The impact of farming practice on soil microorganisms and arbuscular mycorrhizal fungi: Crop type versus long-term mineral and organic fertilization
Introduction
Sustainable agriculture preferably makes use of organic fertilization to improve soil biological properties but the impact of crop rotation and crop identity is often neglected. It is known that fertilizer amendments affect soil microbial community composition (Gunapala and Scow, 1998) with long-term organic management generally resulting in greater soil microbial biomass and diversity compared to conventional mineral fertilization (Toyota and Kuninaga, 2006, Esperschütz et al., 2007). Moreover, microbial biomass C and N were greater in plots amended with organic compared to mineral fertilizer, which in turn affects nutrient availability to plants (Carpenter-Boggs et al., 2000, Fließbach and Mäder, 2000). Microbes also play an important role in nutrient mineralization and the above processes were shown to improve plant performance (Forge et al., 2003, Böhme et al., 2005). In particular, the development of arbuscular mycorrhizal (AM) fungi which is the dominant mycorrhizal symbiosis in agroecosystems is stimulated by manure amendments (Ryan et al., 1994, Miller and Jackson, 1998). Furthermore, organic agriculture supports greater AM fungal root colonization, density and diversity, which in turn enhances the growth of host plants (Mäder et al., 2000, Oehl et al., 2004).
Besides fertilizer amendments, the performance of field crops can be affected by the previous crop cultivated at a field site (Karlen et al., 1994). This is predominantly mediated by physiological mechanisms such as water and nutrient use by plants, and morphological differences in root architecture associated with changes in microbial community structure (Smalla et al., 2001, Garbeva et al., 2008). In particular, the occurrence of AM fungi plays an important role in the effects of crop rotation (Arihara and Karasawa, 2000, Hijri et al., 2006). The presence of host plants increases the density and diversity of AM fungi, while non-host plants decreased spore density (Eom et al., 2000, Karasawa et al., 2002). Bacterial and AM fungal communities are inter-connected in soils since specific bacterial strains promote germination of AM fungal spores and can increase the rate and extent of root colonization (Johansson et al., 2004). This was shown for populations of bacteria functional groups in the rhizosphere of mycorrhizal and non-mycorrhizal sweet corn, subterranean clover and guinea grass (Meyer and Linderman, 1986, Secilia and Bagyaraj, 1987).
Soil microbial biomass and community structure were analyzed via soil phospholipid fatty acids (PLFAs) as essential structural components of living cell membranes. PLFAs have been used to analyze soil microbial communities and fatty acid (FA) biomarkers have been assigned for major bacterial and fungal groups (Frostegärd and Bääth, 1996, Zelles, 1999). Several studies have shown that soil PLFA patterns reflect changes in the community composition of microorganisms in response to agricultural practice (Gunapala and Scow, 1998, Carpenter-Boggs et al., 2000). Biochemical markers have also been used to detect AM fungi in soil with the marker 16:1ω5 PLFAs as a measure for viable fungal hyphae biomass while its occurrence in neutral lipids (NLFAs) resembles fungal spores (Graham et al., 1995, Olsson, 1999, Olsson and Johansen, 2000).
Most studies on inorganic and organic fertilization practices are based on short-term field experiments and little is known about the impact of continuous amendments. Moreover, in many cases several factors such as crop rotation, crop varieties and tillage or plant protection varied. The long-term mineral and organic fertilization system implemented at the Institute for Biodynamic Research (IBDF) in Darmstadt, Germany does not have these restrictions. This allows for the examination of agricultural management practice in a replicated design with no differences except in fertilization (Bachinger, 1996, Raupp and Oltmanns, 2006). Since 27 years, the experimental field is amended either with mineral fertilizer or cattle manure with/without biodynamic preparations. The application rates are low, medium and high amounts, with the medium rate representing common agricultural practice. Two vegetative periods were investigated, firstly with cultivation of mycorrhizal host plant wheat and secondly following a switch to the non-mycorrhizal host plant amaranth. The aims were to assess how microbial communities have developed in the long-term under farming systems with different fertilizer types and amounts, and relate this to short-term effects of preceding mycorrhizal/non-mycorrhizal crops. A special focus was placed on the plant symbiotic AM fungi due to their function in plant nutrition. Comparing 27 years of fertilizer amendment with crop plant identity will enhance knowledge on the impact of farm management practice on soil inhabiting microbial communities and hence nutrient dynamics in the light of sustainable agriculture.
Section snippets
Experimental design
The study was performed at a long-term experimental field site established in 1980 at the Institute for Biodynamic Research (IBDF) Darmstadt, Germany. The site is located at 49° N/8° E and 100 m above sea level, with 9.5 °C annual mean air temperature and 590 mm annual precipitation. The soil type is a haplic cambisol comprising 87% sand, 8% silt and 5% clay in the topsoil. The long-term experiment compares fertilizer amendments with mineral (MIN), cattle manure (CM), and cattle manure with
Microbial biomass
At the wheat plots, total amount of soil PLFAs as a measure for microbial biomass ranged between 10–26 and 5–15 nmol g−1 dry wt soil in 0–5 and 5–10 cm depths, respectively (Fig. 1a,b). Compared to wheat, soil PLFAs under amaranth as crop indicated a slightly higher microbial biomass in 0–5 cm depth (17–33 nmol g−1 dry wt soil) and more pronounced in 5–10 cm depth (13–18 nmol PLFA g−1 dry wt soil). A total of 14 predominant FAs were detected at the different sites with a chain length ranging between 14
Microbial biomass
In agricultural practice, organic and inorganic fertilizers are primarily used to enhance nutrient availability to crops but they also affect microbial communities in the soil (Marschner et al., 2003). In the present investigation, an increase in microbial biomass (i.e. total amount of soil PLFAs) was observed for wheat and amaranth crops following manure application. Similarly, an earlier study at the same site by Bachinger (1996) reported 15% and 31% more Corg content at CM and CMBD plots,
Acknowledgements
We extend gratitude to the Leibniz Society, Brandenburg and Thüringen counties Germany for sponsoring the project. We are also grateful to M. Oltmanns (IBDF Darmstadt) for her assistance during field sampling and to S. Jeserigk (IGZ Großbeeren).
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