Performance of wheat ( Triticum aestivum L . ) crop under different spacings of trees and fertility levels

A field experiment was laid out at Pantnagar (Uttarakhand, India) in Mollisols during rabi season (200910) to study effect of spacing and fertility levels on growth and yield of wheat (Triticum aestivum L.) under different tree species in Terai region. The experiment was conducted in split-split plot design comprising four tree species in main plots, four spacing treatments in sub plots and four fertility levels in sub-sub plot with three replications. The wheat crop var. PBW-502 was sown on November 29, 2009 and harvested on April 21, 2010. Significant higher grain yield was recorded under Poplar (44.60 qha -1 ); however, it was statistically at par with Melia (42.60 qha -1 ) interface at 180-60-40 kg NPK ha -1 fertility level. At 3 × 2.5 m, the wheat growth, yield attributes and yield (grain, straw and biological yield) under Poplar was significantly higher than closer spacing. Application of 180-60-40 kg NPK ha -1 had significant effect on crop growth and grain yield than other levels of fertility. The correlation coefficient (r) studies exhibited that wheat growth and yield attributing characteristics are significantly (p<0.05) and positively correlated with each other.


INTRODUCTION
The pressure from increasing population and urbanization, coupled with land degradation and climate change are the major causes for food insufficiency in developing world.Among different approaches to combat this problem, woody perennial based production systems has the great potential.Historically, agroforestry in India involved two distinct pathways, viz., growing food crops in the forests and establishing tree-crop production systems on arable lands.Agroforestry systems not only arrest land degradation but also improve site productivity through interactions among trees, soil, crops, and livestock (Kumar, 2006).This is the most important way to practice agriculture without deteriorating agrodiseases and environmental degradation is highly appreciable (Garrity, 2004).
Wheat (Triticum aestivum L.) is the most important food crop under agroforestry system in North Indian states, which accounted 88.31 million tones production in 2011-12.In India, it is widely intercropped cereal crop during rabi season (November-April) with Poplar, Eucalyptus and other fast growing short rotation tree species in Uttarakhand, Punjab, Haryana, U.P and Bihar states in north-and-parts of central and eastern states of M.P, Chhatisgarh and W.B. The micro-climate under agroforestry is modified by trees, under such conditions;ecosystems.Its role in the light of combating hunger, the growth response of under story wheat crop may be different from sole cropping system.The wheat production technology in Indo-Gangetic plains is well established but it may require some refinement in technology in mixed land-use systems, like agroforestry particularly nutrient management aspect, where wheat is grown in association with trees.Agroforestry systems have more than two components, which makes it productive and complex in nature as well.The fundamental challenge is therefore to develop a farming system that will be adopted by the farmers.The dynamic nature of nutrient cycling is one of the obstacles in nutrient management in agroforestry systems.It dictates that soil nutrient capital useful for supplying nutrients for plant growth must be equated with short to medium-term, rolling capital (the monthly or annual salary), rather than long-term reserves (gold in the bank).The role of organics is varied and complex, the challenge is to use organics of differing quality in combination with inorganic fertilizers to optimize nutrient availability to plants.A systematic framework for investigating the use of inorganic nutrient sources includes assessment of the fertilizer equivalency value for determining optimal use of nutrient sources.The desired outcome is tools that can be used by researchers and farmers for assessing options of using scarce resource for maintaining soil fertility and improving crop yields.
Usually farmers grow multipurpose tree species on their farmlands to meet their requirements.But the selection of tree species, spacing and the fertility levels are very important to reduce negative tree-crop interactions.Reduction in the yield of annuals arises either due to selection of non-compatible agricultural crop or improper tree spacing.Since no proper spacing as yet has been standardized in different agroforestry systems to avoid the adverse effect of trees on growth and yield of intercrops, therefore, there is a need to determine proper tree spacing for intercropping in agroforestry systems.

MATERIALS AND METHODS
The field experiment was conducted at Agroforestry research center, G. B. Pant University of Agriculture and Technology, Pantnagar, Uttarakhand (29°N Latitude, 79° 30' E longitude and at an altitude of 243.84 masl) during 2009-2010.The plot comprised silty-clay-loam soil with 1.2% of organic carbon, 227, 23 and 230 kg ha -1 available nitrogen, phosphorus and potassium, respectively.The site is characterized by a humid sub-tropical, cold and hot dry summers with 1400 mm mean annual rainfall, of which 80 to 90% is received between June and September.The remaining 10 to 20% rainfall is received during wheat-growing season (November to April).Soybean-wheat crop rotation was followed under short rotation fast growing tree species.
Leaf area was recorded at anthesis (90 DAS) by an area meter and converted to leaf area index (LAI) with dividing total leaf area by land area of the sample.The plant height and the dry matter accumulation were recorded at maturity stage (142 DAS).The number of spikes per meter row length, ear length, spikelets spike -1 , number of grains spike -1 and 1000-seed weight was recorded at physiological maturity.The net plots were harvested to obtain grain/ seed, straw and biological yield.Harvest index was calculated as the ratio of grain to total biological yield.
Data obtained during the course of this investigation, was analyzed by using standard statistical procedure for split-split plot design with the help of computer for analysis of variance (ANOVA) technique (Snedecor and Cochran, 1967).Standard error of mean (SEm±) were computed in each case.The differences among treatments were compared by applying "F" test of significance at 5% probability.Correlation studies (Panse and Sukhatme, 1978) were also performed to study the inter-relationship between various parameters.

Wheat growth
The wheat growth was significantly varied with respect to different tree species at different spacing and fertility levels (Table 1).The leaf area index (LAI) was significantly higher (3.36) under Poplar interface.The LAI increased significantly with widening spacing of all tree species.It was significantly higher (3.18) at 3 × 2.5 m spacing.LAI was significantly increased from 2.30 to 3. 38 and 4.45 as nitrogen dose increases from 120 to 150 and 180 kg ha -1 , respectively.The tallest crop individuals were recorded under Poplar (81.6 cm); however, it was statistically at par with Melia (80.4 cm) interface.The wider spacing (3 × 2.5 m) contributed to produce taller (81.9 cm) crop plants.The crop height was significantly higher at plots fertilized with 180-60-40 kg NPK ha -1 , whereas, it was statistically at par with 150-60-40 kg NPK ha -1 fertilized plots.Dry matter accumulation followed same pattern of the plant height and LAI.The interaction between tree species and fertility level for LAI was also found to be significant, whereas, tree species and their spacing interaction were found to be non-significant.The LAI was significantly higher (5.22) under Poplar with 180-60-40 kg NPK ha -1 fertility level (Figure 1).The interaction among factors was non-significant for plant height and dry matter production.
The crop growth is mainly affected by light and nutrient availability.Leaf litter inputs from agroforestry trees could provide sufficient nutrients and organic matter to sustain crop growth (Lehmann et al., 2002;Bhardwaj et al., 2005).
However, concentrations of foliar nutrients and organic constituents show considerable variation from different tree species.The leucaena added valuable nutritive leaf litter and improved soil properties, which help to improve crop growth.As poplar tree species shed their leaves before sowing of wheat crop, improves soil physicchemical properties and nullified the negative tree-crop interaction for light.Corroborative findings have also been reported by Bhardwaj et al. (2005), Teklay (2004), Patil et al. (2002); Pant (1993) and Sidhu and Hans (1988).The poor wheat growth showed under eucalyptus due to inhibitory effects of allelochemicals.Similar findings were recorded by Fikreyesus et al. (2011) in the case of tomato; Ahmed et al. (2008); Kaushik and Singh (2001) for agricultural rabi crops.Negative tree-crop interactions for light, moisture and nutrients at closer spacing reduces crop growth rate.At closer spacing tree species utilize more resources for their growth and development.Sharma et al. (2000) also reported that close spacing increased amount of leaf litter, which inhibited the crop growth.Corroborative results were reported by Khan and Ehrenerich (1994).Additional supply of inorganic nutrients reduces tree-crop competition for nutrients resulting increase in crop growth.Corroborative results were reported by Ahmed et al. (2002) and Smithson and Giller (2002).
The soil physico-chemical properties improvement and lower down the tree-crop competition for resources resulted that an increases in crop growth and yield.Similar findings were reported by Singh and Sharma (2007).During wheat crop maturity (March-April) poplar start sprouting and shade the crop which decreases light intensity and it becomes one of the limiting factors for reduction in wheat grain yield under poplar interspaces as compared to sole crop (Gill et al., 2009).Apart from nutrient and moisture, light is a major limiting factor for crop growth and yield under tree species.Being an evergreen tree species, eucalyptus reduces light availability and decreased crop yield.Corroborative results were reported by Tripathi et al. (2006), Kumar and Rajput (2003), Kaushik and Singh (2001), Fikreyesus et al. (2011), Willey and Holliday (1971) and Ahmed et al. (2008).Sharma et al. (2001) and Khan and Ehrenerich     (1994) reported that water use of the system increased up to a distance of 6 m from tree line, causes moisture stress to the crop.The tree and crop components compete for nutrients in simultaneous systems, but competition for growth resources is absent or minimal in sequential systems.Inorganic nutrients application may reduce treecrop competition for nutrients.Corroborative results were reported by Yajun et al. (2009), Johannes et al. (2002), Pant (1993) and Steiner et al. (2007).

Correlation study
Correlations between growth, yield components and grain yield under four tree species, spacing and fertility treatments were evaluated for the present study (Table 2).The grain yield was positively significant correlated with crop growth parameters and yield attributes. Straw, biological yield and HI were positively significant correlated with LAI, plant height, dry matter accumulation, ear bearing shoots m -2 , spike length, spikelets per spike and grains per spike.Subhani and Chowdhry (2000) and Attarbashi et al. (2002) reported similar data on these lines.Singh et al. (1995), Rana and Sharma (1997) and Deswal et al. (1996) in wheat also supported these findings.
It may be concluded that the nitrogen fixing deciduous tree species with wider spacing improves growth, yield attributes and yield of wheat.Poplar tree species reduces competition for light with crop by shedding their leaves during crop sowing period which may helps to improve soil properties.
The application of inorganic fertilizers also helps to minimize tree-crop competition for nutrients as added nutrients also utilized by fast growing tree species.

Figure 1 .
Figure 1.Wheat growth influenced by tree species, spacing and fertility levels.

Figure 2 .
Figure 2. Wheat yield attributes influenced by tree species, spacing and fertility levels.

Figure 3 .
Figure 3. Wheat yield influenced by tree species, spacing and fertility levels.

Table 1 .
Effect of tree species, spacing and fertility levels on wheat growth and yield.

Table 2 .
Correlation growth, yields attributes and yield of wheat under agroforestry system.