoza et al., 2016). The increased sorption of DMPP after BC application may be caused by the hydrophobicity of the BC samples (Keiblinger et al., 2018). Also the organic C (OC) content is influenced by pyrolysis temperature, and sorption and OC were also positively correlated.

Further research is needed to determine economically viable conditions for NIs and compare to alternative strategies for improving fertilizer and water use efficiency. This will allow to minimize costs while maximizing positive effects on food security and reducing detrimental environmental effects.