Introduction

Alley cropping is a system of agroforestry where trees and crops are intercropped. The presence of woody species in the alley cropping production system has been shown to contribute to nutrient recycling, reduction in soil nutrient leaching, stimulation of higher soil faunal activities, soil erosion control, soil fertility improvement and sustained levels of crop production (Kumar et al. 2007 and Kasirayi 2014). In order to minimize the competition, hedgerows are maintained to a suitable height through regular pruning. Clippings are returned to the alleys to serve as nutrient source and as mulch. These can also be an important fodder source.

Arbuscular Mycorrhiza (AM) is the most dominant fungal association in agricultural and horticultural cropping systems. These fungi represent an important component by their ubiquity in the soil microbial biomass C and their direct involvement in essential processes at the plant–soil interface (Harley and Smith 1983; Bagyaraj 1984). The benefits of AM fungi in agricultural ecosystems are now widely known. Increase in P uptake and biomass growth response because of AM fungi is well documented (Bagyaraj 2014a). The increased growth of plants inoculated with AM fungi is not only attributed to improved phosphate uptake but also better availability of other diffusion limited nutrients like Zn and Cu (Bagyaraj 2014a).

The soil microbial biomass C constitutes a transformation matrix for all the natural organic materials in the soil and acts as a labile pool of plant–available nutrients (Jenkinson and Ladd 1981). In general, plants serve as a carbon source for the microbial community and in turn microorganisms provide nutrients for plant growth through mineralization of plant and animal residues, and soil organic matter. Agricultural practices have a great influence on the changes in microbial biomass C and community composition in cultivated soils (Lundquist et al. 1999).There have been many studies which have shown that mycorrhizal colonization allows introduced populations of beneficial soil organisms like Azotobacter, Azospirillum and phosphate solubilizing bacteria to maintain in higher numbers and exert synergistic effects on plant growth (Bagyaraj 2014b).

There are some earlier reports on AM fungal inoculation of crops and/or trees on growth and yield of crop plants grown in alley cropping system (Osonubi and Ekanayake 2003). These studies brought out that inoculation with AM fungi enhance the yield of crop plants like cassava, maize, etc. in the alley cropping system (Oyetunji and Osonubi 2007). Higher levels of mycorrhizal propagules in alley-cropped soils and a possible role of the trees in maintaining these sources of inoculum was reported by Diagne et al. (2001). But studies on the effect of alley cropping on the indigenous AM fungi of microbial biomass in soil is scarce (Mazzarino et al. 1993). Therefore, the present investigation was carried out to determine the dynamics of the indigenous AM fungi and microbial biomass in soil as influenced by alley cropping.

Materials and methods

This experiment was an ongoing project for the last 7 years at the Agricultural Research Station, University of Agricultural Sciences, Chinthamani campus located in the eastern dry zone of Karnataka, India. The location is a semi arid tropic (12.97°N and 77.57°E) with a mean temperature of 24.1 °C, annual rainfall of 885 mm at an altitude of 914 m above sea level. The soil in the region is an alfisol of a fine Kaolinitic, isohyperthermic Typic kanhaplustalfs type with 0.54 % organic C, pH 5.4, and 0.06, 0.001 and 0.02 % available N, P and K respectively. The treatments comprised of three spacings (4, 8 and 12 m) as main plots, two perennial leguminous plant species (Gliricidia—Gliricidia sepium and Leucaena—Leucaena leucocephala) as subplots and three field crops [finger millet—Eleusine coracana (Family: Poaceae), peanut—Arachis hypogaea (Family: Fabaceae) and pigeonpea—Cajanus cajan (Family: Fabaceae)] as sub–sub plot treatments laid out in a split–split plot design with four replications.

Recommended varieties of field crops viz., Indaf-5 of finger millet, JL-24 of peanut and TTB-7 of pigeonpea were used. All the alley crops were sown at the same time. Fertilizers @ of 30, 40 and 25 kg N, P and K/ha in the form of urea, single super phosphate and muriate of potash respectively were added to all the crop plants. The crops also received compost @ 7.5 tonnes/ha before sowing. The soil along with the root bits were collected at a depth of 0–15 cm comprising of Gliricidia and Leucaena alleyed with finger millet, peanut and pigeonpea at the harvest of the crop and analyzed for number of AM fungal spores, per cent mycorrhizal root colonization, number of infective propagules of AM fungi and microbial biomass C.

Microbiological analysis

Collection of root and soil samples

Five plants at random in each plot were uprooted along with the adhering soil. The soil samples thus collected from each plant was pooled, mixed and a composite soil sample was obtained by quaternary technique. A portion of this soil sample was stored at 4 °C for microbiological and enzyme activities. The remaining soil sample was air dried, sieved and used for chemical analysis. Similarly the root samples were obtained by spreading the root system evenly on a paper and cutting the central one cm portion of the root system vertically with scissors. The root segments thus obtained from each plant was stained and observed under stereomicroscope for AM colonization. Fifty grams of each of the replicated soil sample was used for enumeration of AM fungal spores and equal quantities of the replicated soil samples were mixed to make 100 g and this pooled soil sample was used for determining the infective propagules.

AM spores in the rhizospheric soil

The number of AM fungal spores in the rhizospheric soil was determined by wet sieving and decantation procedure as outlined by Gerdemann and Nicolson (1963).

Percent AM root colonization

Root segments were stained with 0.2 % acid fuchsin as per the procedure proposed by Phillips and Hayman (1970). The per cent root colonization was determined following the gridline intersect method proposed by Giovannetti and Mosse (1980).

Infective propagules of AM fungi

Enumeration of infective propagules (I.P.) of AM fungi in the rhizospheric soil was done by the most probable number method. It is based on a series of 10-fold soil dilutions where presence of absence of mycorrhizal colonization is recorded and results given as a probability of the number of infective propagules based on a statistical table (Sieverding 1991).

Microbial biomass C

Microbial biomass C in the rhizospheric soil was estimated following fumigation and extraction method as proposed by Carter (1991).

Statistical analysis

The data collected from the field experiments was subjected to statistical analysis. Arcsin transformations were done as per the procedure given by Snedecor and Cochran (1968) wherever necessary. Two-way analysis of variance was used to analyse the data of all the experiments except alley cropping experiment where the data was analysed as factorial RBD by the procedure outlined by Sundar Raj et al. (1972). The treatment means were separated by the least significant difference test (Little and Hills 1978).

Results

The number of spores in the rhizospheric soil of different alley crops significantly increased with spacing of tree species up to 8 m apart. Maximum number of spores occurred in peanut when grown in between Leucaena and in finger millet when grown between Gliricidia 8 m apart. Crops grown in between trees planted 4 and 12 m apart in general harboured lesser AM spores in their root zone compared to crops grown between trees planted 8 m apart. Among the crop species pigeon pea exerted profound influence on spore numbers recording as high as 442 spores 100 g−1 soil. In tree species Leucaena significantly influenced spore production (397 spores 100 g−1 soil) compared to Gliricidia (321 spores 100 g−1 soil) (Table 1).

Table 1 Spacing of tree species on AM spore numbers (100 g−1 dry soil) in an alley cropping system
Full size table

Maximum number of infective propagules (287 I.P. g−1 soil) occurred in the rhizospheric soil of finger millet grown between Leucaena with a spacing of 12 m and less in case of alley crops grown in between Gliricidia (Table 2). Among the different crop species grown as alley crops, finger millet produced a large number of I.P (99 I.P. g−1 soil) compared to pigeonpea and peanut (26 and 25 I.P. g−1 soil respectively). Among the tree species Leucaena supported a larger number of I.P. (86 g−1 soil) compared to Gliricidia (14 I.P. g−1 soil). Spacing of 12 m between trees supported higher number of AM fungal I.P. compared to 4 and 8 m.

Table 2 Spacing of tree species on the number of AM infective propagules (g−1 dry soil) in the alley copping system in the root region soil of different crops
Full size table

Effect of spacing of tree species on AM colonization in the roots of different alley crops is presented in Table 3. The AM colonization was significantly more in the roots of alley crops grown in between Leucaena planted 12 m apart compared to alley crops grown between tree species planted 4 and 8 m apart. But crops grown between Gliricidia 8 m apart showed higher AM fungal colonization. Among the alley crops, roots of finger millet had more AM root colonization (47.0 %) than the roots of peanut (40.8 %) and pigeonpea (39.8 %). Per cent root colonization was more when alley crops were planted in between Gliricidia (45.0 %) compared to Leucaena (39.6 %). Crops grown between trees spaced 12 m apart exhibited significantly higher mycorrhizal root colonization compared to other two spacing’s.

Table 3 Spacing of tree spp. on AM colonization (%) in an alley cropping system
Full size table

In general, microbial biomass C in the root zone of alley crops was more when they are grown in between tree species planted 12 m apart and significantly less in case of alley crops grown in between tree species planted 4 and 8 m apart (Table 4). Maximum microbial biomass C (842 µg g−1 dry soil) was recorded in the rhizospheric soil of finger millet grown in between Gliricidia planted at 12 m apart and least (177 µg g−1 dry soil) in case of finger millet grown in between Leucaena planted 4 m apart. Among the crop species maximum microbial biomass C was recorded in the root zone of pigeon pea. In case of tree species Gliricidia stimulated soil microbial biomass C significantly compared to Leucaena.

Table 4 Spacing of tree species on soil microbial biomass C (SMB) (µg g−1 dry soil) in an alley cropping system
Full size table

There was a significant increase in finger millet grain yield when grown either between Leucaena or Gliricidia spaced 12 m apart (Table 5). In pigeonpea and peanut higher yield was obtained when grown between Gliricidia planted 8 m apart. Among the three crops, finger millet responded best to alley cropping system compared to pigeon pea and peanut. Gliricidia significantly enhanced the yield of finger millet, peanut and pigeonpea compared to Leucaena.

Table 5 Spacing of tree species on grain yield (kg ha−1) in an alley cropping system
Full size table

Discussion

Number of mycorrhizal spores in the rhizospheric soil of finger millet, peanut and pigeonpea plants significantly increased with spacing of tree spp. up to 8 m apart. Maximum spore numbers occurred in finger millet when grown in between Gliricidia and in peanut grown in between Leucaena 8 m apart. Finger millet, peanut and pigeonpea grown in between Leucaena planted 12 m apart in general harboured more I.P. in their rhizospheric soil compared to crops grown between Gliricidia. AM fungal I.P. was highest in rhizospheric soil of finger millet compared to other crops. Occurrence of higher I.P. numbers of AM fungi in the rhizospheric soil of trees under alley cropping system compared to monocrops has been reported earlier (Nobre et al. 2010). This is probably because of increased root density per volume of soil in alley cropping system which favours the rate of spread of AM fungi.

AM root colonization was more in alley crops grown in between Gliricidia planted 12 m apart and it was highest in peanut. Study conducted at Senegal, where soil sampled from around three leguminous tree species (Acacia nilotica, Acacia tortilis and Prosopis juliflora) in 10-year-old alley-cropping plots at two depths and at two distances from the trunk, showed that mycorrhizal inoculum potential and mycorrhizal colonization of the seedlings was greater in soil collected closer to the tree (Okon et al. 2010). This effect is probably because of the fact that legumes are more dependent on AM fungi than non-legumes (Muleta et al. 2008).

In the present study microbial biomass C increased in the rhizospheric soil of alleyed crops grown in between Gliricidia compared to Leucaena. When tree species are taken into account, it could be seen that Gliricidia that stimulated mycorrhizal colonization also harboured higher soil microbial biomass C in the rhizospheric soil. This suggests that the tree species exert a profound influence on microbial biomass C including the mycorrhizal component in it and that of the alleyed crops. Studies conducted by Seiter et al. (1999) in the alley cropping system with alder as the tree and sweet corn as the crop brought out higher fungal and bacterial biomass C in tree rows which declined with the distance from the trees, which is contradictory to with the results of present study. Higher microbial biomass C in the rhizospheric soil of crops (corn and bean) in alley cropping treatment with trees Erythrina poeppiginna and Gliricidia sepium compared to non-alley crops has been reported (Lee and Jose 2003; Henriksen et al. 2004), which is in conformity with the present results.

In the present study there was a significant increase in finger millet grain yield when grown between Leucaena or Gliricidia at a spacing of 8 and 12 m apart compared to the 4 m between trees. This was also true for pigeon pea and peanut. Statistically finger millet had profound increased grain yield grown under Gliricidia compared to Leucaena. Henriksen et al. (2004) studying the influence of three leguminous trees on the growth and yield of bean grown as an alley crop concluded that Erythrina is the best in increasing the yield of beans and also the microbial biomass in the soil. Palada et al. (2005) investigating alley cropping on vegetables with Leucaena observed higher vegetable yield with reduced fertilizer application, and concluded that alley cropping of vegetables with Leucaena is profitable.

It can be concluded that in alfisol soils, growing finger millet as an alley crop in between Gliricidia spaced 12 m apart improves not only the yield of finger millet but also colonization by AM fungi. Future investigations are needed to understand the mechanism of higher AM colonization in this alley cropping system.