Differential responses of grass and a dw

0.25 F *** end of the season. Seasonal changes in N absorption (b)

B × C†

were less obvious ; a small decrease was found at the

0.20 beginning of August, suggesting decreased sink strength for N at that time. The seasonal changes in

the demand of roots for N and P are consistent (negatively correlated) with the soil inorganic nu-

0.10 trient pools observed through the season 3 yr later (Fig. 2), suggesting that the sink strength reflected

0.05 variations in the availability of N and P during the

Leaf P concentration (%) ii

0 growing season.

1.25 C **

(c)

Festuca ovina responded to addition of labile C by an

F ***

1.00 increased root absorption of P in June, and tended to do so in August but not in September (Fig. 6). There

0.75 was no P absorption response to benomyl appli- cation. In August the P demand by

F. ovina roots

0.50 was halved by fertilizer addition, and there was also

a non-significantly ( P ¯ 0.10) decreased absorption

0.25 in September, although the sensitivity of the test was

Leaf K concentration (%)

low due to high variances within treatments.

0.00 Absorption of N by

F. ovina roots after treatments

F *** F **

(d) (d)

(Fig. 6) followed a different pattern to that of the P

1500 1500 B B × × C* C*

demand. Surprisingly, fertilizer addition enhanced

root B B × × F* F*

absorption of N by roots in June, probably because

fertilized plants started growing earlier and therefore had higher demand for N than non-fertilized plants

early in the season. Addition of labile C tended to

15 min

increase the absorption in August but had no effect in early and late season. There was a tendency

P uptake (pg mg

towards an interaction with fertilizer in August

0 0 caused by high absorption in the combined treat-

2 F *** ) (e)

ments. Benomyl caused a decrease in N demand by

©fertilizer d 15 Nª

roots both in June (tendency only) and September.

0 There was no difference in absorption of P between brown (presumably old) and white (more

recently formed) fine roots of

F. ovina. The ab- sorption was 922³204 pg P mg − " brown roots 15

N abundance ( 15 –4

min − ", 1010³182 pg P mg − " white roots 15 min − ", and 941³232 pg P mg − " normal (mixed brown and

Leaf –6

white) roots 15 min − " for roots taken from control

B C BC F BF CF BCF

plots at 4 September. Hence the absorption of P

appears unrelated to root age. Fig. 7. Concentrations in Vaccinium uliginosum leaves of Vaccinium uliginosum showed a similar response to

Non-fertilized

Fertilized

(a) N, (b) P and (c) K, (d) P absorption by excised roots, and (e) leaf "&N natural abundance on 7 August 1994 (year that of

F. ovina in the beginning of August, the only 2). The treatments were : O, control ; B, benomyl ; C, date measurements were performed, with a strong

carbon ; F, fertilizer ; and combination of treatments in a decrease in P demand caused by fertilizer addition, complete factorial design ; n ¯ 6, means³SE. Results of ANOVAs with significant effects of main factors (B, C, F) whereas there were no other main effects on P and their interactions are shown : †, P !0.1 ; *, P !0.05 ; absorption by

V. uliginosum roots (Fig. 7). **, P !0.01 ; ***, P !0.001.

Plant and microbial responses to fertilizer, fungicide and labile carbon 533

Festuca ovina, Year 2

F *** C† C† 7 August

F *** C* 3.0 4 September

Root N concentration (%)

Leaf P concentration (%)

F *** 7 August

F *** C † 3.0 4 September

1.0 Leaf N concentration (%)

Leaf K concentration (%)

B C BC F BF CF BCF Non-fertilized

Fig. 8. Concentrations of (a) root N, (b) leaf N, (c) leaf P and (d) leaf K of Festuca ovina on 29 June (white), 7 August (dark grey) and 4 September (mid-grey) 1994 (year 2). The treatments were : O, control ;

B, benomyl ; C, carbon ; F, fertilizer ; and combination of treatments in a complete factorial design ; n ¯ 6, means³SE. For each of the dates, results of ANOVAs with significant effects of main factors (B, C, F) and their interactions are shown : †, P !0.1 ; * P, !0.05 ; **, P !0.01 ; ***, P !0.001.

spectively), and significantly decreased N in both Tissue nutrient concentrations

pools in August (Fig. 8).

The concentrations of K in leaves of non-fertilized Concentrations of N, P and K in leaves of V.

F. ovina were lower in September than in June and uliginosum were enhanced by addition of fertilizer August, probably due to on average older leaf (Fig. 7), although less strongly than for leaves of F. biomass, leaching losses from yellowing leaves and ovina. The concentrations of P increased pro- translocation to roots. Fertilized

F. ovina showed portionally more than those of K and N. Application peak concentrations of leaf N and K in August of labile C decreased the concentrations of N and K whereas seasonal changes were less for the concen- in leaves of

V. uliginosum.

trations of leaf P and root N (Fig. 8). Root N and leaf N, P and K were all strongly increased by fertilizer application at all sampling "&N of V. uliginosum leaves and added fertilizer times (Fig. 8). The proportional enhancement was Non-fertilized

V. uliginosum had a δ "&N of c. ®5.5^ greatest for leaf K, followed by leaf P, leaf N and root (Fig. 7). Fertilizer application enhanced the value of N. Benomyl tended to enhance the concentration of δ "&N to about ®3.5^, or 2^ higher δ "&N than N in roots sampled in June (Fig. 8), coincident with unfertilized plants, approaching the δ "&N signature low absorption of N in excised roots (Fig. 6). Carbon of the applied fertilizer which was ­0.46³0.06 ^. addition tended to decrease both leaf and root N in The fertilizer NH + -N "&N was determined to June ( P ¯ 0.08 and 0.07, respectively) and leaf K in ®0.92^ and the NO % − -N "&N to ­0.35^ in a August and September ( P ¯ 0.09 and 0.05, re- separate, non-replicated analysis. The variance in $

A. Michelsen et al. plant δ "&N was notably lower in fertilized than in for example, by changes in climate, resulting in

non-fertilized plants. There was no effect of addition time-specific responses to manipulations. of labile C or benomyl (Fig. 7).

Nutrient availability and microbial immobilization 

The seasonally stable pool of inorganic nutrients observed during the growing season (Fig. 2) suggests

Aims that there is no pronounced pulse of inorganic nutrients released from early spring freeze–thaw

The main aim of our manipulations was to in- cycles or from lysed cells of microbes that have died vestigate whether changes in microbial nutrient pool during winter. This is consistent with the moderate sizes in situ would affect the availability of soil fluctuations in the concentrations of soil inorganic nutrients and, in turn, the performance of plants of nutrients observed at a nearby heath and fell field contrasting growth forms. It was possible to change (Michelsen et al., 1996a). However, it is not possible the size of the microbial biomass and its N and P to exclude an early spring pulse of nutrients (Brooks content after 1 (Jonasson et al., 1996a), 2 and 5 yr of et al., 1996) that could have taken place before our treatment, with particular strong effects of fertilizer

first spring sampling.

addition during the first 2 yr and of addition of labile The mean concentrations of NH + and P were

C after 5 yr. Furthermore, as predicted, nutrient %

consistently higher in fertilized plots, and lower in immobilization had different consequences for the plots treated with labile C, both after 2 yr (Fig. 1) cover of the most common graminoid and dwarf and throughout the fifth growing season (Fig. 2), shrub in the community. although these differences were not always stat-

istically significant. However, after 5 yr the con- Carbon and nutrient limitation of microbial growth

centration of NH + in fertilized plots approached that in the controls, suggesting that the N-sink %

The strong increase in microbial C after 5 yr labile C strength of the plant community as a whole was addition (Fig. 3) suggests C limitation of the probably higher at 5 yr than at 2 yr, because after 5 microbial biomass in the longer term. Such C yr all species had had the time needed to respond to limitation of soil microbial biomass is common in fertilizer addition by increased meristem formation. natural ecosystems, although there is also field Hence, in the long term, soil nutrient availability evidence of N limitation, or C and N co-limitation of appeared to be controlled partially by plant-sink microbes (Clein & Schimel, 1995 ; Niklaus & Ko$rner, strength, confirming observations from nearby

1996 ; Zhang & Zak, 1998). In the second year, tundras (Jonasson et al., 1999). However, in cor- nutrient accumulation in the microbial biomass respondence with earlier findings (Chapin et al., occurred after nutrient addition, with no concurrent 1986 ; Nadelhoffer et al., 1992 ; Clein & Schimel, increase in microbial biomass C even when labile C 1995 ; Schmidt et al., 1997a,b), our data suggest that was added. The lack of a response in microbial the availability of soil nutrients is also under biomass C suggests that if there was nutrient or C microbial control. After 5 yr, addition of labile C limitation to microbial biomass formation during the tended to enhance microbial biomass, and decreased second season, this could not be detected at the time the availability of NH + and P more strongly than of soil sampling in late August (Fig. 1). This lack of

after 2 yr.

C limitation on microbial growth after 2 yr might seem surprising, considering that C limitation oc-

Plant-growth responses to soil nutrient enrichment curred both after one season (Jonasson et al., 1996a)

and depletion

and after 5 yr at the same site. However, the rainfall pattern might affect the rate at which microbes We hypothesized that the opportunistic graminoid acquire and metabolize added labile C. In years 1 would be strongly influenced by nutrient limitation and 5, rainfall was very low for the first 2 wk after imposed by microbes, whereas the slower-growing treatment application in July, whereas rainfall was dwarf shrub would be less affected by nutrient higher during this period in year 2, possibly resulting immobilization in saprotrophic microbes. Indeed, F. in faster turnover of added C. In addition, at an ovina responded to fertilizer addition with a strong adjacent heath site, low responses of the microbial increase in cover, and to addition of labile C with a biomass to long-term additions of nutrients and to decrease in cover, after both 2 and 4 yr of treatment warming were probably due to intensified feeding on (Fig. 4), providing the first direct field evidence that bacteria and fungi by nematodes, which are abundant intensified nutrient limitation imposed by in these soils and almost doubled in number in immobilizing microbes can affect the growth of response to these treatments (Ruess et al., 1999). tundra plants. In contrast,

V. uliginosum did not The pool size of microbial biomass C, N and P in show any change in cover, even after 4 yr of addition tundra soils may therefore be strongly influenced by of labile C and fertilizer. Weak responses were, fluctuations in the population size of grazers induced, however, observed in leaves and shoots of V.

Plant and microbial responses to fertilizer, fungicide and labile carbon 535 uliginosum : most pronounced was the increase in earlier observations on tundra graminoids (Chapin &

current-year stem production after fertilizer ad- Bloom, 1976). High root absorption capacity reflects dition, and the opposite response after addition of high above-ground sink strength depleting the roots labile C (Fig. 5). As the most widespread of the few for nutrients, so the increase in N and P absorption opportunistic species with potentially high growth between 7 August and 4 September suggests that the rates in the community, the grass exploited the graminoid had not yet initiated net downward benefits of enhanced resource level, but at the same translocation of N and P by early August. This is in time was very sensitive to a decrease in resource level contrast to graminoids from the middle arctic coastal below that normally found. By contrast, the dwarf tundra, which have a shorter growing season and shrub, the most common plant species in this mixed start net downward translocation of P by late July community, was only slightly sensitive either to (Chapin & Bloom, 1976). increase or decrease in resource levels. This is

In Vaccinium, addition of NPK fertilizer did not consistent with the moderate responses of V. affect cover (Fig. 4), but led to a slight growth uliginosum to 8 yr of fertilizer addition observed in response at shoot level only (Fig. 5) and to ac- nearby non-acidic graminoid tundra (Jonasson, cumulation of N, P and K in leaf tissue (Fig. 7). This 1992), and in dwarf shrub tundra (Graglia et al., promotion of nutrient status was also reflected in the 1997), but differs from the tendency towards a low rate of absorption of P by excised roots. In stronger response in subarctic forest (Press et al., combination with the moderate effects caused by 1998). Our data suggest that this species will respond addition of labile C and the consequent microbial relatively slowly to changes in soil nutrient supply immobilization of nutrients (leading, for example, to driven by environmental changes such as enhanced slight decreases in the concentrations of N and K in summer temperature and CO concentration.

leaves and in current-year stem biomass only), our

data suggest that this species may be more limited by other factors, such as growing-season length and

Seasonal changes in plant nutrient demand : effects of temperature, than by nutrients at this subarctic nutrient enrichment and depletion heath. This behaviour is similar to that of other

The low and similar capacity to absorb P in excised dwarf shrubs without a main arctic–alpine dis- roots of fertilized and non-fertilized

F. ovina early in tribution (Graglia et al., 1997). The independence of the season (Fig. 6), despite the strongly different leaf inorganic nutrient supply may be due to large P concentrations in those treatments (Fig. 8), nutrient stores in the stems which serve as a buffer suggests that available soil P was sufficient to meet against shorter-term fluctuations in the availability

the demand just after the first leaves emerge, except of soil nutrients, and also to the intense colonization after microbial P immobilization when labile C was of roots by ericoid mycorrhizal fungi which promote added. This immobilization led to increased root P nutrient uptake in organic form. That variance in absorption (demand), supporting our hypothesis that leaf "&N abundance was notably higher in non- graminoids are highly sensitive to immobilization of fertilized than in fertilized plants (Fig. 7) might nutrients by microbes. The decreased concentrations suggest that

V. uliginosum is exploiting a range of soil of N in roots and of N and K in leaves throughout N forms (with different δ "&N) in situ. However, the most of the growing season, in response to additions clear shift in leaf "&N abundance of plants from of labile C, also confirm our hypothesis.

fertilized plots towards that of the fertilizer δ "&N The lower absorption rate of P in roots of fertilized demonstrates that

V. uliginosum took up a significant

F. ovina in August and September reflects the low part of its N in inorganic form when fertilizer was demand of plants whose concentration of P in leaves added, despite its ability to acquire organic N had increased three- to four-fold by addition of through the mycorrhizas (Michelsen et al., 1998). fertilizer. In contrast, addition of fertilizer did not Ericoid mycorrhizal colonization was unaffected by decrease the rate of absorption of N, and the fertilizer addition, a response similar to that of concentration of N in shoots increased relatively heather in a temperate heathland (Caporn et al., little, suggesting that the N-sink strength remained 1995). high in fertilized plants, although the concentration of N had increased in the leaves. In the early season,

Effects of benomyl on availability of soil nutrients, nutrient addition even enhanced root N demand, soil microbes, plant performance and mycorrhizal most probably because fertilized plants initiated

function

leafing earlier than non-fertilized plants. The weaker response of N, relative to that of P, following Although benomyl may exert its effect on plants not fertilization was most likely because the N }P ratio of only through depletion of soil pathogenic and the added fertilizer was lower than the tissue N }P mycorrhizal fungi, but also through effects on the ratio.

saprotrophic community (West et al., 1993), its The continued capacity of

F. ovina to take up N effects on microbial biomass and soil nutrients have and P even during early senescence (Fig. 6) confirms rarely been studied. After 2 yr, benomyl amendment

A. Michelsen et al. resulted in approx. 20 % lower concentrations of plication, which suggests that mycorrhizal fungi

microbial C and N in the soil compared to un- were less affected than fungal saprotrophs. Although amended plots, whereas there was no detectable benomyl often decreases arbuscular mycorrhizal effect on N and P availability (Fig. 1). By contrast, function more than its colonization of roots (Larsen after 5 yr the availability of soil NO − and P was et al., 1996), no evidence was found for an impact on

decreased (Fig. 2) while microbial C, N and P were $ ericoid mycorrhizal function. The isotopic com- unaffected (Fig. 3). As the fungicide selectively position of leaf N ( δ "&N) of V. uliginosum was inhibits the growth of some but not all fungi found unchanged by benomyl addition (Fig. 7), whereas in rhizosphere soil (Newsham et al., 1995), shifts the expected response to impaired function of the in the composition of the microbial community associated ericoid mycorrhizal fungi would have are likely. In the fifth season, benomyl addition been a shift towards higher (less negative) plant decreased the rate of mineralization (i.e. microbial δ "&N. This is firstly because less organic N (with low activity) as availability of N and P was lower than δ "&N) and relatively more N (with high δ

"&N) would in plots without added benomyl while microbial

be taken up, and secondly because discrimination biomass C, N and P were unchanged.

against "&N during transfer of N from fungus to plant

The lack of an increase in the concentration of soil would probably decrease if mycorrhizal function was nutrients after two seasons, despite decrease of impaired (Michelsen et al., 1996b, 1998). microbial C and N following addition of benomyl, could be due to plant uptake of nutrients released from dying microbes. This hypothesis is supported  by the decreased absorption of N by excised roots of By adding fertilizer and labile C over 5 yr, we were the non-mycorrhizal

F. ovina in benomyl-treated able to alter the plant and soil microbial nutrient plots in June and August (Fig. 6), and by the demand in a subarctic heath and to confirm that the tendency towards enhanced concentration of N in concentrations of soil inorganic nutrients in heath roots of

F. ovina in June (Fig. 8). Note that direct tundra are under both plant and microbial control. toxic effects of benomyl on plants are unlikely : Paul After 5 yr, the microbial biomass C was enhanced by et al. (1989) did not find any detrimental side effects addition of labile C which led to a non-significant on growth of any of 19 herbaceous wild plants tested. increase in microbial immobilization of N and P, and

Benomyl affected plant P concentration and ab- to a strong decrease of soil inorganic N and P. Together sorption in some instances when labile C was also with the graminoid response showing decreased added, as shown by the significant interaction cover and tissue nutrient concentrations after ad-

between these two factors for absorption of P by F. dition of labile C, this provides the first direct field ovina in June (Fig. 6), and for P concentration and evidence that nutrient limitation imposed by im- absorption in

V. uliginosum in August (Fig. 7). mobilizing microbes can affect the growth of tundra Through its decrease of soil microbial biomass, plants. In contrast to the strong graminoid response, benomyl seemed to relieve the plants from the P the dwarf shrub was only slightly sensitive to changes deficiency imposed by the addition of labile C. For in the resource level. We suggest that the differential the graminoid, there was a main effect on root N responses of the two growth forms are due to absorption and concentration that was independent differences in storage and nutrient uptake pathways, of the addition of labile C.

with the dwarf shrub having large nutrient storage

Our data show that benomyl can affect nutrient capacity and access to organic N forms through its uptake by plants and the soil nutrient source }sink mycorrhizal association. relationship, even in organogenic soils that strongly

The effects of benomyl show (i) that the compound adsorb the compound (Liu & Hsiang, 1994). Effects can decrease soil microbial biomass C and N,

of benomyl on the concentrations of inorganic nutrient mineralization and nutrient availability ; (ii) nutrients in soil have been measured in only a few that plants may respond to those changes by instances (e.g. Fitter & Nichols, 1988 ; Merryweather decreased nutrient absorption and enhanced tissue & Fitter, 1996) and no influence of benomyl on soil concentrations of nutrients without simultaneous nutrients was detected in these studies, perhaps effects on plant cover, shoot and leaf production ; and because the fungicide targeted a microbial biomass (iii) that the compound does not necessarily affect that was presumably much lower than in the system ericoid mycorrhizal function. These findings call for described here. However, consistent with our results cautious interpretation of the effects of fungicides in after two seasons, Jamieson & Killham (1994) natural systems. showed that a decrease of active fungal hyphae by addition of benomyl to pots with intact forest soil profiles led to an increase in N uptake by Sitka  spruce.

We are grateful for the support of the UK NERC Steering The ericoid mycorrhizal colonization of V. Committee of the Stable Isotope Facility at ITE

uliginosum roots was unchanged by benomyl ap- Merlewood. The field work was financed by the Danish

Plant and microbial responses to fertilizer, fungicide and labile carbon 537 Natural Science Research Council, Grant Nos 11–0611–1,

Responses in soil microbes and plants to changed temperature, 11–0421–1 and 95–01046, and the Swedish Environmental

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