Effects of biochar on dry matter production and competitive ability of Rumex obtusifolius L.

Auswirkungen der Biokohle auf die Trockenmasseproduktion und Wettbewerbsfähigkeit des Stumpfblättrigen Ampfers (Rumex obtusifolius L.)

Materials and methods

The pot experiment was performed under natural light and climatic conditions out of doors. We used Mitscherlich pots with a surface diameter of 20 cm and a height of 21 cm. Each pot contained 5 kg of field-moist soil. The pots were arranged on benches in a completely randomised design and periodically rotated as to position to reduce variability in light intensity. The soil used for this study was taken from the A and B horizon of a carbonate-free Cambisol with loamy sand texture. This soil type represents a typical grassland soil in Austria. Soil mate­rial from both horizons was blended and sifted through a 12.7 mm mesh sieve, representing the BC-free grassland soil. The BC-amended soil was prepared by thoroughly mixing the grassland soil with milled BC (particle size of < 2 mm). BC was added once only at a dose of 30 g kg^–1 soil, equating to a field application rate of approximately 30 t ha^–1 if calculated for an incorporation depth of 10 cm and a soil bulk density of 1.0 g cm^–3. In comparison to other BC studies, this means a moderately high rate of BC addition to soil. BC was produced at the thermal power station Dürnrohr in Lower Austria from woody green waste residues through pyrolysis at a temperature of 525°C. We have chosen woody green waste BC because of its relatively wide C:N ratio of 58:1 and moderately high ash content of 15 wt% on a dry weight basis. Some additional basic properties of the BC used in this study are presented in Table 1. Half of the pots was filled with the BC-free grassland soil, representing the control treatment (soil without BC), and the other half was filled with the BC-amended soil, representing the BC treatment (soil with BC).

Among grass species, Lolium multiflorum Lam. (Italian ryegrass) and Dactylis glomerata L. (cocksfoot) were selected for study because under suitable site conditions they are competitive, fast- and tall-growing, high-yielding forage grasses. Moreover, both species benefit from a high soil nutrient supply and consequently they occur mainly on nutrient-rich soils, behaving in this respect similar to R. obtusifolius. D. glomerata was also selected because this grass species is common and widely distributed in grassland and is frequently included in seed mixtures of Austria. For the two-component mixture, L. multiflorum was chosen as competitor because it had been observed that this annual forage grass was able to compete much stronger with R. obtusifolius than other grass species (Niggli et al., 1993).

Seeds of R. obtusifolius were collected from a local meadow which was heavily infested with broad-leaved dock. Germination capacity of the seeds was 76% for R. obtusifolius, 92% for L. multiflorum (variety Aubade) and 88% for D. glomerata (variety Tandem). Seeds of all species were sown at the same time directly into the pots on 22 May 2012.

The pot experiment consisted of five treatments, each with and without BC addition, and five replicates for each treatment. In each treatment, the species were sown at equal seed numbers and they were arranged equidistantly, thus eliminating growth differences due to variations in plant density. To minimise edge effects, Rumex plants were always grown in the centre of each pot. In treatment (1), four individuals of R. obtusifolius were grown in monoculture at 8 cm spacing. This distance was chosen in order to avoid severe intraspecific competition. In treatment (2), four Rumex plants were surrounded by seventeen individuals of L. multiflorum at a distance of 4 cm. In treatment (3), four Rumex plants were surrounded by twenty-one individuals of L. multiflorum at a distance of 3 cm. In this treatment, most of the available space was used by the plants as soon as they became larger, leading to a greater interspecific competition compared to treatment (2). In treatments (4) and (5), twenty-five grasses of L. multiflorum and D. glomerata, respectively, were grown in monoculture in each pot at a distance of 3 cm between the individuals.

The pot experiment was performed for twelve weeks from 22 May 2012 to 20 August 2012. Since we did not observe strong root accumulation at the bottom of the pots, we assume that throughout the duration of our experiment there was no root-restriction stress due to the small pot volume. All plants were harvested when some sprouts of R. obtusifolius started to become infested with plant parasites (fungi). At that time, Rumex plants were at the end of the elongation phase and grasses were in the stage of stem elongation. Throughout the duration of our experiment, weather conditions were relatively warm and humid, with a mean air temperature of +17.8°C (maximum and minimum: +33.5°C and +2.4°C, respectively) and a precipitation sum of 497 mm. The plants were well supplied with water. Only during dry periods the pots were additionally watered with tap water. There was no limitation of plant growth due to shortage or surplus of water. No fertiliser was applied to the pots and appearing weeds were immediately removed by hand.

Results

As a result of BC addition to soil, the above-ground biomass production increased significantly (p < 0.05) in R. obtusifolius and highly significant (p < 0.001) in both grass species as compared to the control (Table 5). The rise in average shoot dry weight was for R. obtusifolius, L. multiflorum and D. glomerata 247%, 65% and 108%, respectively. When grown in competition with L. multi­florum, the shoot dry weight per plant of R. obtusifolius significantly decreased both in the BC-free (p = 0.006) and in the BC-amended (p = 0.011) pots (Fig. 1 and 2). Consequently, the reduced dry matter production due to the interspecific competition with grasses was not eliminated by the addition of BC. Furthermore, it can be seen from Fig. 1 and 2 that with increasing number and proximity of L. multiflorum individuals the above-ground biomass of R. obtusifolius is decreasing, especially in the BC-free pots. In R. obtusifolius, the greatest shoot dry weight per plant was produced in BC-amended pots in the absence of L. multiflorum, whereas the lowest shoot biomass was obtained in BC-free pots at the highest density of competing Lolium individuals (Table 5).

Fig. 1. Effect of Lolium multiflorum on above-ground biomass production of Rumex obtusifolius grown in pots without (on the left) and with (on the right) BC addition after twelve weeks of growth; note the difference in scale on the vertical axis (number in parentheses show the ratio of plant individuals “R. obtusifolius: L. multiflorum”).

Fig. 2. Effect of Lolium multiflorum on above-ground biomass production of Rumex obtusifolius grown in pots without (on the left) and with (on the right) BC addition after twelve weeks of growth; note the difference in scale on the vertical axis (number in parentheses show the ratio of plant individuals “R. obtusifolius: L. multiflorum”).

Discussion

The application of BC to soil changed mineral element concentrations in the shoot biomass of the species investigated. The decrease in concentrations of N, S, Ca and Mg both in R. obtusifolius and in the two grass species was probably caused by the enhanced shoot growth resulting from BC addition, suggesting a dilution effect. In fact, adding BC to soil increased mineral element contents in the shoot biomass as compared to the control (data not shown). This indicates that plants have an increased mineral nutrient requirement, especially for N, after BC addition to soil due to an enhanced shoot growth.

The plant species examined differed greatly in their response to BC addition. The P concentration in the shoot biomass slightly increased in the grass species but not in R. obtusifolius. Surprisingly, shoot P concentration was lower in R. obtusifolius than in the two grass species both in the BC-amended and in the BC-free pots. This indicates that R. obtusifolius has a lower absorption capacity for P when growing in P-deficient soil than L. multiflorum or D. glomerata. Obviously, under conditions of low soil P supply the two grass species are more efficient at acquiring P than R. obtusifolius, presumably due to their comparatively greater root density (Kullmann, 1957), leading to a better exploitation of available P in the soil. The K concentration in the shoot biomass, however, increased only in R. obtusifolius relative to the control, whereas it remained more or less unaffected in the grass species. This indicates that broad-leaved dock has a greater absorption capacity for K when growing in K-rich soil than grasses. A high K absorption capacity is an important adaption of R. obtusifolius to soils with high plant available K supply. Moreover, our results suggest that R. obtusifolius has a very high K requirement to maintain vigorous growth, thus confirming previous findings obtained under field conditions (Bohner, 2001). A similar result from field trials has also been reported by Humphreys et al. (1999), who showed that R. obtusifolius benefits greatly from a high K supply in the soil. The marked increase of the CAL-extractable K content in the BC-treated soil and the concomitant enhanced K uptake by the plants are indications that appreciable amounts of plant available K were introduced into the pots by BC application. Obviously, woody green waste BC is an important source of rapidly plant available K, and acts therefore as a K fertiliser immediately after its application to K-deficient soil. This result is in agreement with Schimmelpfennig et al. (2015), who found in a grassland field experiment in Germany greater K concentrations in the plant biomass from BC-amended plots compared to control plots. In R. obtusifolius, BC addition caused an increase in the C:N ratio and simultaneously a decrease in the ratio of N to other mineral nutrient elements in the shoot biomass, indicating a deficient supply of plant available N in the BC-treated soil compared to the control soil. Obviously, soil N supply could not meet the enhanced plant requirement for N. The deficient soil N supply after BC addition did not inhibit shoot growth of R. obtusifolius. Apparently, under the specific conditions of our pot expe­riment N was not the growth-limiting nutrient in soil, but rather K supply controlled Rumex growth, as indicated by the considerable increase in shoot growth in response to an addition of K by BC application. Our data provide evidence that N shortage has a relatively small effect on shoot growth of R. obtusifolius as long as K is the growth-limiting nutrient element. However, at high levels of plant available K in soil N may become the major factor limiting growth of R. obtusifolius (Jeangros and Nösberger, 1990). An increase in K concentration and a concomitant decrease in N concentration due to BC addition were also observed in Chenopodium quinoa Willd. (Kammann et al., 2011) and in Trifolium pratense L. (Oram et al., 2014).

The significantly higher shoot biomass of all plant species examined in the BC treatment compared to the control demonstrates the potential of woody green waste BC to increase above-ground biomass production of particular grassland species when growing on nutrient-poor soils at least in the year of BC application. The addition of BC to soil enhanced shoot dry weight of R. obtusifolius in monoculture by a factor of 4 and in mixtures with L. multiflorum by a factor of 2 as compared to the control. In contrast, adding BC to soil increased the shoot dry weight of grasses in monoculture only by a factor of 2 relative to the control. Thus, BC promoted the shoot growth of R. obtusifolius more than that of the grasses, indicating a species-specific fertilizer effect of applied BC. This is consistent with the findings of Schimmelpfennig et al. (2015), who showed in a grassland field experiment that BC favours growth of forbs over grasses. In general, species from nutrient-rich habitats are characterised by a high growth rate on fertile soils, they strongly respond to an increased soil nutrient supply and they have high nutrient requirements (Chapin et al., 1986). Therefore, R. obtusifolius can be considered as an indicator species of high soil fertility.

The unexpected considerable enhancement of shoot growth of all plant species examined in the BC-amended pots compared to the control can largely be attributed to an improved K supply to plants provided by the BC. It cannot be excluded, however, that the addition of BC to soil also positively influenced several other growth-controlling factors (e.g., supply of micronutrients), but they were not measured in our pot experiment. It is important to note that Rumex plants showed more morphological plasticity (shoot plasticity) in response to soil K enrichment than the grass species, leading to a greater competitive ability for light (shoot competitive ability). In general, a high degree of shoot plasticity is advantageous on fertile soils because it increases the shoot competitive ability of a plant under nutrient-rich conditions (Grime et al., 1986).

R. obtusifolius seems to benefit from a high content of plant available K in soil more than grass species due to its ability to absorb large amounts of K. Under conditions of high soil K supply, R. obtusifolius has a competitive advantage over grasses mainly because of a comparatively greater above-ground biomass gain. Thus, in K-rich soils Rumex plants, once established, may suppress grasses from the plant community primarily through shading, provided that the supply of all other essential plant nutrients is adequate for a vigorous growth. Under the specific conditions of our pot experiment, K was the most growth-limiting nutrient element in R. obtusifolius. N, however, seems to be of minor importance as long as K is restricting its shoot growth. Thus, K availability in soil appears to be the key factor for Rumex infestation in grassland. Therefore, simply removing established Rumex plants, mechanically or chemically, may provide only temporary control, as long as soil K availability remains high. At an elevated soil K supply the ability of R. obtusifolius to dominate a plant stand is increased, especially if there are numerous gaps in the grass sward. Field observations and the evaluation of many vegetation surveys support this assumption. R. obtusifolius is usually absent from nutrient-poor grassland habitats despite gaps in the grass sward and seed input from the surrounding vegetation, emphasising the importance of soil nutrient status to broad-leaved dock infestation in grassland. To prevent Rumex infestation, a continued excessive application of manure or K fertiliser to grassland soils should be avoided. In seriously infested grassland, the reduction of annual K input in combination with biological control measures (Müller, 2015) may reduce abundance of R. obtusifolius in the long term. A dense grass sward is also very important because it prevents the germination, establishment and spread of R. obtusifolius.

To our knowledge, this is the first report on the possible use of BC as a control measure against weeds in permanent grassland of temperate regions. However, the results of this study may be extrapolated to field conditions only with great caution. Therefore, long-term field trials and on-farm experiments at different sites using various BC types and application rates are needed to confirm our findings.