Nutrient mobilization and acquisition under aridity

Contrary to high leaching losses that plants have to cope with in the humid-temperate forest, plants in the arid shrublands have to deal with low water availability. Water shortage restricts primary productivity (Gherardi and Sala, 2019; Hsu et al., 2012; Huxman et al., 2004), reduces mineralization of OM-derived nutrients (Austin et al., 2004; Dijkstra et al., 2010), hampers abiotic dissolution of minerals (Belnap, 2011; Maher, 2010), and lowers nutrient mobility in soil (Marschner and Rengel, 2012). Therefore, plants need to optimize their acquisition strategy to increase their nutrient gain without wasting water or nutrients. Under arid conditions, low plant N demand (Figure 2.1-6) and sandy, well aerated soils (Bernhard et al., 2018) likely reduce the need but also the potential for N2 fixation from atmosphere by diazotroph bacteria (Aranibar et al., 2004;

Gallon, 2006; Vance and Heichel, 1991). Therefore, it is not surprising that the abundance of diazotrophic bacteria was markedly lower in the arid shrubland than in the humid-temperate forest (Study 3; Figure 2.3-5). Higher abundance of diazotrophs in rhizosphere than in bulk soil (~10 times) underlined that the C requirements of diazotrophs can hardly be met by litter input but highlights the importance of the rhizosphere as hotspot for initial N acquisition by N fixation (Study 3). The importance of root-derived C for the supply of N fixing bacteria was also shown by an increasing proportion of diazotrophs of the total prokaryotic community with increasing depth (Figure 2.3-5).

The generally low OM input drives plants to efficiently reutilize nutrients, as shown by the strong

15N depletion of shoots (Δ ~3‰) and roots (Δ ~11‰) in comparison to bulk topsoil as well as N and K tracer recovery from topsoil (Study 1; Figure 2.1-2, Figure 2.1-6). The ability of nutrient uptake by plants in the arid shrubland, however, is low as plants invest into conservative root traits to cope with low nutrient and water availability (Study 2; Figure 2.2-2; Reich, 2014). A short expansion and a low surface to volume ratio increase the longevity of roots and saves resources but reduces the ability to take up water and nutrients (Reich, 2014). Lowest enzyme activities under arid conditions across the precipitation gradient (Figure 2.4-3) illustrated that water shortage restricts plant C and nutrient investment into extracellular enzyme production. This contributes to a slow acquisition of OM-bound nutrients. Opposite to plants under humid conditions, plants in the arid shrubland have no need to retain nutrients against leaching. Losses that occur are mainly attributed to erosion caused by extreme rainfall events (Turnbull et al., 2011). Therefore, a slow acquisition is not disadvantageous in terms of nutrient retention.

Nutrient uptake by roots is slow, but the relative importance of the rhizosphere as hotspot of OM input (Figure 2.4-2), microbial abundance (Figure 2.3-4), and enzymatic breakdown (Figure 2.3-3;

Figure 1.3-2) in the remaining water film around roots increased strongly with increasing aridity (Ahmed et al., 2018, 2014; Holz et al., 2018). Especially in the studied dry soils with low aboveground litter input, OM-input from roots can equal or surpass the input from aboveground biomass (Freschet et al., 2013; Wang et al., 2018; Zechmeister-Boltenstern et al., 2015), and serve microbial decomposers and diazotrophs as highly needed C and nutrient resource (Jones et al., 2009; Kuzyakov and Domanski, 2000; Pausch and Kuzyakov, 2018).

Aridity restricts the potential of plants to build up long, thin fine roots for scavenging, but forces them to invest into short, thick roots that sustain under prolonged water-shortage in topsoil (Adams et al., 2013; Comas et al., 2013; Padilla et al., 2013), As drought is not a periodical event but a permanent condition in the arid shrubland, plants need not only to be adapted to survive dry periods but to continue growth under permanent water shortage.By increasing the specific root length with depth (i.e. increasing the fine root proportion or expanding the fine root system; Figure 2.2-2) they

Figure 1.3-2: Activities (Vmax) of chitinases, aminopeptidases, and phosphatases per nmol of SOC in absolute soil depth, corresponding to 0-50%, 50-100% and

>100% solumn depth (sampling set 3). Data are presented as means with standard errors.

can scavenge for deep water resources and avoid drought (Comas et al., 2013; Muñoz et al., 2008;

Sala et al., 2012b).

Scavenging with and maintenance of an (extensive) root system, however, is C costly despite following a conservative resource management, and restricted in such an ecosystem with low plant primary productivity (Brunner et al., 2015; Poorter et al., 2012). Thick roots explore less soil volume and absorb less nutrients, but they have a greater potential of being colonized by AMF (Eissenstat et al., 2015; McCormack and Iversen, 2019). The investment in AMF is an important strategy to scavenge for nutrients when environmental conditions restrict root proliferation. This was indicated by an increase of the AMF extraradical mycelium when the extent of the fine root network decreased (Figure 2.2-5), which highlighted the trade-off of plant C investment between either network (Study 2). Especially the direct connection with organic matter and mineral surfaces that hyphae form, and microbes’ ability to respond to and utilize brief precipitation fast, is likely of particular importance in water limited systems with low nutrient mobility (Austin et al., 2004;

Schwinning and Sala, 2004; Taylor et al., 2009).

Plants provide the extraradical mycelium of AMF with photosynthetic C to scavenge for and transfer P to their host plant (Study 2; Ryan et al., 2012). But contrary to AMF in the humid-temperate forest, AMF seemed not to be involved in plant N acquisition in the arid shrubland (Study 2). Instead, plants at the arid site invested into their fine root system to exploit N-rich patches (Figure 2.2-5; Hodge, 2009; Lynch and Ho, 2005), indicating a functional shift in the role of AMF with increasing aridity (Antunes et al., 2011; Bennett and Classen, 2020; Johnson et al., 2003). The main function of AMF seemed to be the acquisition of P, which was present mainly in inorganic form (Study 5). AMF abundance relative to SOC was as high in the arid shrubland compared to the humid-temperate forest (Study 2; Figure 2.2-3; Table 2.2-2), which might indicate that AMF are involved in the biological weathering of minerals. Since the abiotic dissolution of minerals in the arid shrubland is low (Study 5:), biological weathering can be an important strategy for plants and microbes to cover their demands of rock-born nutrients (Drever and Stillings, 1997;

Maher, 2010; Taylor et al., 2009).

The acquisition of mineral-bound P from subsoil and saprolite by biological weathering was indicated by more plant-released organic acids in the presence of inorganic P species (Figure 2.5-4) and by higher LMWOA:MBC ratios with increasing depth (Table S 2.5-8). Those ratios pointed to an intended excretion of organic acids by plant roots to utilize inorganic P (Study 5). Indications for an ongoing biological weathering by fungi and plants were also given by the high K tracer recovery from subsoil and saprolite (Figure 2.1-2), demonstrating that plants scavenge in subsoil and saprolite to acquire weathering-released K next to the reutilization of K from topsoil (Study

1). The reutilization of OM-derived nutrients is also not restricted to the recycling of litter, but extends to the recycling of belowground biomass, displayed by the equal ¹⁵N recovery from topsoil, subsoil, and saprolite (Study 1; Figure 2.1-2). Plants in the arid shrubland reutilize the organic matter associated with their rhizosphere and explore deeper soil for additional N sources such as microbial necromass. Microbial DNA abundance in saprolite was highest in the arid shrubland compared to the other sites along the gradient (Oeser et al., 2018), which can serve as a valuable resource in this strongly limited ecosystem.