Mulch Inoculation and Placement Influenced Barley ( Hordeum Vulgare ) Growth and Soil Nitrate Levels * 1

Article No.: 022713504 DOI: 10.15580/GJAS.2013.5.022713504 We investigated the effect of point of mulch placement and use of leaf mulches from plants inoculated with rhizobia on growth, nitrogen concentration and content of barley, and soil nitrate and pH changes. Mulches placed on soil surface enhanced barley heights and vegetative biomass in all leaf types used. However, nitrogen concentration was relatively higher in both barley vegetative plants and ears of barley grown in mixed mulches of both inoculated and uninoculated leaves. Mixing mulch types with soil caused a quick nitrate release within the first four weeks, which sharply dropped before week 6. Placing mulches on the soil surface resulted to a gradual nitrate release over the study period. Further, soil pH in all mulch treatments decreased within the first four weeks. Results from this study indicated that point of mulch placement was more effective than rhizobia inoculation of mulch on barley growth, nitrogen concentration and content, soil nitrate and pH changes. The results for nitrate levels revealed that it might be necessary for farmers to understand nitrogen requirements of crop so as to know where to place mulches. Results on pH revealed that care should be taken when mulching crops that are sensitive to small changes in pH. Submitted: 27/02/2013 Accepted: 22/05/2013 Published: 29/05/2013


INTRODUCTION
Many tropical soils are low in both total and plantavailable nitrogen (N) and crop yields are often limited by N supply (Ladha et al., 1993).In agricultural systems crop yields are often increased by applying N fertilizers (Danso et al., 1993;Brady 1974;Simpson, 1986).However, scarcity of N fertilizers and their increased selling prices in developing countries have led to tremendous search for alternatives to N fertilizer sources for provision of N to crops (Araya and Stroosnijder, 2010;Broschat, 2007;Döring et al., 2005;Mafongoya and Nair, 1996).
Substantial leaf mulches can be obtained in alley cropping systems, where annual crops are grown between rows of trees and branches are pruned regularly to provide leaf mulch for the interplanted crops (Atta-Krah, 1990;Balasubramanian and Sekayange, 1991).Sometimes the legume leaves are ploughed into the soil (Catchpoole and Blait, 1990) to facilitate decay and release of nutrients to the soil for crops.Benefits of mulching include the reduction of erosion hazards, better infiltration of rain water and less evaporation, lower soil temperature, supply of organic matter and nutrients, higher soil biological activity, better root growth and suppression of weeds (Araya and Stroosnijder, 2010;Billeaud and Zajicek, 1989;Jodaugienė et al., 2006;Lal, 1974;Schroth et al., 1992).
Studies on mulching have been documented (Ahaiwe et al., 2010;Gutteridge, 1992;Mafongoya and Nair, 1996;Sinkevičienė et al., 2009;Yobterik et al., 1994).However, these studies were carried out using mulches from trees that had not been inoculated with rhizobia and all mulches were incorporated into the soil.Therefore the purpose of this study was to investigate the effect of leaf mulches from rhizobia-inoculated trees and their point of placement on barley early growth, nitrogen uptake and, soil nitrate and pH changes.

MATERIALS AND METHODS
Leaves of both rhizobia inoculated and uninoculated multipurpose Leucaena leucocephala and Robinia pseudoacacia shrubs were obtained from 7-month old plants.The leaves were either placed on the surface or mixed up with soil in 3 L pots at a rate of 4.835 g per pot (equivalent to 2.5 tones dry matter per hectare, pot surface area) (Gatteridge, 1992).Soils used in each pot weighed about 3 kg.Barley (Hodeum vulgare, variety ahadi, obtained from Kenya Seed Company, Kenya) was planted in all soil treatments (which had been watered slowly to eliminate differences due to soil moisture content and allowed to equilibriate for 24 hours).Enough pots were planted to allow for a destructive sampling of barley three times in four replicates.The pots were randomized in a greenhouse bench and kept moist by gentle watering.The greenhouse was maintained at temperatures between 19 and 24 o C and 16 hours of day light and 8 hours of night.Heights of the two tallest barley plants' highest leaf in each pot in all treatments were taken after every two days for five days starting from the third day after barley germinated (six days after planting).After two weeks barley was thinned to four plants per pot.The pots were randomized the bench twice every week.Barley straws were cut at the soil level and surface soil samples were taken at the upper 5 cm.This was a destructive sampling which was done at two, four and six weeks after barley germination.The barley straws were dried at 75 o C for 48 hours, weighed and ground to pass through an 810 µm, No. mesh sieve.The ground barley was analyzed for total N (%) using a Technicon TM AutoAnalyzer TM II.Soil pH in water, 1:5 (soil: water) was determined with pH meter (Cole-Permers).Soil nitreate was extracted using 2N potassium chloride (KCl) by shaking soil in a 250 ml conical flask for one hour, at a ratio of 1:10 (soil: KCl); nitrate concentration was determined as total N above, and then expressed on an oven dry basis (µg.100g -1 soil).

Statistical analysis
The data of barley growth, nitrogen concentration and content, soil nitrate and pH changes, was analyzed as a randomized complete block design using the Statistical Analysis System (SAS, 2008).Means were separated by least significant difference (LSD) and simple't' tests at p<0.05.

RESULTS
Barley heights recorded between days 6 and 14, after germination, showed that surface placed mulches of either Leucaena or Robinia relatively enhanced barley heights compared to mixed mulches (Figure 1and 2).On days 12 and 14 significant differences (p<0.05) in barley heights were noted in barley grown in surface placed mulches and that grown in mixed uninoculated Leucaena (Figure 1).In addition, heights of barley grown in surface placed mulches of Robinia had significantly (p<0.05)higher heights than barley grown in mixed mulches of Robinai on days 6, 8. 12 and 14 (Figure 2).
As observed in growth in heights, barley biomass was relatively higher in all mulches placed on soil surface for all leaf types (Figure 3 and 4).However, barley mulched in surface placed mulches of inoculated Leucaena resulted in significantly (p<0.05)higher biomass relative to other mulch types during week 6 (Figure 3).Further, during week 6, mulching barley with uninoculated Robinia leaves significantly (p<0.05)increased barley biomass relative to other mulch types (Figure 4).
Nitrogen concentration in barley straw decreased with the age of barley (Figure 5 and 6).However, significantly (p<0.05)higher nitrogen concentration in barley straw was recorded after mulching with mixed leaves of uninoculated Leucaena relative to other Leucaena leaf treatments (Figure 5).Further, relatively lower straw N concentration was recorded when barley was mulched with surface placed inoculated Leucaena leaves (Figure 5).When Robinia mulches were used, relatively higher N concentration was recorded in barley mulched with mixed uninoculated Robinia.In addition, there was a significant difference between N concentrations of barley mulched with uninoculated mixed Robinia and barley mulched with surface placed inoculated Robinia mulch (Figure 6).Mixing mulches with soil generally enhanced barley ear biomass (Table 1).In addition, ears of barley planted in all mixed mulches had generally higher nitrogen concentration and nitrogen content than those of barley planted in surface placed mulches (Table 1).
Generally nitrate released peaked in week four and drastically dropped in week 6 in all mixed mulch types (Figure 7 and 8).In addition, nitrate release in all surface placed mulches increased in week 4 through week 6 (Figure 7 and 8).In mixed Leucaena mulches, 1250 mg.g -1 soil nitrate was released from uninoculated leaves in week four compared to only about 500 mg.g -1 soil nitrate released from inoculated leaves during the same week (Figure 7).Further, during the same week surface placed inoculated leaves of Leucaena released1250 mg.g -1 soil nitrate compared to 900 mg.g -1 soil released by uninoculated surface placed Leucaena leaves (Figure 7).
As observed in Leucaena mulches, highest nitrate among the Robinia mixed mulches was released from mixed uninoculated Robinia leaves (1500 mg.g -1 soil nitrate), and the lowest by mixed inoculated leaves (900 mg.g -1 soil nitrate), during week 4.In addition, surface placed mulches of Robinia released lower nitrate levels compared to the mixed mulches, with surface placed uninoculated Robinia releasing lowest nitrate amounts (Figure 8).Soil pH generally dropped between weeks 2 and 4 and started rising during week 6.Generally, highest pH drops were observed in mixed uninoculated mulches of both Leucaena and Robinia (Figures 9 and 10).

DISCUSSION
Relatively higher barley heights were recorded when leaf mulches were placed on the soil surface most probably because the presence of leaves on soil surface during barley germination might have increased cell elongation in barley plumule in response to limited light supply to the germinating barley.However, the effect of mulching on barley heights varied with tree species from which leaves were obtained and type of leaves used implying that point of mulch placement and mulch type could have determined barley heights.Barley straw biomass were relatively higher in surface placed uninoculated leaves of both Leucaena and Robinia probably indicating a prolonged growth effect due to lack of light caused by placing leaf mulches in the soil surface during barley germination in these mulch treatments.However, highest biomass were noted in surface placed mulches of inoculated Leucaena and in surface placed mulches of uninoculated Robinia, which corresponds to soil pH of about 6 and soil nitrate levels of 1450 and 600 mg.g -1 , respectively.It was not clear whether the low nitrate levels in the uninoculated Robinia treatments were due to plant uptake or slow mulch decomposition.Gutteridge (1992) found out that S. sesban was effective in increasing maize stover when mixed with soil, whereas L. leucocephala, C. callothyrsus and A. cunninghamii resulted in higher maize (Zea mays) stover biomass when placed on surface.Further, Jakobsen and Jensen (1981) reported an 85% barley growth increase after mulching.
Barley ear results indicated that mulching with mixed leaves of Leucaena and Robinia enhanced ear total N and N content both of which indicated that these treatments may be necessary in increasing grain yield and grain N content.This observation may then imply that mixing mulches of Leucaena and Robinia with soil may be necessary to improve barley yield.
Barley straw nitrogen concentration decreased with time, probably due to allocation of N to growing parts from old leaves (Marschner, 1986) and therefore dilution of N in most old leaves of barley.However, N concentration varied with mulch type and point of mulch placement and implied that mixing mulches with soil enhanced barley nitrogen concentration in all mulch types.However, it was not clear why mixed mulches resulted to relatively higher nitrogen concentration in barley plants.It was hypothesized that high N concentration noted in the two week old barley plants might have been contributed by acquisition of N from seed endosperm but it was not very clear why barley from mixed mulch treatments had relatively higher N concentration over the study period.
Soil nitrate data indicated that mixed leaves were effective in releasing nitrate within a month after which nitrate supply drastically dropped as opposed to surface placed leaves which gradually released nitrate with time.Broschat (2007) found no effect of mulching on soil nitrate after samples were taken 6 weeks after mulching.The differences in results could probably be due to differences in mulch types, types of plants mulched, nitrate leaching and rates of mulches used.Based on ear total N and N content observations made after using mixed leaves of inoculated Leucaena and mixed leaves of uninoculated Robinia, it might be tempting to say that the time at which nitrate is supplied to the plant, leaf type, point of placement and species may commonly affect barley performance.
Soil pH decrease in week 4 coincided with high nitrate levels mostly in mixed leaves but increased at week 6.Brady (1974) suggested that during decomposition of organic matter carbonic acid is too weak to cause low pH often detected in most soils.So, he further suggested that inorganic acids such as sulphuric acid and nitric acids (from decomposing matter and sometimes from certain fertilizers such as ammonium sulphate under microbial activity are potential suppliers of hydrogen ions in the soil).Studies have shown that soil pH decreases when organic mulches are used and that this decrease is proportional to the depth of these mulches (Billeaud and Zajicek, 1989).However, Broschat (2007) found no effect of mulch on soil pH.Soil pH in this study rose after week 4 probably due to reduced fresh organic matter in the soil, especially mixed treatments, which slowed mulch decomposition.Increase in soil pH after mulch application was reported by Ahaiwe et al. (2010), andHue andAmien (1989) and was attributed to the release of organic chelates from decomposition products of green manure which formed complexes with Al (OH)3.

CONCLUSION
Results of this study indicated that placing mulches on the soil surface enhanced barley heights and biomass implying that it might be advisable to place mulches on the soil surface if the purpose of barley planting is biomass production.However, if the intention of barley planting is to produce seeds, which may relate to ear nitrogen content, it may be advisable to mix mulches with soil.To enhance ear nitrogen content it might be advisable to use mulch leaves from inoculated Leucaena leucocephala and leaves from uninoculated Robinia pseudoacacia.

Figure 1 :Figure 2 :Figure 3 :Figure 4 :Figure 5 :Figure 6 :
Figure 1: Effect of using mulches from inoculation and uninoculated Leucaena on barley height during the first fourteen days after barley germination

Figure 7 :Figure 8 :Figure 9 :Figure 10 :
Figure 7: Effect of using mulches from inoculation and uninoculated Leucaena on soil nitrate (mg.g -1 soil) during the first six weeks after barley germination