Root competition for phosphorus between the tree and herbaceous components of silvopastoral systems in Kerala, India. Suman Jacob George I , B. Mohan ...
189
Plant and Soil 179: 189-196, 1996.
© 1996KluwerAcademic Publishers. Printedin the Netherlands.
Root competition for phosphorus between the tree and herbaceous components of silvopastoral systems in Kerala, India S u m a n J a c o b G e o r g e I , B. M o h a n K u m a r 1, R A . W a h i d z and N.V. K a m a l a m z I College of Forestry and 2Radiotracer Laboratory, Kerala Agricultural University, Vellanikkara, Thrissur 680654, India Received 27 October 1994. Accepted in revised form 3 November 1995
Key words: forage grasses, multipurpose trees, 32p recovery, root activity pattern
Abstract Root competition in polyculture systems involving combinations of four tree species and four grass species was evaluated based on 32p recovery by each species in mixed and sole crop situations. The tree species were: Leucaena leucocephala, Casuarina equisetifolia, Acacia auriculiformis and Ailanthus triphysa, and the grass species were: Pennisetum purpureum (hybrid napier), Brachiaria ruziziensis (congo signal), Panicum maximum (guinea grass) and Zea mexicana (teosinte). Four lateral distance (25 and 50 cm) and depth (15 and 50 cm) treatments were included in the study to characterize the relative fine root distribution of trees. Absorption of 32p was monitored through radioassay of leaves. Regardless of the species, 32p uptake from 50 cm soil depth was lower than that of 15 cm depth. Absorption of 32p from 50 cm lateral distance was also less than that of 25 cm distance in Acacia and Casuarina. Grass species in sole crop situations absorbed more 3Zp than in mixed systems. None of the grass species when grown in association with tree components affected the absorption of 32p by trees. All grass species exerted a complementary effect on 32p absorption by Casuarina. Leucaena also benefited in the same way when grown in association with congo signal and/or teosinte. Of the tree species, Acacia and Leucaena adversely affected the 32p uptake by grass species.
Introduction
Competition for native and applied resources among component crops is an important factor that limits productivity of agroforestry systems. Although the success of many of these systems relies heavily on the exploitation of component interactions, according to Nair (1993), information on the underlying mechanisms is often limited. Among the negative (production-decreasing) interactions common to these systems, competition for light, water and nutrients is perhaps the most important. Root system morphology and fine root distribution are cardinal factors in determining the magnitude of interspecific competition (below ground) in mixed species systems. Root competition in polyculture systems is of special significance in view of its implications on yield
reduction. Root competition in intercropping systems involving annual crops had been evaluated using 32p tracer technique by a few workers (Ashokan et al., 1988; Cooper and Ferguson, 1964; De et al., 1984). However, studies on these aspects concerning integrated tree-crop systems are very few and scattered. This study aims at evaluating the root competition between grass and tree species in integrated land-use systems based on the relative absorption of applied 32p. Furthermore, many tree species were found to have most of their feeder roots concentrated in the surface soil layer (Jonsson et al., 1988; Rubigwa et al., 1992; Sankar et al., 1988; Wahid et al., 1989). However, such information on silvopastorai systems is scarce. Hence an attempt was made to characterize the relative distribution of fine roots in four fast growing tree components of such a system.
190 Materials and methods A field experiment, initiated in June 1988, involving combinations of four tree species and four grass species at the Livestock Research Station, Thiruvazhamkunnu, Palakkad district, Kerala, India was used for the present study. The location has an elevation of 6070 m above mean sea level and is situated between 11°21'30" and 11°21'50 '' N latitude, 76°1J50 '' E longitude. Mean annual rainfall is about 2570 mm, bulk of which is received during the south-west monsoon season (June to August). Mean maximum temperature ranges from 28.4 °C (October) to 38.0 °C (April) and mean minimum temperature, from 19.5 °C (January) to 25.9 °C (November). The soil at the experimental site is acidic oxisol with a pH of 5.1. The four tree species used in the study were: Leucaena leucocephala (Lam.) de Wit., Casuarina equisetifolia J.R. and G. Forst., Acacia auriculiformis A. Cunn. ex Benth. and Ailanthus triphyhsa (Dennst.) Alston., and the four grass species were: Pennisetumpurpureum Schumach. (hybrid napier), Brachiaria ruziziensis Germain and Everard. (congo signal), Panicum maximum Jacq. (guinea grass) and Zea mexicana (Schrad.) Reeves and Mangelsd. (teosinte). The experimental variables thus included 16 tree-grass combinations and their respective monocultures (a total of 24 treatments arranged in a randomised block design, having three replications). The trees were planted in plots of 6 m x 6 m in two rows, 4 m apart. Each row consisted of six trees spaced at 1 m distance, thus giving a population of 12 trees per plot. The plots were demarcated by 50-cm wide risers ("bunds") on all sides. The grass species were planted in the alleys of trees on 15th June 1992. A spacing of 60 c m x 3 0 cm was adopted for hybrid napier and congo signal, 40x20 cm for guinea grass and 30x 15 cm for teosinte (KAU, 1992). A schematic layout representing the tree-grass system is given in Figure 1. The tree species were five years old and the grass species, five months when 32p treatments were imposed. Tree growth characteristics are given in Table 1.
Characterization of root competition To assess the nature and magnitude of root competition, a 32p tracer technique which allowed the comparison of relative absorption of the radio-label by tree and grass species in their mixed and monoculture systems was employed. Phosphorus-32 was applied at two lateral distances (25 and 50 cm) and two depths
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(15 and 50 cm) with respect to the tree species. Each lateral distance-depth combination formed a 32p treatment. Thus, there were four 32p treatments. Plants of the same species and/or the other species next to a treated plant served as border plants. The minimum distance between two treated trees was 2 m. Although this distance may appear too small for experiments with trees whose roots are likely to extend over longer distances, excavation studies have shown that these species have only limited lateral extension of roots. The lateral spread of the roots of eight-year-old Acacia, Casuarina, Leucaena and Ailanthus were 137, 97, 108 and 76 cm respectively at the same location (Jamaludheen, 1994). Therefore, it was safe to assume that there would not be any cross-feeding between adjacent 32p treated trees. In sole-grass situations, the depth and lateral distances chosen for 32p application were the same as that of their respective tree-grass systems. Each treatment was replicated thrice and thus the experiment had a total number of 288 units (including 24 tree-grass systems and their monocultures, besides four lateral distance-depth combinations) laid out in a completely randomised design. For soil application of 32p, six equi-spaced holes were dug to the required depth and lateral distance according to the treatment protocol (Fig. 2). Into each soil hole, a PVC access tube was inserted leaving about
191
Table 1. Allometric data of the five-year-old fast growing multipurpose trees grown in a silvopastoral system used for the study Species
Acacia auriculiformis Casuarina equisetifolia Leucaena leucocephala Ailanthus triphysa
Height
DBHa
Crown
(cm)
(m)
diameter biomass (cm) (Mg ha- t)
Mean root
10.91 8.23 9.05 4.18
9.28 5.54 6.70 5.63
4.28 3.00 3.00 1.66
18.9 2.8 12.6 4.7
Leaf area index
14.52 0.05c 1.77 1.06
aDBH - Diameter at breast height (1.37m). bMeans of the dry weights of the coarse root fractions of three trees per species destructively sampled. CAreacorrespondingto one side of the needles.
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P V C a e e e s s tube
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Tree
All units are in era Figure 2. Diagram showing experimental units for radio-phosphorus application in tree-grass systems.
10 cm of the tube length above the soil surface. The open end of the tube was closed with a plastic cap to prevent filling-up during rains. Phosphorus-32 solution at a carrier level of 1000 mg L -1 P was dispensed into the access tube at the rate of 4 mL per hole on October 17, 1992 (during the north-east monsoon season) using a device fabricated for the purpose (Wahid et al., 1988). After dispensing, the radioactivity adhering to the inner walls of the access tube was washed down with a jet of
about ! 5 mL water. The total radioactivity applied per plant was 1.25 mCi (46.25 MBq). Carrier in the 32p solution was required to reduce the soil fixation of the applied radio-label (IAEA, 1975).
Leaf sampling and radioassay The most recently matured leaves were sampled separately for radioassay from the treated plant and from
192 those surrounding it in each experimental unit. Leaf samples were collected twice, at 15 and 30 days after 32p application. The samples were dried at 75 °C, wet digested (HNO3 and HC104) and the digests were radioassayed for 32p employing Cerenkov counting technique (Wahid et al., 1985). During the experiment, the Cerenkov counting efficiency remained constant and hence the count rates were not converted into absolute activity but were expressed as counts per minute (cpm). The count rates were corrected for background and decay and subjected to log transformation before statistical analysis (analysis of variance).
Results and discussion
Recovery of 32p in the leaf as a function of depth and lateral distance of 32P application Data on radioactivity recovered in the leaves of four tree species following 32p placement at different lateral distances and depths are given in Table 2. Considerable differences were observed for 32p absorption in relation to the lateral distance of placement. The quantity of 3ap absorbed by the trees increased with time as was evident from the greater recovery of 32p in the leaves at 30 days after application of the radio-label. The tree species differed in the pattern of absorption of 32p from the two lateral distances of application (25 and 50 cm from the pianO. Since the experiment was conducted during the monsoon season when soil moisture was not limiting, the extent of absorption of 32p could be considered to reflect the amount of root activity (Wahid et al., 1989). Based on this reasoning, the data showed that both in Acacia and Casuarina, the root activity was more concentrated at a lateral distance of 25 cm than 50 cm. In the other two cases, the differences in root activities at 25 and 50 cm lateral distances were not significant suggesting a uniform distribution of active roots at these distances. However, only Casuarina and Leucaena followed a consistent pattern in this respect (Table 2). In contrast to the lateral distribution, the vertical spread of feeder roots decreased at 50 cm depth in all the tree species studied. Here also, absorption of 32p increased with time. The data showed that for Casuarina and Ailanthus, the surface 15 cm soil layer had three to four times more root activity than in the lower layer (50 cm). In the other two cases namely, Acacia and Leucaena, the differences in root activity between 15 and 50 cm depths were relatively less. In agroforestry,
stratification of roots of different species at different depths is desirable. Casuarina appears to have some of these traits, though a large fraction of its root activity is in the top 15 cm, much of it is concentrated near the trunk (Table 2). Although direct measurement of root density/distributron would be desirable to make positive conclusions in this regard, several workers (Nye and Tinker, 1977; Vose, 1980) have suggested that using 32p would be a precise method for characterizing root interactions. Moreover, this tehnique was adopted by many for characterising root activity patterns of tree crops such as cacao (Theobroma cacao L.; Wahid et al., 1988), cashew (Anacardium occidentale L.; Wahid et al., 1989) and intercropping systems (Ashokan et al., 1988). Studies using 3Zp on agroforestry systems involving fast growing multipurpose tree species are, however, rare. Root excavation studies with tree species are also limited. Dhyani et al. (1990) studied the root systems of five agroforestry trees (28-months old) following the excavation method. They observed that the maximum lateral spread of fine roots ranged from 95 to 135 cm and length of tap root from 0.17 to 1.2 m, albeit bulk of the roots was concentrated near the surface. In another excavation study aimed to characterise the root system morphology of 2 year-old poplar clones, Friend et al. (1991), observed marked variations in the horizontal spread of roots. A distinction may, however, be made between excavation studies and isotopic studies of the type reported here. In general, the former method attempts to characterise the morphology/biomass accumulation in the coarse roots (structural) as opposed to the characterization of fine root (physiologically active) activity in the latter. Interpretation of the present data, however, is limited by the fact that the fine roots are in a state of constant flux, with death and replacement taking place simultaneously. Many tree species are reported to exhibit marked temporal variations in spatial distribution of fine root mass (Persson, 1983; Srivasthava et al., 1986) depending on edaphic and climatic factors. Therefore, seasonal variations in the root activity patterns of tree species included in the present study are possible. However, since the study was carried out during the rainy season when soil moisture availability was not limiting, biomass allocation to fine roots is presumably at its peak. Therefore, root activity characterization at this time may probably represent root interactions of the largest possible magnitude.
193
Table 2. Radioactivity (log cpm g - l ) recovered in the leaves of tree species at 15 and 30 days after application of 32p to the soil 32p
Acacia
Casuarina
Leucaena
Ailanthus
placement 15
30
15
30
15
30
15
30
2.021 (104.9) 1.977 (94.8) 0.0497 NS
2.353 (224.9) 2,224 (167.5) 0.0385 0.1095
1.487 (30.69) 1.180 (15.13) 0.0680 0.1935
1.892 (77.98) 1.541 (34.75) 0.0643 0.1829
1.916 (82.41) 1.880 (150.7) 0.0677 NS
2.405 (254.1) 2.388 (385.5) 0.0683 NS
1.684 (48.3) 1.255 (18.0) 0.0703 0.2003
1.907 (80.7) 1.889 (77.4) 0.0823 NS
2.162 (145,2) 1.836 (68.5) 0.0497 0.1414
2.483 (304.1) 2.093 (123.9) 0.0385 0.1095
1.780 1.910 (60.25) ( 8 1 . 2 8 ) 0.886 1.523 (7.69) (33,34) 0.0680 0.0643 0.1935 0.1829
2,178 (150.7) 1.618 (41.5) 0.0667 0.1898
2.586 (385.5) 2.207 (161.1) 0.0683 0.1943
1.913 (81.8) 1.027 (10.6) 0.0703 0.2003
2.235 (171.8) 1.561 (36.4) 0.0823 0.1829
Lateral distance (cm) 25 50 SEM (~) CD (0.05)
Depth (cm) 15 50 SEM (-4-) CD (0.05)
Retransformed values given in parantheses; NS-Non-significant, SEM-Standard error of mean, CD-Critical difference.
Table 3. Radioactivity (tog cpm g - 1) recovered in the leaves of tree species grown in association with grass species at 15 and 30 days after application of 32p to the soil Acacia
Casuarina
Leucaena
Ailanthus
Grass crops
Congo signal Guinea grass Hybrid napier Teosinte Control (no grass crop) SEM (+) CD (0.05)
15
30
15
30
15
30
15
30
1.929 (84.9) 1.977 (94.8) 1.965 (92.2) 1.971 (93.5) 2,155 (142.9) 0.0785 NS
2.324 (210.9) 2.249 (177.4) 2.265 (162.5) 2.211 (162.50) 2.392 (246.6) 0.0608 NS
1.482 (30.3) 1.360 (22.9) 1.204 (16.0) 1.098 (12.5) 1.523 (33.3) 0.1075 0.3072
1.666 (46.3) 1.981 (95.7) 1.948 (88,7) 1,948 (88.7) 1.305 (20.2) 0.1017 0.2907
1.919 (83.0) 1.918 (82.8) 1.793 (62.1) 1.990 (97,7) 1.871 (74.3) 0.1054 NS
2.611 (408.3) 2.318 (208.0) 2.328 (212.8) 2.562 (364,7) 2.163 (145.5) 0.1301 0.3720
1.545 (35.1) 1.433 (27,1) 1.366 (23.2) 1.685 (48,4) 1.318 (20.8) 0.1122 NS
2.092 (123.6) 1.606 (40.4) 1.848 (70.5) 1,997 (99,3) 1.947 (88.5) 0.1301 NS
Retransformed values given in parentheses. NS-Non-significant, SEM-Standard error of mean, CD-Critical difference.
Absorption of applied 32p by tree species as influenced by the component grass species A c o m p a r i s o n o f the absorption o f applied 32p by different tree species as influenced by the associated grass species is g i v e n in Table 3. Trees differed in the absorp-
tion o f applied radiophosphorus d e p e n d i n g on the grass species g r o w n in association with them. A b s o r p t i o n o f r a d i o p h o s p h o r u s by Acacia and Ailanthus was not influenced by the associated crop. This s h o w s that as far as these two tree species are c o n c e r n e d , the grass species tested in this study did not exert any c o m p e t -
194 itive or complementary effects. With the other two trees, however, considerable differences existed in the absorption of 32p depending on the associated grass species. Casuarina and Leucaena when grown mixed with any one of the four grass species, showed higher uptake of 32p compared to their respective sole-crop situations. Absorption of 32p by Casuarina was highest when grown mixed with guinea grass. Leucaena, on the other hand, showed greater absorption of 32p when grown mixed with teosinte or congo signal than when grown with guinea grass or hybrid napier. These results suggest that both Casuarina and Leucaena are benefited by mixing with grasses although no clear trend was discernible with respect to the grass (interspecific) effects. Higher proportion of plant cycling suggested by Nair (1984) because of the increased plant cover and root density might be responsible for this observed favourable effect of the mixed species systems on nutrient uptake. In this context, Nair (1984) reported that transport of nutrients below the rooting zone, a major avenue for the loss of nutrients in sedentary agriculture, can be considerably reduced in mixed species systems where the total volume of root exploitation will be larger and consequently the amounts of nutrient loss less. Although this may be more applicable for mobile soil nutrients such as N and K, it might hold good for P as well, because of the P reversion mechanisms operating in soil. Nonetheless, Ailanthus, possibly because of its poor growth and low root biomass accumulation (Table 1), did not exhibit such a beneficial interaction. Lack of a positive influence on 32p uptake in the acacia based cropping system can be explained by the general depressing effect of acacia on understorey growth owing to its dense foliage and rapid growth rate. Such a differential response in 32p uptake depending on the growth habit of the component crops in an intercropping system was reported by Ashokan et al. (1988) also. Although strictly not comparable with the present species-mix, they observed a decrease in 32p uptake by elephant foot yam (AmorphophaUus compnulatus Blume) when it was grown in association with banana (Musa (AAB) 'Mysore') and/or cassava (Manihot esculenta Crantz). However, for banana, an increase in 32p uptake was observed when it was mixed with elephant foot yam or cassava, indicating competitive and/or complementary interactions in 32p uptake depending on the nature of the associated crop species. The results of the present study also suggest the possibility of such competitive and/or complementary inter-
actions in agroforestry depending on the nature of the woody perennial components. The differences in 32p absorption could also be argued to be due to the differences in the spacing followed for the different grass species. Among the grass species, wider spaces were adopted for hybrid napier and congo signal than for the other two species. This would mean that the influence of hybrid napier and congo signal roots would be less felt by the trees than that of guinea grass or teosinte. However, such a distinction was not apparent from the results suggesting that differences in 32p absorption by the trees were not due to the variable spacing adopted for grass species.
Absorption of applied 32p by grass species as influenced by tree components The data relating to the absorption of 32p by the grass species when grown mixed with tree species are given in Table 4. A comparison of the foliar 32p content of the sole crop with that of the mixed crop revealed lower absorption of 32p by grass species when grown in association with any of the tree crops tested in the study. Absorption of 32p by grass species was greater in the sole crop situations. In general, Acacia and Leucaena exhibited lower foliar 32p content of the component grass crops. Clearly these trees competed with the understorey grass species due to the surface concentration of their root activity (Table 2). In Casuarina, however, root activity was more concentrated within the 25 cm lateral distance and, therefore, competitive interactions were of a lower magnitude. It recorded the highest 32p recovery for hybrid napier (30 days after application) and guinea grass (15 days after application). Further it recorded the second highest value for these two crops at 30 and 15 days after application respectively. Therefore, these grass species in association with Casuarina were found promising. The results, therefore, suggest both competitive and/or complementary interactions in nutrient uptake depending on root morphology of the tree species grown in association. Severe reduction in the nutrient uptake and yield of the associated crop can be expected if the tree component of the system has a shallow spreading root system. Possibly competition for other resounces such as water may be implied. Regarding the understorey species evaluated in the present study, however, the differences in uptake pattern did not follow a predictable pattern. Probably the tolerance (shade) relationships of the understorey species are implicated.
195 Table 4. Radioactivity(log cpm g- ]) recovered fromthe leaves of grass speciesgrownin associationwith fourmultipurpose tree species, at 15 and 30 days after applicationof 32p to the soil
Congo signal
Guineagrass
Hybrid napier
Teosinte
Tree species
Acacia Casuarina Leucaena Alianthus
Control (no tree crop) SEM (q-) CD (0.05)
15
30
15
30
15
30
15
30
3.188 (1541.7) 2.884 (765.6) 2.845 (699.8) 2.920 (831.8) 3.210 (1621.8) 0.0856 0.2446
3.300 (1995.9) 2.249 (177.4) 2.716 (520.0) 3.325 (2113.5) 4,071 (11776.1) 0.1231 0.3518
2.993 (984.0) 3.762 (5781.0) 3.204 (1599.5) 3.389 (2449.1) 3.730 (5370.3) 0.0796 0.2275
3.327 (2123.2) 3.314 (2060.6) 3.280 (1905.4) 3.357 (2275.1) 3.990 (9772.3) 0.1354 0.3870
3.280 (1905.5) 3.409 (2564.5) 3.082 (1207.8) 3.422 (2642.4) 3.531 (3396.2) 0.1181 NS
3.623 (4197.6) 4.266 (18450.1) 3.635 (4315.2) 3.500 (3162.3) 3.944 (8790.2) 0.1083 0.3095
2.324 (210.9) 2.187 (153.8) 1.900 (79.4) 2.251 (178.2) 2.904 (801.7) 0.1099 0.3141
2.708 (510.5) 2.545 (350.7) 2.693 (493.2) 2.927 (845.3) 3.619 (4159.l) 0.1541 0.4404
Retransformed valuesgiven in parentheses. NS-Non-significant,SEM-Standarderror of mean, CD-Criticaldifference.
Conclusions The results of the present study suggest the nature and magnitude of root competition occurring in tree-grass systems, during the absorption of P and perhaps other nutrients as well. Most of the absorbing roots are concentrated near the trunk of the tree for Casuarina suggesting its desirability for silvopastoralism. In general, grass species did not adversely affect the absorption of 32p by the trees tested in this study. They, however, exerted a complementary effect on 32p absorption by Casuarina. Casuarina was most benefited when grown in association with guinea grass, and Leucaena, when grown with congo signal or teosinte. Furthermore, in the tree-grass cropping systems tested, it was always the tree species that dominated, often suppressing the absorptron of 32p by the grass components.
Acknowledgements The authors are grateful to Dr C C Abraham, former Special Officer, College of Forestry and Dr P Ramachandran, Professor and Head of the Livestock Research Station, Thiruvazhamkunnu and Dr S Chinnmani, former Assistant Director General (Agroforestry), Indian Council of Agricultural Research for the various facilities provided. Dr K V Suresh Babu, Mr Thomas Mathew and Mr K Umamaheswaran, who
were involved in the initial layout of the experiment are also gratefully acknowledged.
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