3 Results > Discussion
3.1.6 Assessment of Grassland Stands
3.1.6.1 Water and Nutrient Use Efficiencies
In 2002, the slight differences in ET of our grassland stands (Table 50) were not reflected in mean water use efficiency based on accumulated biomass (WUEbm). It ranged slightly from 2.4 to 2.6 g dm l watertransp-1. No effects of functional diversity were found for WUEbm of our grassland stands.
Table 50 Mean water- and nutrient use efficiencies for aboveground biomass in experimental grassland stands I-V in 2002
Significant distinctions between stands are indicated by different letters (one way-ANOVA: dF = 5; Tukey HSD-Test; stand I-V n = 5)
^ raw data: NEßHÖVER >BEIERKUHNLEIN, unpublished
The mean nitrogen use efficiency based on accumulated aboveground biomass (NUEbm) ranged slightly from 66 to 71 g dm g Naccum-1. I (H. lanatus + A. elatius), II (H. lanatus) and V (H. lanatus + P.
lanceolata) showed tendentiously higher NUEbm than other stands. Stand III and IV (H. lanatus + P.
lanceolata) showed lower grass contribution (Appendix, Table XIV). N contents in grass biomass (1.4 %) were significantly lower than in herb biomass (1.7 %; one way-ANOVA: dF = 1, F = 66.25, p ‹ 0.001;
Tukey HSD ***). Since the dominant herb species P. lanceolata only showed slightly higher N contents (1.5 %; one way-ANOVA: dF = 7; F = 19.36, p ‹ 0.001, Tukey HSD ns), differences in NUEbm
between stands were only tendentious. Higher NUEbm in grass dominated stands were reflected in tendentiously lower Nstand.
Parameter I II III IV V Tukey HSD F P WUEbm^
[g dm l watertransp-1] 2.5 2.5 2.5 2.6 2.4 ns 1.54 0.229
NUEbm^
[g dm g Naccum-1] 72 71 66 67 70 ns 1.38 0.275
KUEbm^
[g dm g Kaccum -1] 42a 40a 34b 34b 37a *** 5.96 0.002
MgUEbm^
[g dm g Mgaccum-1] 790a 770ab 670b 704ab 741ab * 3.14 0.028
CaUEbm^
[g dm g Caaccum -1] 335a 327a 154b 173b 184b *** 115.05 0.000
The mean K use efficiency (KUEbm) ranged from 34 to 42 g dm g Kaccum-1. Stand I, II and V showed significantly higher KUEbm than any other stand. This effect could not be attributed to functional diversity. Grasses showed significantly lower K contents (23.8 mg K g dm-1) as herb species (34.9 mg K g dm-1; one way-ANOVA: dF = 1, F = 82.14, p ‹ 0.001; Tukey HSD ***). For P.
lanceolata, K contents (32.5 mg g-1) were significantly higher than for H. lanatus and A. elatius (25.1/22.6 mg K g-1, one way-ANOVA: dF = 7, F = 40.30, p ‹ 0.001; Tukey HSD ***).
The mean Mg use efficiency (MgUEbm) of our grassland stands ranged from 670 to 790 g dm g Mgaccum-1. Stand I showed a significantly higher and stand II and V tendentiously higher MgUEbm than any other stand.Stand III had a significantly lower MgUEbm. Grasses showed sig-nificantly lower Mg contents (1.2 g Mg g dm-1) than herbs 2.5 mg Mg g dm-1; Appendix, Table XVI).
For H. lanatus and A. elatius (1.2 / 1.1 mg Mg g dm-1), the Mg contents were significantly lower than compared to P. lanceolata, Mg contents (1.7 mg Mg g dm-1; one way-ANOVA: dF = 7, F = 65.16, p ‹ 0.001; Tukey HSD ***).
The mean Ca use efficiency (CaUEbm) of our grassland stands ranged from 154 to 192 g dm Caaccum-1. Grass dominated stands I and II showed significantly higher CaUEbm than any other stand. This finding was reflected in herb species showing significantly higher Ca contents (10.2 mg Ca g-1 ) than grass species (2.9 mg Ca g dm -1; Appendix, Table XVI). For P. lanceolata, Ca contents (9.1 mg Ca g dm -1) were significantly higher than for any other species (one way-ANOVA: dF = 7, F
= 65.16, p ‹ 0.001; Tukey HSD ***).
Due to lower base cation contents in aboveground biomass, grass dominated stands had signifi-cantly higher base cation use efficiencies and Kstand, Mgstand and Castand. Hence, herb contribution to stand biomass was a factor determining KUEbm, MgUEbm and CaUEbm of our grassland stands in 2002.
The mean WUEbm of our grassland stands decreased significantly from 2002 to 2003 by 50%
(Appendix, Table VIII). This finding is likely due to the hot and dry summer 2003. Luxury consumption of water due to irrigation, likely physiological mechanisms for enhanced WUEbm made unneces-sary grassland plants. No implications of functional diversity on mean WUEbm (Table 51) could be found. Stand I (A. elatius + H. lanatus) and II (H. lanatus + G. pratense) had significantly higher WUEbm than stand V (P. lanceolata + A. elatius + T. officinale).
Table 51 Mean water- and nutrient use efficiencies for aboveground biomass in experimental grassland stands I-V in 2003
Significant distinctions between stands are indicated by different letters (one way-ANOVA: dF = 5; Tukey HSD^ Test; stand I-V n = 5)
raw data: TÜNTE >BEIERKUHNLEIN, unpublished
However, OPITZ VON BOBERFELD (1994) reported enhanced WUEbm at higher levels of fertiliza-tion (10 g N m-2 yr-1) for A. elatius stands. SCHINDLER ET AL. (2001) confirmed enhanced WUEbm at lower N fertilisation (4-6 g N m-2 yr-1) for Hordeum vulgare (Barley). In contrast to these findings, TSIALTAS ET AL. (2001) reported of decreased WUE for grass and herb species with increased N contents in leaf biomass.
LINDHAUER (1983) found higher water use efficiency of Hordeum vulgare seedlings due to higher K+ supply. High availability of K+ in soil solution may also have caused a higher WUEbm. BERGMANN (1992) reported a higher capacity of grass species for K acquisition due to their homorhizal root system. No correlations were found between NUEbm, KUEbm and WUEbm. Grass species might maintain enhanced WUEbm at fairly increased K acquisition.
The mean NUEbm increased tendentiously from 2002 to 2003 by 4 % (Appendix, Table VIII). Better supply with base cations might have affected NUEbm to some extent. The mean NUEbm in our grassland stands ranged from 64 to 77 g dm g Naccum-1. In 2003 stand II showed tendentiously lower NUEbm than any other grassland stand. This finding was well reflected in higher Nstand. Due to higher biomass yields, stand I had highest Nstand.
Parameter I II III IV V Tukey HSD F p WUEbm^
[g dm l watertransp-1] 1.7 a 1.5 a 1.1 b 1.0 b 1.0 b *** 28.44 0.000
NUEbm^
[g dm l g Naccum-1] 77 64 76 72 72 Ns 2.68 0.061
KUEbm^
[g dm g Kaccum-1] 43 a 37 b 35 b 37 b 35 b ** 11.80 0.000
MgUEbm^
[g dm g Mgaccum-1] 769 a 464 b 468 b 504 b 488 b *** 48.55 0.000
CaUEbm^
[g dm g Caaccum-1] 306 a 131 b 91 c 111 bc 110 bc *** 137.99 0.000
Herb species showed slightly higher N contents (1.6 %) than grass species (1.4 %; Appendix; Table XVI). General correlations between herb contribution and NUEbm could not be found. The vari-ability of N contents of G. pratense in different grassland stands was low. It showed only tenden-tiously higher contents in stand II than in stand IV and V (one way-ANOVA: dF = 1, F = 1.06, p › 0.05).
H. lanatus only had tendentiously higher N contents in stand I (one way-ANOVA: dF = 1, F = 3.15, p › 0.05). Hence, lower NUEbm of the species contributing to stand II seemed to be plant characteris-tics for both species and not strictly dependent on increased N availability of stand II.
The mean KUEbm of our grassland stands ranged from 35 to 43 g dm g Kaccum-1. Stand I showed significantly higher KUEbm than any other grassland stand. In 2003, grass species showed only tendentiously lower K contents than herb species (Appendix, Table XVI). However, Stand I dominating A. elatius showed significantly lower K contents (22.7 mg K g dm-1) than P. lanceolata and T.
officinale (27.6 / 50.6 mg K g-1; one way-ANOVA: dF = 7, F = 83.05, p ‹ 0.001; Tukey HSD ***). Due to higher biomass production, stand I showed similar Kstand than the other stands.
The mean MgUEbm of our grassland standsranged considerably from 488 to 769 g dm g Mgaccum -1. The range of MgUEbm was lower in 2002 than in 2003. Differences in MgUEbm likely occur more pronounced at higher Mg2+ concentrations in soil solution. (Appendix, Table XIII). In 2003, grass dominated stand I showed higher MgUEbm. Herbs had significantly higher Mg contents (2.6 mg Mg g dm-1) than grass species (1.4 mg Mg g dm-1; Appendix Table XVI).
The mean CaUEbm of our grassland stands ranged in 2003 from 40 to 138 g dm g Caaccum-1. It was considerably lower than in 2002. Analogous to K and Mg, higher soil solution concentra-tions of Ca in 2003 led to decreased use efficiencies of Ca.
Stand I showed a significantly higher CaUEbm for both years, whereas stand II showed a only tendentiously higher CaUEbm in 2003.In 2003,grass species showed significantly lower Ca con-tents (4.2 mg Ca g dm-1) than herbs (12.0 mg Ca g dm-1; Appendix, Table XIII).Hence, higher availabil-ity of Mg and Ca in soil solution reflected differences in physiological characteristics of plants more pronounced than other nutrients.
Water- and Nutrient Efficiencies in Grassland Stands 2002 / 2003
In 2003, WUEbm was two times lower than in 2002 (Appendix, Table XIV). Due to the dry and hot summer in 2003, stands were irrigated, which in turn led to higher ET, because plants were not forced to save water. In temperate climate, WUEbm for grasslands can range from 1.3 to 4.7 g dm l watertransp-1 at high precipitation (FIELD ET AL., 1997; ARP ET AL., 1998; LUCERO ET AL., 2000).
NELSONET AL. (2004) reported of WUEbm ranging from 2 to 7 g dm l watertranspir-1 at low precipi-tation (320 l m-2 a-1). WUEbm of our grassland stands in was rated low for 2002 and 2003.
Higher WUEbm in stand I (A. elatius + H. lanatus) and II (H. lanatus + G. pratense) between grassland stands could be explained by different factors. The contribution of A. elatius increased in stand I from 2002 to 2003 from 11 to 83 % of stand aboveground biomass (Appendix, Table X). Due to its enhanced resilience towards water stress (ELLENBERG, 1991) A. elatius might have enhanced WUEbm to some extent.
TSIALTAS ET AL. (2001) found higher WUEbm for grass species compared to herb species. They also underpinned the importance of the physiological traits in concern of species abundances in grasslands under water limitation. Significant negative correlations between herb biomass and WUEbm were also found for 2003. Since correlation was based on the presence of stand I and II, mere coinciding of herb contribution and WUEbm must be assumed.
EBDON ET AL. (1998) showed the importance of canopy temperature for transpiration and thus for WUEbm. Since H. lanatus is adapted to better water supply (ELLENBERG, 1991), it may provide lower canopy temperatures for A. elatius by enhanced transpiration as suggested for Trifolium repens and Lolium perenne (HOGH-JENSEN >SCHJOERRING, 1997). Besides modification in can-opy temperature and moisture, H. lanatus may have profited from shading provided by broad leaves of G. pratense in stand II.
Increased WUEbm in stand I and II may have been due to slightly higher availability of N in these stands.SCHINDLER ET AL. (2001) reported of increased WUEbm due to slightly higher N availabil-ity for Hordeum vulgare. In contrast to this, TSIALTAS ET AL. (2001) showed that WUE of grass-land plants under water limitation is negatively correlated with N contents in aboveground bio-mass. The correlations between higher N availability and WUEbm were dependent on the pres-ence of stand I and II.
Since correlation between NUEbm and WUEbm were also not found, a mere coinciding between the high N availability in soil solution and WUEbm in our grassland stands is likely. In agreement with TSIALTAS ET AL. (2001) WUEbm was identified as important factor determining biomass production (Figure 39). The correlation was highly significant for both years, even when stand I and II were excluded from analysis.
Negative correlations between herb contribution and WUEbm were likely biased by higher H.
lanatus detritus inputs in stand I (A. elatius + H. lanatus, zI) and II (H. lanatus + G, pratense, z II). Due to the lacking of permanent monocultures, single plant species or functional groups with higher WUEbm could not be identified.
High K concentrations in soil solution likely enhance WUEbm (BERGMANN, 1992). Significant correlation between soil solution concentration of K+ or KUEbm and WUEbm could not be found for both years.
Figure 39 Correlation between water use efficiency and above-ground biomass yield of ex-perimental grassland stands in 2002 / 2003
I
I
I I
I
II II
II II II
0,5 1,0 1,5 2,0 2,5 3,0
Wate r U se Efficie ncy [g dm l watertransp.-1]
400 600 800 1000 1200
Aboveground Biomass Yield [g dm m-2 yr-1 ]
WUEbm:above bm: r2 = 0.87; p = 0.000;
y = 320.67 + 264.17*x 2002
2003
SCHILS ET AL. (1999) gave NUEbm ranging from 37 to 49 g dm g Naccum-1. Hordeum vulgare showed NUEbm from 30 to 49 g dm g Naccum-1 (GORNY >SAKIEWICZ, 2001; DAEPPET AL., 2001).
Considering these values, NUEbm for all stands were regarded as very high for both years.
Correlations between herb contribution and NUEbm were not found. The biomass production seemed to be rather independent of NUEbm.
KOUTROUBAS ET AL. (2000) reported of KUEbm ranging from 66 to 180 g dm g Kaccum-1 under laboratory conditions. In dependence on high or low availability, P. lanceolata showed a range of 69 to 153 g dm g Kaccum-1. MARSCHNER (2002) gave values of optimal nutrient contents for Lolium perenne. Deduced KUEbm ranged from 29 to 40 g dm g Kaccum-1. Regarding these values, KUEbm of our grassland stands were medium to high for both years.
Deduced MgUEbm from values given by MARSCHNER (2002) ranged from 200 to 500 g dm g Mgaccum-1
. CaUEbm of Lolium perenne at optimal nutrition can range from 83 to 167 g dm g Ca ac-cum-1 (deduced after MARSCHNER, 2002). Regarding these values, MgUEbm and CaUEbm of our grassland stands were rated very high for both years.
KUEbm, MgUEbm and CaUEbm decreased significantly with herb contribution in our grassland stands (Figure 40). This finding is likely explained by lower base cation demands of grass species (MARSCHNER, 2002). Significant correlations between soil solution concentrations and use effi-ciencies of Mg and Ca were identified as coinciding due to detritus inputs in stand I and II in 2003.
Figure 40 Correlation between herb contribution in above-ground biomass of experimental grassland stands and nutrient use efficiencies for K, Mg, Ca in 2002 / 2003
0 20 40 60 80 100
Herb Contribution [%]
0 200 400 600 800 1000
Base Cation Use Efficiency [g dm g cationaccum.-1 ]
He rb [%]:
2002 2003 KUEbm: r2 = 0.40; p = 0.000; y = 40.64 - 0.07*x 2002 2003 MgUEbm: r2 = 0.59; p = 0.000; y = 777.63 - 3.31*x 2002 2003 CaUEbm: r2 = 0.89; p = 0.000; y = 304.72 - 2.65*x
DACCORD ET AL. (2001) confirmed considerably lower K and Ca contents and tendentiously lower Mg contents of grass in comparison to herb species. Hence lower use efficiencies of K, Ca and for some extent of Mg for herb species compared to grass species are indicated for European grasslands. Deduced values gave for herbs mean use efficiencies for K 22, Mg 312 and for Ca of 73 g dm base cation accum-1. In contrast to this, grass species showed for K 31, Mg 588 and for Ca 270 g dm g base cationsaccum-1. The findings in our grasslands highly agree with values given by DACCORD ET AL. (2001) for K and Ca, whereas the MgUEbm of the experimental grasslands was considerably higher. MgUEbm for herb species exceeded the given values by the two-fold and for grass species only slightly. These differences may be due to differences in species traits, but they also hint at low Mg supply in our grassland stands.
Neither KUEbm, nor MgUEbm or CaUEbm showed significant correlation to aboveground biomass production. A dilution of base cation contents due to enhanced growth could not be found.