3.1 Site specific peat soil characteristics
ZV peat soil (limed) had a higher pH than BF soil (non-limed), as was expected from its liming history (5.57 and 4.36 respectively, p = 0.002). Organic matter content and bulk density indicated that both soils were degraded peat soils (Liu and Lennartz 2019). BF soil also contained higher extractable NH4+ (p = 0.026), Fe (p < 0.001) and P (p = 0.033), and had a lower C:N ratio (p < 0.001) than ZV peat soil, although BF soil had a lower total-K content (p < 0.001) (Table 1).
Table 1 Site characteristics of the two different locations. Values indicate means ± standard errors (n=4). Asterisks indicate significant differences between soils (* p<0.05, ** p<0.01, *** p<0.001).
|
Limed (ZV)
|
Non-limed (BF)
|
H2O-pH
|
5.57 ± 0.05
|
4.36 ± 0.24**
|
C:N
|
11.16 ± 0.14
|
18.15 ± 0.88***
|
C (%)
|
20.2 ± 0.8
|
19.9 ± 2.9
|
organic matter (%)
|
44.5 ± 1.8
|
48.6 ± 5.7
|
wet bulk density (g FW l-1 FW)
|
790 ± 16.5
|
948 ± 22.5**
|
dry bulk density (g DW l-1 FW)
|
314 ± 12.4
|
448 ± 29.3**
|
NaCl-extractable NO3- (μmol l-1 FW)
|
130 ± 34.4
|
104 ± 67.7
|
NaCl-extractable NH4+ (μmol l-1 FW)
|
201 ± 46.3
|
1017 ± 273*
|
H2O-extractable K (μmol l-1 FW)
|
204 ± 36.3
|
113 ± 42.5
|
H2O-extractable P (μmol l-1 FW)
|
6.29 ± 3.67
|
13.17 ± 3.41*
|
H2O-extractable Fe (μmol l-1 FW)
|
26.2 ± 3.9
|
83.2 ± 12.2***
|
Olsen-P (µmol l-1 FW)
|
616 ± 22.4
|
1201 ± 109**
|
Total-N (mmol l-1 FW)
|
405 ± 10.9
|
348 ± 38.7
|
Total-K (mmol l-1 FW)
|
20.3 ± 1.64
|
5.04 ± 0.52*
|
Total-P (mmol l-1 FW)
|
20.4 ± 1.3
|
15.4 ± 1.3
|
3.2 Biomass yield and nutrient removal
T. latifolia produced more biomass than P. australis in all treatments of the vegetation experiment: on average 12.7 g DW mesocosm− 1 and 6.9 g DW mesocosm− 1, respectively (p < 0.001; Fig. 1). A positive relationship between N load and biomass production of T. latifolia was found on limed ZV soil (p = 0.001) increasing from 5.3 to 10 ton ha− 1. Biomass production of P. australis on ZV soil increased from 2.6 ton ha− 1 to an optimum of 5.5 ton ha− 1 at 150 kg N ha− 1. However, biomass production did not increase any further at 450 kg N ha− 1 (p < 0.05; Table 2), and algae were observed in the surface water.
In the pH experiment, average biomass production of T. latifolia was 40% higher on limed ZV soil compared to non-limed BF soil (12.7 and 7.6 g DW mesocosm− 1, respectively; p < 0.001). The positive relationship between N load and biomass production of T. latifolia was also found on BF soil (p < 0.001; Table 2), increasing from 2.5 to 5.9 ton ha− 1. On both soils NUE of T. latifolia was lowest in the treatments receiving 450 kg N ha− 1 (p < 0.05).
Nutrient removal largely followed the same trends as the biomass results (Table 2). Plants stored 31 up to 205 kg N ha− 1 in aboveground biomass, depending on N load, soil type and plant species. N removal was positively related to N load for both T. latifolia (on both soils; p < 0.001) and P. australis (p = 0.008). P removal (5 to 21 kg P ha− 1) and K removal (27 to 141 kg K ha− 1) by aboveground biomass did not significantly increase with N load. N, P, and K removal by P. australis was highest at an N load of 150 kg ha− 1 compared to lower N loadings (p < 0.05). At all N loads of the vegetation experiment, P and K removal rates of T. latifolia were higher than P. australis on ZV soil (p < 0.001 for both). In the pH experiment, P and K removal rates were also higher for T. latifolia on ZV soil than on BF soil (p = 0.003 and p < 0.001, respectively).
Table 2 Extrapolated aboveground biomass yield, nitrogen use efficiency (NUE) and nutrient sequestration by T. latifolia on different soils and P. australis on ZV soil at different N loads (removable by harvesting). Values shown are mean ± SE (N=4). Letters indicate significant differences between N treatments where applicable (P<0.05).
|
|
T. latifolia
|
|
P. australis
|
|
N load (kg ha-1)
|
ZV soil
|
BF soil
|
ZV soil
|
Yield (g DW mesocosm-1)
|
0
|
9.3 ± 1.7ᵃ
|
4.5 ± 0.4ᵃ
|
4.6 ± 0.4ᵃ
|
|
50
|
9.7 ± 1.3ᵃ
|
7.1 ± 0.8ᵃ
|
5.2 ± 0.8ᵃ
|
|
150
|
14.1 ± 1.3ᵃᵇ
|
8.5 ± 0.8ᵃᵇ
|
9.6 ± 0.8ᵇ
|
|
450
|
17.7 ± 3.3ᵇ
|
10.3 ± 1.3ᵇ
|
8 ± 1.3ᵃᵇ
|
NUE (g DW g N-1)
|
0
|
86.1 ± 6.2ᵃ
|
82.2 ± 8.2ᵃ
|
63.5 ± 13.7
|
|
50
|
76.1 ± 7.1ᵃ
|
83.3 ± 11.1ᵃ
|
69.3 ± 9.5
|
|
150
|
86.6 ± 3.3ᵃ
|
70.6 ± 5.9ᵃ
|
45.9 ± 4.2
|
|
450
|
50 ± 2.9ᵇ
|
47.8 ± 7.3ᵇ
|
51 ± 14.2
|
N removal (mg N mesocosm-1)
|
0
|
107 ± 13ᵃ
|
55 ± 3ᵃ
|
80 ± 14ᵃ
|
|
50
|
132 ± 23ᵃ
|
91 ± 17ᵃ
|
76 ± 14ᵃ
|
|
150
|
164 ± 18ᵃ
|
122 ± 13ᵃ
|
217 ± 30ᵇ
|
|
450
|
362 ± 75ᵇ
|
221 ± 15ᵇ
|
192 ± 47ᵃᵇ
|
P removal (mg P mesocosm-1)
|
0
|
25.3 ± 10.5
|
9.5 ± 1.2
|
8.6 ± 1.8ᵃ
|
|
50
|
24.9 ± 2.8
|
18.6 ± 3.1
|
12 ± 2.5ᵃᵇ
|
|
150
|
32.9 ± 4.3
|
16.6 ± 2.4
|
19.8 ± 3.7ᵇ
|
|
450
|
37.5 ± 9.5
|
22.8 ± 3
|
17 ± 2.1ᵃᵇ
|
K removal (mg K mesocosm-1)
|
0
|
204 ± 77
|
48 ± 3
|
68 ± 6ᵃ
|
|
50
|
198 ± 18
|
113 ± 17
|
70 ± 16ᵃ
|
|
150
|
251 ± 37
|
121 ± 26
|
178 ± 22ᵇ
|
|
450
|
222 ± 35
|
116 ± 27
|
119 ± 20ᵃᵇ
|
Table 3 Nutrient content and nutrient ratios of T. latifolia and P. australis at different N loads. Values are shown as mean ± SE (n=4). Letters indicate significant differences between N treatments where applicable (p<0.05).
![](https://myfiles.space/user_files/83064_0857a92044b57365/83064_custom_files/img1630999211.png)
3.3 Nutrient stoichiometry
N loading increased the N content of T. latifolia (p < 0.001) (Table 3), which increased to more than 2% in aboveground biomass on both soils. The N content of P. australis also increased to more than 2%, although this effect was not significant (p = 0.273). For T. latifolia, P content was not affected by N loading, but K content decreased in the 450 kg N ha− 1 treatment. N:P and N:K ratios also increased significantly in T. latifolia receiving 450 kg N ha− 1 (p = < 0.001), reaching average N:P ratios of 10 and N:K ratios of 1.7 to 2.2. P. australis showed a higher K content at 150 kg N ha− 1 compared to 50 kg N ha− 1 (p = 0.033), but N loading had no significant effect on P content (p = 0.392) and N:P (p = 0.255) or N:K (p = 0.567) ratios. C:N ratios strongly decreased in T. latifolia treatments receiving 450 kg N ha− 1 (p < 0.001), with no differences between soil types (p = 0.242), from 38 to 22 on average. C:N ratios in P. australis did not differ significantly between different N loads (p = 0.134).
3.3 Pore and surface water nutrients
In the vegetation experiment, N (mainly NH4+) accumulated in the pore water in all unvegetated controls as opposed to vegetated treatments (p < 0.001; Fig. 2), with concentrations that became 5–40 times higher than at the start of the experiment. This led to surface water N concentrations of 142, 347, 1909, and 7933 µmol/l, respectively, for the increasing N loads, at the end of the experiment (Fig. 3). Pore water NO3− in control mesocosms increased after N application, but was depleted in all treatments towards the end of the experiment.
In vegetated mesocosms, NH4+ was depleted or strongly decreased in surface water and pore water towards the end of the experiment as the plants increased in biomass, leading to a 97–100% reduction in the T. latifolia mesocosms on ZV soil, a 90–98% reduction in the T. latifolia mesocosms on BF soil, and a 52–100% reduction in the P. australis mesocosms on ZV soil. Surface water NO3− was also depleted in all vegetated mesocosms at the end of the experiment, leading to a 97–100% reduction in all treatments.
Pore water P concentrations decreased in T. latifolia treatments (81% on ZV soil; 26% on BF soil; p < 0.001), but not in P. australis or controls (p < 0.001; Fig. 4). P concentrations in pore water were highest in BF soil, between 100 and 300 µmol/l at the start of the experiment, but no PO43− mobilization to the water column occurred (Figure S1). In ZV soil, PO43− mobilization correlated negatively with N load, meaning that mobilization to the water layer occurred mostly in the low N treatments. Added K accumulated to concentrations of around 1500 µmol/l in surface water of unvegetated mesocosms, but was depleted in vegetated mesocosms in all treatments (Figure S2). K did not accumulate in the pore water.
3.4 N budget
N budgets show the division of N over different compartments at the end of the experiment (Table 4). Net N loss, defined as the amount of N not covered in any compartment, correlated positively with N load. Net N losses were up to 62% for treatments receiving 450 kg N ha− 1. Net N mineralization occurred at the lowest two N loads. Plants (aboveground and belowground) took up 73–389 mg N mesocosm− 1 on average, positively related to N load. In unvegetated mesocosms, up to 98 mg N mesocosm− 1 accumulated in surface water, and up to 135 mg N mesocosm− 1 in pore water. Soil available N also increased in unvegetated mesocosms (up to 229 mg N mesocosm− 1), and, less so, in P. australis mesocosms receiving 0 and 450 kg N ha− 1 (up to 96 mg N mesocosm− 1). In all other vegetated mesocosms, soil N stocks decreased slightly.
3.5 P budget
P budgets show the division of P over different compartments at the end of the experiment (Table 5). Net P mineralisation occurred in all vegetated mesocosms, and was higher at the highest N loads. In most control treatments net P loss occurred, which is defined as the amount of P not covered in any compartment. This net P loss was lower at higher N loads and negative at the highest N load, meaning that P mineralisation occurred in the latter case. Plants (aboveground and belowground) took up 15–52 mg P mesocosm− 1, with higher uptake rates at the highest N loads. In unvegetated mesocosms, up to 0.25 mg P mesocosm− 1 accumulated in surface water, and up to 2.67 mg P mesocosm− 1 in pore water.
Table 4 N budget for different paludicrops, N loads and soils. Values are shown as mean ± SE (n=4) and are expressed in mg N mesocosm-1. N loss was calculated by subtracting start values from end values in soil (NH4+ and NO3- determined by salt extraction), surface water, pore water, and plant fractions. Positive values represent a net N loss and indicate the proportion of N available at the start that is lost from the system, i.e. not found in any other fraction in the end (e.g. gaseous losses of N2 and N2O). Negative values represent net N mineralisation (e.g. mobilization and organic matter decomposition). Letters represent significant differences between N load treatments.
![](https://myfiles.space/user_files/83064_0857a92044b57365/83064_custom_files/img1630997889.png)
Table 5 P budget for different paludicrops, N loads and soils. Values are shown as mean ± SE (n=4) and are expressed in mg N mesocosm-1. P loss was calculated by subtracting start values from end values in surface water, pore water, and plant fractions. Positive values represent a net P loss and indicate the proportion of P available at the start that is lost from the system, i.e. not found in any other fraction in the end (e.g. binding to the soil adsorption complex). Negative values represent net P mineralisation (e.g. mobilization and organic matter decomposition). Letters represent significant differences between N load treatments.
![](https://myfiles.space/user_files/83064_0857a92044b57365/83064_custom_files/img1630997959.png)