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Observations of plant uptake of organic nitrogen (N), particularly amino acid N, in soils with large amounts of organic N but insufficient net mineralization rates to satisfy the annual N demand (e.g., Schimel and Chapin 1996; Näsholm et al. 1998) have refueled the debate on the competitive interaction between plants and soil microorganisms (Kaye and Hart 1997). As plants from ecosystems with relatively high net N mineralization rates also take up amino acids (Näsholm et al. 2009), it is possible that organic N may be another shared and mutually limiting N source over which plant–microbial competition may occur, especially during periods when inorganic N is scarce.
Competition for N may come out differently depending on the access to various N sources, their concentration in the soil (Bardgett et al. 2003), and other soil conditions, such as spatial distribution of roots and microorganisms (Wang and Bakken 1997; Hodge et al. 2000; Xu et al. 2011), decomposability of the soil C (Månsson et al. 2009), and soil moisture (Schimel et al. 1989; Lipson and Monson 1998). However, little is known about the conditions favoring the uptake of mineral N versus amino acid N in plants and microorganisms. The ratio of glycine-to- uptake varies from 0.2 to 7.7 in microorganisms and from 0.6 to 2.1 in plants, in arctic systems (Schimel and Chapin 1996; Henry and Jefferies 2003) and temperate grasslands (Bardgett et al. 2003). In the latter system, the microbial uptake was dominated by glycine during plant growth in May but shifted to in September, possibly in response to an increased pool of easily decomposable C as plant senescence started. This preference for to glycine in microorganisms was also evident in their response to glucose addition to soil, which increased respiration, at the same time as grasses in the same soil reduced their uptake of glycine more than the uptake of (Dunn et al. 2006). When soil moisture conditions are constant, microorganisms show no uptake discrimination between and amino acids or prefer simple amino acids to , whereas most grass species and shrubs seem to prefer and glycine to more complex amino acids, although species differences exist (Weigelt et al. 2005; Harrison et al. 2007; Sørensen et al. 2008).
Short drying and rewetting cycles occur frequently during a growth period in temperate forests (Ladekarl 1998; Subke et al. 2003). When the soil is drying, bacteria accumulate osmotic active internal solutes, such as free amino acids and their derivates, to maintain the internal water potential in balance with the surrounding environment (Csonka and Hanson 1991). Following rewetting, concentrations of amino acids and mineralized N increase in the soil due to cell lysis (Marumoto et al. 1982; Pulleman and Tietema 1999) and microbial decomposition of soil organic matter (Van Gestel et al. 1993; Appel 1998; Lipson and Monson 1998).
As the soil is rewetted, the efflux of easily decomposable C from roots (Neuman and Römheld 2001), microorganisms (Marumoto et al. 1982), and soil organic matter (Pulleman and Tietema 1999) will increase. This easily available C may fuel fast-growing microorganisms (Bottner 1985; Van Gestel et al. 1993) and increase rates of microbial N mineralization and net immobilization of (Schimel et al. 1989; Pulleman and Tietema 1999; Bengtsson et al. 2003; Kaiser et al. 2011), to the detriment of N uptake in plants. Under constant and high soil moisture conditions, fast-growing microorganisms will be less active (Bottner 1985; Van Gestel et al. 1993) and possibly open a window of competition with plants for limited quantities of mineralized N.
Plants can also respond rapidly to rewetting of a dry soil and restore the and NO3− uptake within hours to a few days after moderate-to-severe drought (Brady et al. 1995; Cui and Caldwell 1997; Buljovcic and Engels 2001). Furthermore, organic N is potentially a more cost-effective N source than inorganic N (Schmidt and Stewart 1999), e.g., by also supplying carbon to a plant that has been energy limited during drought (Raab et al. 1996).
According to the resource competition model (Burkholder 1952), competition for the same and limiting resource is assumed to bear a cost to all competitors. Both plants and microorganisms qualify for part of the definition, as they can use the same sources of N. However, just like removal, introduction or both of one or more potential competitor is a necessary criterion to demonstrate competition (Schoener 1983). Solely relying on uptake data, as in most work on plant-microorganism competition for nitrogen sources so far, is not a proof for plant–microbial competition for N. But uptake data will, in combination with the removal of one or the other group of organisms, be useful in testing the second part of the condition for resource competition, namely the limitation of the availability of N by one potential competitor to the other. Excluding the microorganisms from a competition assay in soil without changing the soil conditions is not an alternative, so we made an effort to exclude the plant and yet provide the same soil conditions as if the plant had been present.
To address the issue of competition for different N forms between plants and microorganisms under varying moisture regimes, we developed two hypotheses to test in this project:
- Microorganisms will take up less in the presence than in the absence of the plants in a constantly moist soil.
- Immediately following rewetting of a dried soil, microorganisms increase their total uptake of N, especially , in response to the increased N pool in the soil, while plants turn to an increased uptake of energy providing N sources, that is, glycine and glutamate.
To test the hypotheses, plant-microbial competition for , glycine and glutamate, which represent less and more complex amino acids, respectively, was studied in a 24 h assay, in which the 15N uptake in the organisms was measured in planted soils and unplanted reference soils that had been either constantly moist or dried and rewetted over a short period of time. Dual labeled amino acids (13C-15N-amino acids) were used to estimate the uptake of intact amino acids by the plants. The soil ATP content was analyzed to estimate the soil microbial biomass and activity.
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Drying and rewetting of a soil is known to induce a burst of mineralization of organic N, resulting in high soil concentrations of (Kieft et al. 1987; Van Gestel et al. 1993; Pulleman and Tietema 1999) and amino acids (Lipson and Monson 1998). Our experiment was no exception. Notwithstanding the fact that the soil concentration increased, there was strong evidence for the drying and rewetting treatment favoring the uptake in microorganisms, especially in the presence of plants. The microbial uptake increased to almost 30% (cf. Figs. 1, 3and Table 3) of the pool at the beginning of the 24-h assay, at the expense of the uptake in the plants, which decreased by 60%. The reduced plant uptake of in the dried and rewetted soil may depend on superior uptake by fast-growing bacteria and microfungi, which have higher surface-to-volume ratios and growth rates than plants, triggered by rhizodeposition and exudates from the plants (Henriksen and Breland 1999; Neuman and Römheld 2001; Kaiser et al. 2011). Those changes in the uptake pattern in combination with the simultaneous shift toward an amino acid dominated N uptake in the plants, supposedly to partly compensate for the negative effects of the increased microbial immobilization, may be taken as a pretext for a temporary competition for under conditions of net mineralization exceeding immobilization.
The microbial uptake was stimulated by the drying–rewetting treatment also in the absence of roots, suggesting that concentrations of easily degradable substrate with high C:N ratios increased when the soil was dried and then moistened, possibly by release of physically protected organic matter (Van Gestel et al. 1993; Pulleman and Tietema 1999) and extraction of C when water was added. This result is consistent with previous studies that also found increased microbial 15 immobilization from 24 h (Pulleman and Tietema 1999; Bengtsson et al. 2003) to 1 week (Schimel et al. 1989) after rewetting a dry soil. The lack of correspondence between variations in microbial N uptake and the ATP content in soil may depend on the insensitiveness of the ATP measurement to subtle variations in microbial activity (Contin et al. 2000). In a study by Raubuch et al. (2002), soil drying and rewetting was found to lower the ATP content in the soil but increase the respiration rates compared with a constantly moist soil, suggesting a decrease in microbial biomass rather than a lowered activity. Compared with our experiment, their soils were dried fast (15 h) and at high temperature (40°C), which may have caused cell lysis and consequently a greater reduction in microbial biomass and ATP content.
Amino acid N is potentially a more cost-effective N source under conditions when the leaf net CO2 uptake decreases because of stomatal closure, for example during drought (Cornic and Massacci 1996; Schmidt and Stewart 1999). Drought causes a decline in the activity of some enzymes, for example nitrate reductases (Cornic and Massacci 1996), and induces others, such as root proteases, facilitating amino acid uptake (Kohli et al. 2012). That may provide an additional explanation to the higher amino acid uptake in the dried–rewetted soil. The plant seemed to have a preference for glycine-to-glutamate, which is in agreement with previous studies on preferences for less complex amino acids (Lipson et al. 1999; Weigelt et al. 2005; Harrison et al. 2007), whereas the microorganisms showed no discrimination between the two. This suggests that F. gigantea used the different amino acids independent of the immobilization by the microorganisms. The short-term uptake of glycine-15N was similar in F. gigantea and grass species from other ecosystems (Henry and Jefferies 2003; Näsholm et al. 2009), but the drying–rewetting effect on the amino acid uptake was opposite to that found in the alpine sedge Kobresia myosuroides (Lipson and Monson 1998). That may have been the result of a longer drying period (45 days) for the soil with K. myosuroides, causing more damage to the roots than in our study.
The lower 13C:15N ratio in F. gigantea in the dried and rewetted treatments compared with the constant moist soil can be explained by either an uptake of mineralized amino acid-15N parallel to the intact amino acids, or by a higher amino acid catabolism and respiration by plants in the dried–rewetted treatment. Under conditions when carbohydrates are in low supply, for example when water is limiting, plants can deaminate glutamate and catabolize glycine and use the C as an energy source (Buchanan et al. 2000). The 15, produced from the deamination, may be used to synthesize new amino acids, while some of the 13C may be lost by respiration, thus reducing the 13C:15N ratio in the plant. The slopes of the regression lines for excess 13C versus excess 15N in the plants were not as steep as the slope of the intact tracer, which suggests that the proportion of 13C-to-15N decreased with the plant uptake of 15N. This would mean that the proportion of mineralized amino acid-15N in relation to 15N from intact amino acids taken up by the plants increased with increasing 15N uptake or, alternatively, that the amino acid catabolism increased with increasing 15N uptake. If , for various reasons, is more easily taken up by plants than amino acids (Nordin et al. 2001), then, the plant uptake of mineralized to intact amino acid 15N would be positively related to the 15N uptake in plants. On the other hand, it is well documented that increased plant nutrient uptake rates also increase the energy demand (Marschner 2002). That would provide an alternative explanation to the lower excess 13C:15N ratios in plants with higher 15N uptake, as amino acid-13C may be used as energy source.
At the oak-dominated site, the in situ gross immobilization rate was autocorrelated within a range of 2.7 m and varied spatially by two orders of magnitude within a 100 m2 plot (Bengtson et al. 2006), suggesting that the small-scale spatial pattern of competitive outcome may be quite heterogeneous. As the half-lives of and common amino acids is less than 24 h even at soil temperatures below 10°C (Henry and Jefferies 2003), the spatial pattern may also have a profound short-term temporal variation. Temporal changes of soil conditions add another dimension of heterogeneity in competition. For instance, short pulses of carbon and nutrients following addition of labile carbon and periods of drying and wetting tend to favor N uptake in microorganisms at the expense of the plant (Dunn et al. 2006; Månsson et al. 2009; this study). This immobilization is probably temporal, as turnover time is short for microorganisms (few days; Schmidt et al. 2007) and dissolved organic carbon, the main source of respired carbon (few hours; Bengtson and Bengtsson 2007). Portions of the mobilized N are suggested to be continuously relocated to the plant in a mainly unidirectional flow and immobilized in roots and aboveground tissues over longer periods (Kuzyakov and Xu 2013).