Rhizodeposit Solution Characteristics

pH in Rhizodeposit Solution

The median pH in rhizodeposit solution obtained from swards containing H. lanatus (not shown) ranged slightly from pH 7.1 to 7.3. Significant differences between stands were not found. The pH in rhizodeposit solution of swards containing P. lanceolata (not shown) ranged from pH 6.6 to 7.1. Dicultures with grass species H. lanatus and A. odoratum showed a significantly higher pH.

The distinction between stands with grass species and pure herb cultures merely accounted for a total difference of 0.2 µM H⁺ l^-1.

3.2.2.1 DOC Concentration

Prior to 1^st harvest, the median DOC concentration in rhizodeposit solution of swards containing H. lanatus (Table 68) ranged from 15.2 to 29.7 mg C l^-1. The predominant amount of DOC was likely derived from degradation of the organic growth medium. No significant differences were found for H. lanatus swards.

Table 68 Median (25^th/75^th Percentile) DOC concentration in rhizodeposit solution of H. lanatus swards without Fe supply prior to and after 1^st harvest

Significant distinctions between swards are indicated by different letters (Kruskal-Wallis-ANOVA; Mann-Whitney U-Test; stand treatment n = 4)

After 1^st harvest, the DOC concentration in rhizodeposit solution of swards containing H. lanatus ranged from 21.1 to 25.6 mg C l^-1.

Compared to samples prior to 1^st harvest, dicultures with P. lanceolata and R. acris showed dis-tinctly higher DOC concentrations at after 1^st harvest (^{Table 68}). A comparison of purge solution obtained before collection of rhizodeposit (not shown) showed a significant decrease in DOC from 27.0 to 20.2 mg l^-1 after 1^st harvest for all swards of H. lanatus and P. lanceolata (Appendix, Table XXVII). Hence, increases in DOC concentrations are unlikely derived from degradation of root biomass repelled after harvest, but indicate enhanced rhizodeposition.

Sampling Ref H. lanatus + A. odoratum + P. lanceolata + R. acris ^{MW-U KW-H} P

Prior to the 1^st harvest, the median DOC concentration in rhizodeposit solution of swards con-taining P. lanceolata (Table 69) ranged from 20.8 to 30.0 mg C l^-1. P. lanceolata monocultures showed slightly higher DOC concentrations than diculture swards. However, the differences be-tween mono and diculture swards were merely tendentious.

Table 69 Median (25^th /75^th Percentile) DOC concentration in rhizodeposit solution of P. lanceolata swards without Fe supply prior to and after 1^st harvest

Significant distinctions between swards are indicated by different letters (Kruskal-Wallis-ANOVA; Mann-Whitney U-Test; stand treatment n = 4)

After 1^st harvest, the DOC concentration in rhizodeposit solution ranged considerably from 23.8 to 51.0 mg C l^-1. Dicultures with H. lanatus showed significantly lower DOC concentrations than the other swards. For P. lanceolata monocultures increased DOC concentrations may indicate higher Fe stress compared to samples taken prior to 1^st harvest.

Higher DOC concentrations in H. lanatus and P. lanceolata swards compared to Ref clearly in-dicate rhizodeposit release of soluble carbon compounds. Low rhizodeposition was observed for H. lanatus with A. odoratum prior to and with P. lanceolata after 1^st harvest. In P. lanceolata swards monocultures and dicultures with A. odoratum showed higher rhizodeposition. However, rhizodeposition increased and differences were only significant after 1^st harvest. P. lanceolata and A. odoratum showed enhanced rhizodeposition. Whereas P. lanceolata showed enhanced rhizodeposition under intra- as well as under interspecific competition, A. odoratum only showed 15.6 mg C l^-1 prior to and 21.1 mg C l ^-1after 1^st harvest in monoculture.

Sampling Ref P. lanceolata + R. acris + H. lanatus + A. odoratum ^{MW-U KW-H p}

---[DOC mg l^-1

]---prior to

1^st harvest (13.2/18.5)^14.7

30.0 (25.7/32.9)

24.9

(17.2/33.5)

20.8 (19.1/24.3)

29.1 (25.2/32.6)

ns 9.24 0.056

after

1^st harvest ^10.4

c

(9.7/11.7) 51.0 ^a

(39.6/61.3) 23.3 ^ab

(19.7/33.9) 25.6 ^ab

(23.7/29.5) 31.8^a (27.2/55.3)

* 14.04 0.007

3.2.2.2 Spectral Characteristics of DOC

Prior to1^st harvest, the median intensity of specific UV-absorbance⁷ (280 nm ) in rhizodeposit solution (^{Table 70}) of H. lanatus swards ranged from 0.09 to 0.14. After 1^st harvest, the median intensity of UV-absorbance ranged from slightly from 0.13 to 0.14. The differences between H.

lanatus swards were rather low prior to and after 1^st harvest.

Table 70 Median (25^th /75^th Percentile) of specific UV absorbance (280 nm) of rhizodeposit solution from H. lanatus swards 3 weeks prior to and after 1^st harvest

Significant distinctions between swards are indicated by different letters. (Kruskal-Wallis-ANOVA; Mann-Whitney U-Test; treatment n = 4)

The median intensity of emission spectra (435-480 / 300-345 nm) of H. lanatus swards (^{Table 71}) ranged prior to 1^st harvest from 2.3 to 5.6. Dicultures with A. odoratum and P. lanceolata showed ten-dentiously lower intensities than the other swards. After 1^st harvest, intensities ranged from 4.9 to 6.7. Analogous to prior to 1^st harvest, intensities were significantly lower in dicultures with A.

odoratum and P. lanceolata.

Table 71 Median (25^th /75^th Percentile) of emission spectra (E2/E1) of rhizodeposit solution from H. lanatus swards 3 weeks prior to and after 1^st harvest

Significant distinctions between swards are indicated by different letters. (Kruskal-Wallis-ANOVA; Mann-Whitney U-Test; treatment n = 4)

According to KALBITZ ET AL (2000; 2003), this finding indicates higher contribution of lower complexity compounds and thus hints at enhanced rhizodeposit release of aliphatic compounds.

7 Spectral data had to be corrected by y = 1,304 +1,469(x), chapter 2.4.2

Sampling Ref H. lanatus + A. odoratum + P. lanceolata + R. acris ^{MW-U KW-H} p

Prior to 1^st harvest, the median intensity of specific UV absorbance of rhizodeposit solution (Table 72) from P. lanceolata swards ranged from 0.09 to 0.12. It was tendentiously higher in dicultures with R. acris. After 1^st harvest, the absorbance ranged from 0.08 to 0.14. Analogous to H. lana-tus swards, the differences in specific UV-absorbance between P. lanceolata swards were rather low prior to and after 1^st harvest.

Table 72 Median (25^th /75^th Percentile) of specific UV absorbance (280 nm) of rhizodeposit solution from P. lanceolata swards prior to 1^st harvest

Significant distinctions between swards are indicated by different letters (Kruskal-Wallis-ANOVA; Mann-Whitney U-Test; treatment n = 4)

The median intensities of emission spectra of rhizodeposit solution prior to 1^st harvest (^{Table 73}) ranged from 2.3 to 4.4. They were lower in monocultures and in dicultures with A. odoratum prior to and after 1^st harvest. After 1^st harvest, the intensities ranged from 4.9 to 7.1 This indi-cated higher contribution of low complexity compounds (KALBITZ ET AL., 2000; 2003), which may be released as rhizodeposits in these stands.

Table 73 Median (25^th /75^th Percentile) of emission spectra (E2/E1) of rhizodeposit solution from P. lanceolata. swards 3 weeks after 1^st harvest

Significant distinctions between swards are indicated by different letters (Kruskal-Wallis-ANOVA; Mann-Whitney U-Test; treatment n = 4)

Low intensities of emission spectra prior to (2.4) and after 1^st harvest (3.1) for rhizodeposits from A. odoratum monocultures may suggest a considerable contribution of A. odoratum to the release of low complexity compounds in diculture with P. lanceolata.

Sampling Ref P. lanceolata + R. acris + H. lanatus + A. odoratum ^{MW-U KW-H p}