W. A. J. M. De Costa

Agronomic regeneration of soil fertility in tropical Asian smallholder uplands for sustainable food production

Smallholder cropping systems are a vital component in the agricultural sectors of tropical Asia, especially in the developing nations. These systems are important for producing food and providing a livelihood to a major proportion of the populations of Asia. While the rice systems, usually cultivated under puddled conditions are considered stable, the upland or highland units, which provide a range of food, fibre and feed commodities, are generally situated on marginal lands. Low soil fertility, erosion, sub-optimal crop management and subsistence farming conditions characterize these units. Hence, these units require improved but low cost strategies to reverse the trend of lower productivity, loss of sustainability and most importantly, the loss of livelihood for the farming populations. Among the many strategies available, the regeneration of soil fertility through agronomic measures would be a useful and easily adoptable method of enhancing productivity of tropical Asian upland small-holding farming units. The methods available include the revival of age-old traditions such as adding organic matter directly to soils, green manuring and mimicking forest ecosystems through agrofor-estry. Agronomic measures using plants to enrich the soil and regenerate its supporting capacity are presented in the present paper. Relevant examples are cited to highlight the potential of these methods for regenerating productivity of the upland smallholder farming units and more importantly the agro-based livelihood of a large majority of the populations of tropical Asia.

Factors controlling transpiration of mature field-grown tea and its relationship with yield.

The objective of this experiment was to determine the factors influencing the transpiration rates of mature, clonal tea (Camellia sinensis L.) and estimate its transpiration efficiency. The heat pulse technique was used to measure transpiration rates of tea plants growing in the field as part of extensive canopies at Talawakelle, Sri Lanka during the period between 1 January and 19 February 1997. Irrigation and shading treatments were used to determine the influence of soil water content (S) and irradiance on transpiration rate. The transpiration rate declined only slightly when S decreased from field capacity (44%) to 33%. However, when S declined below 33%, the transpiration rate showed a rapid decline from 1.6 to 0.7 l per plant per day at 15% S. When S was near field capacity, maximum transpiration rates of 0.53‐0.93 l plant 1 h 1 occurred between 1000 and 1500 h. The corresponding maxima when theS was near permanent wilting point (i.e. at 1.5 MPa matric potential) were 0.27‐0.53 l plant 1 h 1 . Transpiration decreased linearly with decreasing irradiance throughout the range of radiation levels tested (i.e. from 100 to 15% of full sunlight) at a rate of 0.031 l per plant per day per % reduction in solar irradiance. The daily transpiration rates of tea plants (0.42‐1.07 l per plant per day) under the natural shade of Grevillea robusta were considerably lower than the value of tea plants in the open, 3.511 l per plant per day. Spraying of an antitranspirant, Kaolin, decreased canopy temperature by 2‐4C and especially around mid-day. Kaolin also decreased transpiration slightly during the period between 1000 and 1500 h. Transpiration efficiency ( TE) was 9.637 kg ha 1 (made tea) mm 1 of water transpired. The relationship between total dry matter yield and the ratio between transpiration and mean saturation vapour pressure deficit also was linear with a proportionality constant of 6.9 g kg 1 kPa.

Tree-crop interactions in hedgerow intercropping with different tree species and tea in Sri Lanka: 2. Soil and plant nutrients

The primary objective of the present study was to determine the capacity for soil fertility improvement in hedgerow intercropping systems (HIs) involving six different hedgerow species (Calliandra calothyrsus Meissner, Senna [Cassia] spectabilis (DC.) H. Irwin and Barneby, Euphatorium innulifolium (R.M. King and H. Rob.) H.B.K., Flemingia congesta Aiton F., Gliricidia sepium (Jacq.) (Kunth.) Walp. and Tithonia diversifolia (Hemsl.) A. Gray) and tea (Camellia sinensis (L.) O. Kuntz) on sloping terrain in the high rainfall zones of Sri Lanka. Incorporation of hedgerows and addition of their prunings as mulch decreased topsoil bulk density and increased organic matter content and cation exchange capacity. In the majority of mulched HIs, most of the essential soil nutrients, with the exception of phosphorus, were higher than in sole tea crops. In HIs involving all tree species, the mulched treatment had significantly greater soil nutrient contents than the respective unmulched treatment. When prunings were not added as mulch, there was evidence of competition for nutrients between the tea bushes and hedgerow species. The amounts of nutrients added through hedgerow prunings exceeded the recommended K requirement of tea, but were lower than the N and P requirements. In a majority of HIs, the amounts of nutrients exported as tea yield were lower than in the control under both mulched and unmulched conditions. It is concluded that incorporation of contour hedgerows has the potential to regenerate soil fertility and sustain tea yields on sloping terrain in Sri Lanka, when prunings are added as mulch.

Tree-crop interactions in hedgerow intercropping with different tree species and tea in Sri Lanka: 1. Production and resource competition

Incorporation of tree hedges along contours has been proposed as a means of reducing soil erosion and increasing soil fertility of tea (Camellia sinensis (L.) O. Kuntz) plantations on sloping terrain in high-rainfall zones of Sri Lanka. Tea yields in these hedgerow intercrops are determined by the balance between the positive (i.e., increased soil fertility) and negative (i.e., resource competition) effects of hedgerows. Therefore, the objectives of the present study were to determine: (a) the net effect of six different hedgerow species (Calliandra calothyrsus Meissner, Senna [Cassia] spectabilis (DC.) H. Irwin and Barneby, Euphatorium innulifolium (R.M. King and H. Rob.) H.B.K., Flemingia congesta Aiton F., Gliricidia sepium (Jacq.) (Kunth.) Walp. and Tithonia diversifolia (Hemsl.) A. Gray) on long-term tea yields; (b) the effect of incorporating hedgerow prunings as mulch; and (c) the relationship between hedgerow characteristics (i.e., biomass production, canopy and root growth) and tea yield variation. Tea yields, measured over one complete pruning cycle from October 1998 to September 2001, showed reductions relative to a sole tea crop under all hedgerow species except Euphatorium. The yield reductions ranged from 22 to 40%. Tea yields under Euphatorium showed increases up to 23% relative to the sole crop control. Addition of hedgerow prunings as mulch increased tea yields in all hedgerow intercrops. The yield increases ranged from 11 to 20%, with the highest being under Euphatorium. Tea yields showed a negative relationship (R2=0.38) with the pruned biomass of hedgerows. Limitation of environmental resources (e.g., water and light) and hedgerow characters which intensified resource competition (i.e., greater canopy lateral spread and height and greater root length densities, especially in the top soil layer) were responsible for observed tea yield reductions in hedgerow intercrops.

The physiological basis of increased biomass partitioning to roots upon nitrogen deprivation in young clonal tea (Camellia sinensis (L.) O. Kuntz)

Deprivation of nitrogen (N) increases assimilate partitioning towards roots at the expense of that to shoots. This study was done to determine the physiological basis of increased root growth of tea (sCammellia sinensis L.) under N shortage. Nine-month-old clonal tea (clone TRI2025) was grown in quartz sand under naturally lit glasshouse conditions. Three levels of N (0, 3.75 and 7.5 mM N) were incorporated in to the nutrient solution and applied daily. Plant growth, photosynthesis, root respiration and plant N contents were measured at 10-day intervals over a 45-day period. Root dry weight showed a sharp increase during the first 15 days after the plants were transferred to 0 mM N, whereas no such increase was shown in plants transferred to 7.5 mM N. In contrast, shoot dry weight increased at 7.5 mM N and was significantly greater than at 0 mM N, where no increase was observed. Due to the above changes, root weight ratio increased and leaf weight ratio decreased during the first 15 days of N deprivation. Leaf photosynthetic rates did not vary between N levels during the initial 15-day period. Thereafter, photosynthetic rates were greater at 7.5 mM and 3.75 mM N than at 0 mM N. Root respiration rate decreased at 0 mM N, whereas it increased at 3.75 and 7.5 mM N, probably because of the greater respiratory cost for nitrate uptake. Root respiratory costs associated with maintenance (Rm) and nitrate uptake (Ru) were calculated to investigate whether the sharp increase of root growth observed upon nitrogen deprivation was solely due to the reduced respiratory costs for nitrate uptake. The estimated values for Rm and Ru were 3.241 × 10−4 μmol CO2 g−1 (root dry matter) s−1 and 0.64 mol CO2 (mol N)−1, respectively. Calculations showed that decreased respiratory costs for nitrate uptake could not solely account for the significant increase of root biomass upon N deprivation. Therefore, it is concluded that a significant shift in assimilate partitioning towards roots occurs immediately following N deprivation in tea.

Decomposition and nutrient loss from prunings of different contour hedgerow species in tea plantations in the sloping highlands of Sri Lanka

Contour hedgerows of multipurpose tree species in the sloping tea lands of Sri Lanka are expected to reduce soil erosion and also add significant amounts of plant nutrients to the soil via periodic prunings. The objective of this experiment was to characterize the biomass decomposition pattern and quantify the amount of nutrients added through prunings of six tree species (Calliandra calothyrsus, Senna spectabilis, Euphatorium innulifolium, Flemingia congesta, Gliricidia sepium and Tithonia diversifolia) currently being used in hedgerows associated with tea. Withered leaf and stem prunings (50 g) were enclosed in 2-mm litter bags, placed at 5-cm depth and retrieved after one, three, six, nine and 12 weeks. Loss of initial dry weight, N, P and K was measured. Single exponential decay function adequately described both dry weight and nutrient loss. Tree species differed significantly in their rate of breakdown with decomposition constants (k) varying from 0.0299 to 0.2006 week−1 for leaves and from 0.0225 to 0.0633 week−1 for stems. Gliricidia showed the highest k for leaves with the rest in the following descending order: Senna > Tithonia ≥ Euphatorium > Calliandra > Flemingia. A similar pattern was observed for loss of all nutrients with Calliandra and Flemingia always having lower k values than the rest. Although N immobilization was not observed, immobilization of P and K was observed during the first week of incubation in some species, particularly in stem prunings. Annual biomass of prunings differed significantly between tree species in the following descending order: Calliandra > Senna > Flemingia > Tithonia > Gliricidia > Euphatorium. Calliandra added the greatest amount of nutrients annually to the soil with Euphatorium adding the least. Calliandra prunings provided the annual total K requirement and 49% of the N requirement of mature tea. However, none of the species provided more than 5% of the P requirement. It is concluded that among the tree species tested, Calliandra and Flemingia are the most suitable for contour hedgerows in tea plantations of this agroclimatic region because of their higher soil nutrient enrichment capacity and slower decomposition rates which would minimize leaching losses.

GENOTYPIC VARIATION IN CANOPY PHOTOSYNTHESIS, LEAF GAS EXCHANGE CHARACTERISTICS AND THEIR RESPONSE TO TAPPING IN RUBBER ( HEVEA BRASILIENSIS )

The main objective of this study was to quantify the genotypic variation of photosynthetic and gas exchange parameters of Hevea and to examine their relationships to dry rubber yield and its components. Canopy photosynthesis (P c ) of the genotype RRISL 211 was 20 % greater than that of RRIC 121. This was primarily due to RRISL 211s greater light-saturated leaf photosynthetic rates and a greater leaf area index in the top canopy stratum. Tapping significantly increased P c in RRISL 211, but not in RRIC 121. The genotypic variation in photosynthetic capacity was not reflected in the overall dry rubber yield, which did not differ between the two genotypes. However, analysis of yield components showed that while RRISL 211 may have preferentially partitioned a greater proportion of its additional assimilates to increasing the latex volume and extending the root system, RRIC 121 partitioned more assimilates to increasing its dry rubber content through greater biosynthesis of rubber. The higher plugging index and the greater post-tapping girth increment of RRIC 121 were probably responsible for observed increases in its dark respiration following tapping. Although RRISL 211 had a greater transpiration efficiency, this did not provide a yield advantage as the trees were growing in an environment with adequate rainfall throughout the year.

Resource competition in contour hedgerow intercropping systems involving different shrub species with mature and young tea on sloping highlands in Sri Lanka

Tea (Camellia sinensis) in Sri Lanka is grown predominantly on sloping highlands. Incorporating trees as hedgerows along contours is aimed at reducing erosion and improving soil fertility through addition of prunings as mulch. However, there could be significant competition for essential growth resources between the hedgerows and tea. The primary objective of this study was to determine the influence of six hedgerow species (Calliandra calothyrsus, Senna [Cassia] speclabilis, Eupatorium inulifolium, Flemingia congesta, Gliricidia sepium and Tithonia diversifolia) on mature (6-year-old) and young (6-month-old) tea. This study had three on-farm, long-term (from Nov 1998 to Dec 2002) field experiments. Experiment 1 had 12 treatment combinations with the six shrub species and two mulching treatments (i.e. hedgerow prunings added to the tea plot as a mulch, and unmulched) on mature tea plus a sole tea crop as control. The same was repeated on young tea in Experiment 2 to determine whether resource competition on young tea was greater than that on mature tea. Experiment 3 examined the effects of removing tree root competition on tea by cutting a 1-m deep trench between hedgerows and tea. In all experiments, total tea yields of hedgerow intercrops, cumulated over the 50-month experimental period, were significantly lower (by 3-50%) than sole tea crops, thus indicating significant resource competition except in the case of mulched hedgerow systems involving Eipatorium and mature tea (18% yield increase). Removal of below-ground competition significantly increased tea yields by 11-19%. Addition of hedgerow prunings as a mulch significantly increased yields of both mature and young tea by 13-21 %. Tea yields of hedgerow systems with Calliandra, Flemingia and Eupatorium showed greater yield reductions in young tea than in mature tea, but the opposite was shown with other hedgerow species. There was a significant negative linear relationship between tea yield and pruned biomass of hedgerows. Tea yields of all experiments showed significant negative correlations with several hedgerow characters, which are indicators of their competitive ability. These included hedgerow root density, canopy lateral spread, height and cross-section. In a majority of hedgerow systems, the available phosphorus content of topsoil (0--20 cm depth) was up to 51 % lower compared to sole crops. It is concluded that incorporation of contour hedgerows in to an existing tea crop could result in significant resource competition with tea and thereby cause tea yield to decrease. However, there is scope for selection of hedgerow species that minimize competition through spatial and temporal complementarity with tea in resource capture.

Growth, root adaptations, phosphorus and potassium nutrition of rice when grown under the co-limitations of phosphorus, potassium and moisture

ABSTRACT Growth of rice when soil phosphorus (P), potassium (K), and moisture are co-limiting is poorly known. An experiment was conducted using two rice varieties [long (LD) and short (SD) duration], two moisture conditions (continuous flooding and top-soil drained after flowering), and five fertilizer treatments (with and without the applications of P and/or K). Phosphorus uptake rate, translocation of P and K from roots, and retranslocation of P and K from senescing leaves were reduced in drained soil compared with flooded condition. LD had greater- root length, root dry weight, specific root length (m g−1), and increased availability of P in soil than SD. Even though LD was better adapted to co-limitations of P, K, and moisture in soil than SD, those adaptive responses were not sufficient to increase the arrested growth, P and K nutrition when P, K and moisture supply was co-limited than their optimal supply.

Physiological basis of yield variation of tea (Camellia sinensis) during different years of the pruning cycle in the central highlands of Sri Lanka.

The objective of this study was to elucidate the physiological basis of the significant yield decline that occurs during the fourth year of the pruning cycle of tea. Biomass partitioning, which was hypothesized to be a major factor in causing this yield decline, was measured by destructive harvests of entire tea bushes, in two contrasting, mature, field-grown tea cultivars (TRI 2025 and DT1) at the end of different years of the pruning cycle. In both cultivars, yield showed continuous increases from year 1 to 3, followed by reductions of 44% and 35% in TRI2025 and DT1 respectively in the fourth year. Patterns of biomass partitioning to roots, stems or branches did not correlate with the above yield variation whereas harvest index, canopy leaf area index and mature leaf dry weight showed variations which paralleled the yield variation. The fourth-year decline in harvest index was brought about by reductions in both shoot number per m 2 and mean individual shoot weight, which indicate a reduction in sink strength. Both cultivars showed reductions in light-saturated photosynthetic rate of maintenance foliage during the second half of the pruning cycle, indicating reduced source capacity. Hence, a combined reduction of both sink strength and source capacity during the fourth year could have brought about the significant yield reduction in tea. A significant increase of root starch in the fourth year indicated a down-regulation of physiological activities of the bush towards the end of the pruning cycle. Mechanisms responsible for this down-regulation need to be elucidated by further research.