Abstract. To acquire information on the nature of belowground interactions in intercropping system involving ginger, Zingiber officinale Roscoe, and Ailanthus ...
Agroforestry Systems 41: 293–305, 1998. 1998 Kluwer Academic Publishers. Printed in the Netherlands.
Root competition for phosphorus between ginger and Ailanthus triphysa in Kerala, India J. THOMAS1, B. MOHAN KUMAR1,*, P. A. WAHID2, N. V. KAMALAM3 and R. F. FISHER4 1
College of Forestry and 3 Radiotracer Laboratory, Kerala Agricultural University, Vellanikkara, Thrissur 680 654, India; 2 College of Agriculture, Padanakkad, Nileshwar 671 353, Kerala, India; 4 Department of Forest Sciences, Texas A&M University, College Station, TX 77843-2135, USA (*Author for correspondence) Key words:
32
P soil injection, 32P uptake, root activity, tree density
Abstract. To acquire information on the nature of belowground interactions in intercropping system involving ginger, Zingiber officinale Roscoe, and Ailanthus triphysa (Dennst.) Alston, their root activity was evaluated based on 32P recovery by each species in mixed and sole crop situations. Treatments included two Ailanthus densities (1,111 and 3,333 tress ha–1) and four lateral distances of 32P application (10 and 20 cm from the treated ginger plant and 20 and 40 cm from the treated Ailanthus trees). Recovery of 32P in ginger foliage increased with time, irrespective of tree population density and lateral distance of isotope application. Lack of significant variations in 32P recovery as a function of tree population density suggests that tree density is probably not a strong determinant of belowground competition in the well-fertilised, manured and mulched system studied (at least till four years after tree planting). Nonetheless, neighbouring Ailanthus trees absorbed a substantial potion of the 32P supplied to ginger. This, in turn, suggests that the effective root zones of ginger and Ailanthus may overlap. Data on 32P uptake of Ailanthus suggest that 41% to 59% of the root activity is concentrated within a zone of about 40-cm distance from the trunk. Neighbouring trees in the high density stands absorbed significantly more 32P than those in low density stands (P < 0.01 at 30 and 45 days after 32P application). Our observations also suggest that competition between the tree and the herbaceous crop for nutrients applied to the tree component is unlikely in the Ailanthus-ginger mixed species system studied. Therefore, from a crop management perspective, it is better to fertilise the herbaceous component of the mixed species system adequately, as it will also benefit the tree component. Nutrient use efficiency may be higher under such situations.
Introduction Ginger (Zingiber officinale Roscoe, Family: Zingiberaceae), is an important spice crop of the humid tropics. Being a premier flavourant in culinary preparations and an ingredient of medicines, ginger enjoys immense popularity. India is the world’s chief producer and exporter of this spice. Pakistan, Bangladesh, USA, Morocco and Saudi Arabia are the principal importers. In India, approximately 55,000 ha of ginger are cultivated with an annual production of 156,180 tonnes of dry ginger (Spices Board, 1995). Being a shade-loving plant, ginger is grown in association with a variety of trees (Jaswal et al., 1993; Spices Board, 1995). Although ginger is traditionally grown in association with many trees, Ailanthus triphysa (Dennst.)
294 Alston, a fast growing multipurpose tree, predominates in the ginger-based agroforestry systems of Kerala. Owing to its compact crown (Mathew et al., 1992), moderate root spread (George et al., 1996) and deep rooting tendency (Jamaludheen, 1994), Ailanthus is thought to be less competitive with associated field crops. Nonetheless, literature relating to the performance of ginger in tree-based cropping systems and the magnitude of interspecific competition, especially for belowground resources, is scarce. While research reports on crop management and protection abound, they largely relate to ginger in monospecific stands (Sreekumar et al., 1981; Korla et al., 1989; Das et al., 1990; Balasubramanian and Gopalan, 1992; Mohanty et al., 1993). In mixed species systems, roots of different species frequently intermingle and often this overlap of the roots can be extensive (Clements et al., 1929). Furthermore, if soil resources are limited and neighbouring plant species have active root systems in the same location of the soil profile, one species may be more effective in acquiring these scarce resources than the other. In general, tree root systems can potentially out-compete field crops grown in association with them. Yet, little is known about the characteristics of root systems that enable one species to acquire resources more efficaciously than others in a competitive situation (sensu. Caldwell and Richards, 1986). According to Caldwell (1987) competition belowground is known more by its manifestations than by its mechanisms and even the most basic questions have received little attention. Although there is increasing information on root-system biomass and root production in various ecosystems, there is a paucity of information on root competition and rooting density in tropical agroforestry systems (George et al., 1996). The present study aims to characterise the nature of belowground interactions between ginger and Ailanthus in integrated tree-crop system, based on the relative absorption of 32P. Caldwell and Richards (1986) also observed that when considering competitive relationships, the distribution and degree of overlap of neighbouring root systems are of particular interest. However, data on overlapping root systems in agroforestry are scarce, owing to methodological problems. An attempt is made here to estimate the relative distribution of physiologically active Ailanthus roots.
Materials and methods Design and installation The study was conducted at Vellanikkara, Thrissur district, Kerala (13°31¢ N latitude and 76°13¢ E longitude and at an elevation of 40 m above sea level). Vellanikkara experiences a warm humid climate, having a mean annual rainfall of 2,670 mm, most of which is received during the Southwest monsoon (June to August). A total of 4,307.5 mm of rainfall was received during the ginger trial period (May 1994 to June 1995). The isotope study reported here was
295 carried out from September to November, 1994, when 724 mm of rainfall was received. Mean maximum temperature at Vellanikkara ranges from 29.1 °C (July) to 36 °C (May) and the mean minimum temperature varies from 21.9 °C (January) to 25 °C (May). Soil of the experimental site is an Ultisol (Typic Plinthustult – Vellanikkara series midland laterite – Ustic moisture regimes and Isohyperthermic temperature regimes). The soil physico-chemical properties (prior to ginger planting) were as follows: total N: 0.11% to 0.15%, available P: 8.8 mg L–1 to 14.1 mg L–1, available K: 44 mg L–1, organic C: 1.3% to 2.8% and pH: 5.7 to 5.9. A split plot experiment with Ailanthus triphysa was initiated in June 1991. It consisted of three replications of eight treatments. Main plot treatments consisted of four population densities of Ailanthus: (D1 – 3,333 trees ha–1 – 3 ´ 1 m spacing, D2 – 25,000 trees ha–1 – 2 ´ 2 m spacing, D3 – 1,600 trees ha–1 – 3 ´ 2 m spacing, and D4 – 1,111 trees ha–1 – 3 ´ 3 m spacing). Four fertiliser levels (F1 – 0:0:0, F2 – 50:25:25, F3 – 100:50:50 and F4 – 150:75:75 Kg N, P2O5 and K2O ha–1) formed the sub plot treatments. Each subplot was 20 ´ 10 m. Ailanthus trees were fertilised according to this treatment protocol in August 1992 and September 1993. Ginger was planted as an understorey crop in May 1994 following a package of recommended practices (KAU, 1993). We used the ginger cultivar Kuruppampady, a dry type known for its tolerance to shade, disease and pest incidence (KAU, 1993). Ginger rhizome bits (15 g each; treated with Dithane M 45 @ 3 g L–1 and Ekalux @ 1 mL L–1 as a prophylactic measure against disease and pest incidence) were planted in small pits (4 to 5 cm depth) at a spacing of 25 ´ 25 cm on beds (9 ´ 1 m size) in the interspaces of Ailanthus. There were six beds each in all plots. Monospecific ginger plots (three) having six beds each were established in the contiguous area (envisaged as per the original experiment design). Immediately after sowing, farm yard manure (FYM) at the rate of 30 t ha–1 (wet weight basis) was broadcast-applied on the beds as recommended (KAU, 1993). Average elemental composition of FYM is a follows: 0.4% N, 0.3% P and 0.2% K. The recommended fertiliser dose (KAU, 1993) of 75:50:50 kg N, P2O5 and K2O ha–1 year–1 was applied as follows: entire P and 50% of K as basal dose, half the quantity of N at 60 days after planting and the remaining N and K at 120 days after planting. The beds were also mulched (at the rate of 15 t ha–1; fresh weight basis) with green leaves (Gliricidia maculata, weeds such as Chromolena odoratum and other locally available herbs and shrubs). Mulching at the rates of 7.5 t ha–1 (fresh weight basis) was repeated along with the second (60 days) and third (120 days) split doses of chemical fertilisers (KAU, 1993). Results relating to the performance of ginger as an understorey crop as influenced by tree density and fertiliser regimes are presented elsewhere (Kumar et al., in preparation).
296 Tracer studies to characterise root interactions To assess the nature and magnitude of root competition in the ginger-Ailanthus agroforestry system, two field experiments involving ginger and Ailanthus as treated plants were super-imposed on the Ailanthus experiment, described earlier. Root interaction was studied only at two Ailanthus densities: 1,111 trees ha–1 3,333 trees ha–1 (highest), besides the treeless control and one fertiliser level (50:25:25 kg N, P2O5 and K2O ha–1). A 32P soil injection technique (George et al., 1996) was employed for this purpose, wherein the radio-label was placed at specified points in the root zone of both species (Figure 1) in the mixed and monospecific systems. In the first experiment (ginger as the treated plant), 32P was applied at distances of 10 and 20 cm from the ginger plants in monospecific and mixed species stands. A split plot design was employed for this purpose with distance from the plant in the subplot and tree density in the main plot. This trail was intended to provide insights into the extent of root competition between ginger and the associated tree component for nutrients applied to the former. In a second experiment (Ailanthus as the treated plant), 32P was applied at distances of 20 and 40 cm from the Ailanthus trees in both monospecific and mixed species stands. For this a split-split plot design was used (tree density in the main plot, cropping system in the sub plot and lateral distance of 32P application in the sub-sub plot). This study was aimed at evaluating root competition for nutrients applied to the tree component of the system. The 32P was placed at a uniform depth of 20 cm from the surface of ground to units consisting of single tree/ginger clumps (Figure 1). The experimental units for 32 P application were selected on the basis of uniformity of growth and spatial isolation from each other. The minimum distance between any two treated tree/ginger clumps was 7 m. All treatments were replicated four times. At each site (experimental unit) for injection of 32P, eight equally spaced 2.5 cm diameter by 20 cm deep holes were drilled into the soil at the appropriate distance (10 and 20 or 20 and 40 cm, as the case may be) from the plant a day in advance of 32P injection (Figure 1). PVC access tubes were inserted into these holes and their open ends covered with plastic caps to prevent entry of rain water. During the moderate rainfall season in September (240.5 mm out of the total 4,307.5 mm rainfall received during the experiment period from May 1994 to June 1995, was received during this month), 2 mL of 1,000 mg L–1 phosphorus solution containing 0.5 mCi of 32P was dispensed into the access tubes around (Figure 1) Ailanthus trees/ginger plants using a field dispenser (Wahid et al., 1988). After dispensing, the radioactivity remaining on the sides of the access tube was washed down with a jet a about 15 mL of distilled water. The high concentration of P in the carrier solution was used to reduce chances of soil fixation of the radioisotope.
297
Figure 1. Schematic diagram showing experimental units for 32P application in the Ailanthusginger mixed species system and the sampling strategy for neighbouring trees/ginger beds (sectional views of ginger beds only are shown with access tubes for 32P application; not to scale) at Thrissur, Kerala, India. A: A single unit for 32P application in experiment 1 with ginger as the treated plant (a – treated ginger clumps; b and c – neighbouring ginger plants at 25 and 50 cm away from the treated plants; 1 and 2 represents neighbouring Ailanthus trees respectively at 1.375 and 1.625 m away from the treated clumps. Access tubes for 32P soil injection were installed at a lateral distance of either 10 cm or 20 cm on either side of the treated ginger clumps), B–C: A single unit for 32P application in experiment 2 involving Ailanthus as the treated plant flanked by adjacent ginger beds (1 – treated Ailanthus tree, 2 – neighbouring trees in the north-south direction – spacing their 3 m or 1 m depending on tree population density; 3 – neighbouring trees in east-west direction (3 m apart); a–d represents ginger plants at 1.125, 1.375, 1.625 and 1.875 m away from the treated tree respectively. Access tubes for 32P soil injection were installed at a lateral distance of either 20 cm or 40 cm or either side of the treated trees).
Leaf sampling and radioassay The most recently matured leaves from the treated and neighbouring ginger plants or Ailanthus trees were sampled for radioassay. Sampling was done at 15, 30 and 45 days after application of 32P. In the second experiment, the neighbouring Ailanthus trees were also sampled. Foliage samples form neighbouring plants, similarly situated were pooled to make composite samples, row-wise (Figure 1). Leaf samples were dried at 70 °C and radio-assayed for 32P content.
298 The method consisted of wet digestion of one gram of plant sample using a 2:1 mixture of HNO3 and HClO4. The digest was then transferred into a counting vial and made up to 20 mL volume. Vials were counted in a liquid scintillation counter (Pharmacia-LKB, Finland) by the Cerenkov counting technique (Wahid et al., 1985). Statistical analysis Count rates (counts per minute, cpm) were corrected for background and decay and subjected to log10 transformation. The data on recovery of 32P activity in the leaves of ginger and adjacent Ailanthus trees (experiment 1) were analysed following the analysis of variance technique for split plot design, using MSTAT with tree density (0, 1,111 and 3,333 trees ha–1) as the main plot factor and lateral distance of 32P application (10 and 20 cm) as the sub-plot factor. Data from the second experiment were analysed following ANOVA for splitsplit plot design, using MSTAT with tree density (1,111 and 3,333 trees ha–1) in the main plot, cropping system (monospecific Vs mixed) in the sub plot and lateral distance of 32P application (20 and 40 cm) in the sub-sub plot. Assuming that recovery of radioactivity in the foliage is a reflection of the density of active roots at different depth/lateral distances, the relative root activity at a particular lateral distance was computed from the following equation (Wahid et al., 1989a; Jamaludheen et al., 1997): Relative root activity (%) = 32P activity (cpm) recovered in the leaf for a particular lateral distance ´ 100 = –––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– Total cpm for all treatments
Results and discussion Foliar recovery of 32P applied to ginger as a function of tree density and lateral distance The quantity of 32P absorbed by ginger increased with time (Table 1), regardless of tree population density and lateral distance of 32P application. Similar increase in 32P recovery has been previously reported by Wahid et al. (1989a, 1989b) for cashew (Anacardium occidentale) and cacao (Theobroma cacao) and George et al. (1996) for Acacia auriculiformis, Casuarina equisetifolia, Leucaena leucocephala and Ailanthus triphysa. This increasing trend in the absorption of radio-label with time suggests active growth of ginger roots. Our studies on belowground biomass accumulation (data not presented here) also indicated a concomitant increase in dry weight of ginger roots (Thomas, 1996) implying that radiophosphorus application corresponded to the grand growth phase of ginger. Nutrients applied during the grand growth phase are absorbed quickly, when soil moisture is adequate. As our experiment was
299 Table 1. 32P activity recovered in the leaves of ginger at 15, 30 and 45 days after application of the label, as influenced by Ailanthus tree population density and lateral distance of 32P application at Thrissur, Kerala, India. Treatments
15 days TR
30 days N 25
N 50 32
TR
45 days N 25
N 50
TR
N 25
N 50
–1
P activity (cpm g dry weight)
Density (trees ha–1) 3333 525 1111 173 0 151 F test NS
324 288 195 NS
33 08 43 NS
708 955 501 NS
1202 1122 0346 0NS
0
