Does a Specific Location of Composted Poultry Litter in Soil Influence Nutrient Use Efficiency and Vegetable Production? A Mesocosm Experiment

Animal wastes may be promoted as an alternative to mineral fertilizers that remain unaffordable to the overwhelming part of smallholder farmers in Sub-saharan Africa. However for an efficient use, mechanisms that underly their impact on crops should be well understood. This study was conducted in mesocosm to evaluate impacts of two ways of composted poultry litter (CPL) addition on growth and nutrient use efficiency by cucumber. It included three treatments with five-bucket replicates each: Control, CPL applied on soil surface (CS) or buried to 10 cm-depth (CB). Dry CPL was added at the rate of 0.5 kg bucket. At harvest, root distribution was examined in the 0-5, 5-10 and 10-20 cm depths. Dry biomasses of roots, shoot and fruits were also determined and allowed for calculation of diverse indexes of biomass allocation (root:shoot ratio, root weight ratio, stem weight ratio, leaf weight ratio) and nutrient use efficiency (factor productivity of the compost, partial factor productivity of nutrients, agronomic efficiency of compost, and apparent agronomic efficiency of nutrients). The results showed that application of CPL led to a significant improvement of all considered parameters except for the leaf weight ratio which was higher in the control (44.1±3.3) than in CS (28.1±1.9) and CB 31.2±3.5). Total lateral root length was significantly higher in CS than in CB (113.5±10.7 cm vs. 75.5±9.0 cm). The number of lateral roots per plant in the 0-5 cm soil layer was higher in CS than in CB (5.4 vs. 1 root plant); the reverse was observed in 5-10 cm (1.2 vs. 4.4 root plant). Both fresh fruit yield and total dry mass were positively correlated to root attributes. These were themselves negatively impacted by soil acidity. All nutrient use efficiency indexes were higher in CS than CB. The CPL improved the agronomic performance of cucumber particularly when applied at soil surface.


Introduction
In sub-Saharan Africa, market gardening is an income-generating activity for urban and peri-urban farmers.This activity essentially supplies urban markets with fresh vegetables (Onana, 2006) such as cucumber (Cucumis sativus L.), tomato (Solanum lycopersicum), carot (Daucus carota), cabbage (Brassica oleracea), lettuce (Lactuca sativa), etc., thereby contributing to food security.Peri urban agriculture is more and more intensive as observed in other sub-Saharan African countries, because of increasing food needs (Kasongo et al., 2013).In such situation, soil nutrient removals should be offset by inputs to maintain the soil fertility.Thus, conventional solutions in particular mineral fertilization were proposed.This way, although effective in the short term remains unaffordable to the overhelming part of small farmers because of the relatively high cost (Haefele et al., 2013).In addition, the use of this kind of fertilizer leads to environmental concerns (Miranda et al., 2015;Celik et al., 2010;Lee et al., 2013).In this context, organic fertilization may be considered as an appropriate solution for sustainable crop production (Chang et al., 2014).
Intensified livestock development in periurban zone as poultry livestock offers large resource of organic matter to be recycled.Market gardening soils are being increasingly fertilized with poultry droppings, which are likely to provide an interesting contribution to cultivated soils.About 70% of the nitrogen and phosphorus ingested by poultry is returned through droppings (Chabelier et al., 2006).Also, poultry droppings are increasingly preferred by farmers compared to other animal droppings because of their high macroelement content (Duncan, 2005).Although animal wastes have been used as manure for long times world-wide (López-Masquera et al., 2008), very few studies have focused on the potential impacts of composted poultry litter (CPL) on vegetable crop yield.Indeed, the composted form is known to provide more uniform substrate and stable organic matter that promotes greater soil C sequestration as opposed to noncomposted poultry manure (Peltre et al., 2010;Bouajila & Sanaa, 2011).Also, studies that focused on the efficiency of nutrient use according to the way of CPL application are scarce.This gap should be filled in since the location of organic fertilizer at the soil surface or deeper in the soil may influence the dynamic of soil organic matter and nutrient with implication for crop mineral nutrition (Adekya & Agbede, 2016).
Organic matter is known to influence soil properties in terms of structure and water dynamics (Islam et al., 2017;Liu et al., 2017).In organic fertilization, the location of organic residues has important implications for the soil organic matter building up and soil fertility.The fate of this organic residue may be linked to its status.Fresh materials were reported to be subjected to faster decomposition and release of N when buried in the soil (Jahanzad et al., 2016).However, concerning more stable residue such as composts, burial may appear less favourable for decomposition compared to surface application (Wang et al., 2014).In the latter case, key nutrient such as nitrogen is preserved from loss through volatilization and may result in improved crop production (Adekya & Agbede, 2016).Here, the research question relates to the influence of the way of CPL application on plant root development, biomass allocation and yield.The main hypothesis was that the agronomic efficiency of CPL differs according to location in the soil.

Study Site
The study took place at the experimental site of the University Nangui Abrogoua, Abidjan (5°00′ and 5°30′ N, 3°50′ and 4°10′ W) (Figure 1).The climate is of equatorial type with atmospheric humidity of 90% to 100%.Average annual rainfall and temperature are 1700 mm and 27.3 °C respectively.

Poultry Manure Composting
Poultry litter was composted in aerobic conditions at the agricultural training center of Tshanfeto (5°20′12″ N and 4°7′57″ W) located at the eastern outskirt of Abidjan.Watering and turning were carried out once every two weeks to ensure good conditions for substrate decomposition.After about 10 weeks, the mature compost was harvested, dried and transferred to Nangui Abrougoua University.The concentration and total amount of nutrients supplied are shown in Table 1.
Prior to filling the buckets, the bottom was drilled with five 2 cm-diameter holes to allow for good drainage.The buckets were then exposed to open air for cucumber plants to grow under ambient weather.The experiment included three treatments with five replicates each: buckets with CPL spread on entire soil surface (CS) or buried in holes (20 cm length, 20 cm width and 10 cm deep) (CB), and a control (without any addition).Dry CPL was applied at the rate of 0.5 kg per bucket for both CS and CB treatments.

Cucumber Growth Conditions and Yield Measurement
One day after CPL addition, cucumber (variety Tokyo) was sown at the rate of three seed holes per bucket.The experimental plot and buckets were maintained weed-free through regular weeding.Buckets were watered every two days with 1 L water throughout the growing period (February to March 2017).However, when rainfall event occurred during obout 30 min, buckets were watered four days after.When fruit reached maturity (60 days after sowing), cucumber plants were harvested and oven-dried at 60 °C to constant weight.Then, dry root, shoot and fruit dry yields were determined.

Root Development and Distribution
At harvest, root development and distribution was examined on one individual plant per bucket.For each bucket, one side was uncovered over an arc of 20 cm rope, then the block of soil at that side was pickled to bring out roots along the soil profile.This profile was subdivided into three layers (0-5 cm, 5-10 cm and 10-20 cm) where lateral roots (or first order ramification, directly attached to the taproot) were counted.Their length was measured using a graph paper.

Determination of Biomass Allocation and Nutrient Use Efficiency Indexes
Allocation of resources by cucumber was assessed through diverse ratios: -Root weight ratio (RWR); -Stem weight ratio (SWR); -Leaf weight ratio (LWR); -Root:shoot ratio (R:S) = DM root /DM shoot(stem, leaf and fruit) ; -Harvest Index (HI): Dry fruit mass to total dry plant mass: HI (%) = (MS fruit /MS plant ).
Diverse nutrient use efficiency was calculated through production efficiencies indexes.In all these equations, fertilizer refers to CPL.

Soils Sampling for pH Measurement
Water pH was measured on the initial soil used for the experiment using a glass electrode in 1/2.5 soil/water ratio as described by Thomas (1996).At the end of the cucumber cycle, soil was sample in each bucket at the 0-10 and 10-20 cm depths and also subjected to pH measurement.Prior to this, samples were air-dried at ambient temperature, crushed and sieved at 2 mm.

Statistical Analyses
Yields, biomass allocation ratios and nutrient efficiency indexes were compared using one-way analysis of variance (ANOVA) after verification of the homogeneity of the variances (Levene test) and log-transformation of data where needed.When differences were significant, mean separation was done using Student-Newman-Keuls test at 5%.All these tests were done using the Statistica 7.1.Results were significant when p < 0.05.

Soil pH in the Different Treatments at the End of Trials
Soil pH at the end of trials significantly differed between treatments (p < 0.001).Values were higher in soil with compost addition compared to the control (Table 2).For the same soil layer, there was no significant difference between CS and CB.In each of the treatments, pH in 0-10 cm layer was significantly higher than in the 10-20 cm (CS: 5.5±0 vs. 4.5±0 for CS; CB: 5.4±0.1 vs. 4.6±0.1).

Root Development
Addition of CPL resulted in a significant increase of taproot length and collar diameter relative to the control (Table 3).Density, total length and mean length of lateral roots also increased significantly.The collar diameter of the taproot and total lateral roots length in CS was significantly higher than in CB.Note.In the same row, means with different letters are significantly different at the 0.05 level.CS: compst applied at the soil surface, CB: buried compost.

Root Distribution
In general, addition of CPL led to increased number of lateral roots (Figure 1).Root distribution was heterogeneous throughout the soil profile, and was influenced by the CPL application way.In the 0-5 cm soil layer, the number of lateral roots was significantly higher (p < 0.0001; F = 46.3) in CS (5.4±0.5)than in the control (1.2±0.2) and CB (1±0.3).
Figure 1.Roots distribution in the soil profile under the different treatments Note.CS: compst applied at the soil surface, CB: buried compost.
In the 5-10 cm soil layer, the highest number (p < 0.0001; F = 48.15)was rather observed in CB (4.4±0.2),followed by CS (1.2±0.4) and the control (0.6±0.2).The number of lateral roots was lower in the 10-20 cm soil layer than in 0-10 cm and did not show any significant difference (p = 0.2; F = 1.7) among treatments.Pearson correlation test showed that the number of cucumber roots strongly increased with soil pH (Figure 2).

Cucumber Biomass Yield
Both fresh fruit and total dry biomass yields were significantly higher on amended soils (CS, CB) as opposed to the control (fresh fruit yield: p = 0.008; Total dry biomass yield: p = 0.007) (Figure 3 a, b).However, no significant difference was observed between CS and CB although fresh fruit yield and total dry biomass were 22.1% and 51.2% higher on the former than on the latter, respectively.Both fresh fruit and total dry biomass yield were very low on control (0.04±0.00 kg per bucket and 0.007± 0.00 kg per bucket, respectively).
Pearson correlation tests revealed that both fresh fruit and dry shoot biomass yields were significantly positively correlated to roots development parameters (Figures 4a to 4f), particularly dry root mass and mean lateral root length.

Biomass Allocation Indexes
No significant difference was observed between treatments for Root:Shoot ratio, root weight ratio and stem weight ratio (p = 0.19; p = 0.18; p = 0.26 respectively) although these parameters were relatively higher in the control (Table 4).In turn, leaf weight ratio was significantly (p = 0.006) higher in the control than in CS and CB.Note.In the same row, means with different letters are significantly different at the 0.05 level.CS: compst applied at the soil surface, CB: buried compost.

Harvest Index
Harvest index under CPL addition (CS and CB) was significantly higher (p = 0.01) than in the control (Figure 5).However, no significant difference was observed between CS and CB.
Figure 5. Cucumber harvest index on the different treatments Note.CS: compst applied at the soil surface, CB: buried compost.

Nutrient Use Efficiency (NUE) Indexes
With respective increments of 33.3% and 100%, productivity efficiency and agronomic efficiency of CPL were higher in CS compared to CB (Table 5).Partial factor productivities for CPL-N, -P and -K were 53.1%, 54.5% and 40% higher in CS than CB.The same trends were observed for apparent agronomic efficiencies (AAE) that were 70.8%, 72% and 100% higher in CS relative to CB.The agronomic efficiency of the CPL was also 100% higher in CS relative to CB.

Discussion
The low fresh fruit and total dry biomass yields on the control indicates that original soil at Tshanfeto was nutrient-poor and unsuitable for adequat cucumber development.Acidic soil condition could also be the reason for poor cucumber growth and development.Indeed, data of the present study showed that low soil pH led to limited lateral root development.In addition such soil condition is known to reduce the availability of nutrients, particularly exchangeable cations that are determinant in cucumber growth and fruit production as recently reported by Abobi et al. (2018).Note.CS: compst applied at the soil surface, CB: buried compost.
Conversely, the increase in fresh fruit and total dry biomass yields following CPL addition indicated that this organic residue was able to overcome the above-mentioned constrains.Concentrations of exchangeable cations, especially in K + (63.4 g kg -1 ) make it a good fertilizer for cucumber which is known to be potassium-demanding (Morgan, 2016).The CPL was rich in terms of organic matter and nutrients that are known to promote root development (Nakamura et al., 2008).Root development is conducive to water and mineral nutrition, hence plant growth and biomass production.This is supported by our data since (i) density of lateral roots was the highest in the 0-5 cm layer in CS while it was so in the 5-10 cm layer CB, and (ii) both fresh cucumber fruit yield and total dry mass were significantly and positively correlated to roots parameters (dry mass, mean length and number of lateral roots).
Another reason for the positive impact of CPL on cucumber growth and yield could be the significant rise of soil pH from very acidic to acidic on CB and CS, probably as a result of the supply of cations, particularly calcium as reported by Boating et al. (2006).As a matter of fact, soil acidity is known to control root development (Marschner et al., 2005) and this is confirmed by the significant correlation observed in the present study.The positive impact of CPL was also shown by Essehi et al. (2016) on rubber seedling growth, by Iren et al. (2015) on water-leaf growth and yield.Kimuni et al. (2014) found that cabbage yield obtained with 60 t ha -1 of CPL was similar to that with 350 kg ha -1 of mineral fertilizers (NPK and urea).
Since root development proved influential on cucumber growth and biomass production, this parameters also may explain the difference observed between the two CPL application ways or locations in the soil.As cucumber roots are essentially superficial (FAO, 1988), spreading CPL on the entire surface of the CS buckets probably fostered root proliferation (lateral roots) and growth in length.This also allowed taproot growth in and width, explaining the higher collar diameter in CS than in CB.However, burying the CPL at 10 cm depth in CB buckets probably put the nutrients out of reach during the early stage of seedling development.Thus, the taproot had to grow in length to reach the nutrient source, resulting in increased length and lower collar diameter in CB as opposed to CS.Since the CPL was applied locally and at depth, lateral root development was restricted in CB as shown by the lower total lateral root length as compared to CS (Table 6).As a result, the efficiency of CPL-nutrient use by cucumber was higher in CS compared with CB, as reflected by values of the considered indexes.The better soil aeration at surface which is favorable to nutrient assimilation by roots (Huber & Schaub, 2011) could also explain the higher agronomic efficiency of CPL in CS compared to CB. Lower mineralization rate of CPL incorporated into soil as reported by Kimuni et al. (2014) could also be explanatory.Therefore, it could be stated that soil surface is the right place for CPL application in cucumber cropping owing to a maximized effectiveness in nutrient use (Fixen & Garcia, 2006).
Values of Harvest index that are higher than 0.5 reflect effective translocation of assimilates from the other parts of the plant to fruits (Lucas, 1984).The present study showed that CPL addition greatly contributed to this performance since HI associated with the control was quite lower while HIs associated with CS and CB were higher than this threshold.However, the similarity in HI values in CS and CB is attributable to the one in total dry biomass and dry fruit yields.
According to Meziane (1997), Root:Shoot ratio is strongly influenced by the soil nutrient status.Therefore, in the original nutrient-poor soil from Tshanfeto, cucumber plant seems to have favoured root development for soil exploration rather than shoot development.This may explain why values of the ratio were to some extent higher in the control compared to the treatments with CPL addition.The same explanation might apply for the lower (even not significantly) values recorded in CS relative to CB.In CS, no significant effort was needed by the plant to access the nutrient source (CPL) since cucumber lateral roots are naturally shallow.In contrast, in CB, the plant had to make an extra effort to develop roots in an unusual soil depth to access the nutrient source.Another reason for the higher Root:Shoot ratio was that fruit yield was lower, contributing to a lower aboveground biomass.In poor-soil conditions, other plant species such as leguminous were reported to direct greater effort in fruit production instead of non-reproductive parts (stem, roots, leaves) while in good soil conditions, they direct effort in non-reproductive organ production that allow them accessing light for the photosynthetic carbon assimilation (Meziane et al., 1997;Koné, 2009).However, in the present study with a species of the Cucurbitaceae family, the reverse phenomenon was observed as reflected by the higher values of Root weight ratio and the Leaf weight ratio in the control compared to CS and CB.The same reason could be mentioned for the higher values recorded in CB as opposed to CS.

Conclusion
This study showed that composted poultry litter greatly improved cucumber production owing to the quantity of nutrient it provided and the stimulation of root proliferation and development.Root development in turn was favoured by the reduction of the soil acidity and the presence of nutrients, and led to improved nutrient use efficiency.
The location of the CPL in the soil influenced the nutrient use efficiency as values of the indexes were higher when CPL was applied at the soil surface.This was favoured by the fact that cucumber roots are essentially shallow, thus no significant extra effort was directed towards root development at the expense of fruit.Although value of cucumber fresh fruit yield in CS was above that of CB, statistical significance was not reached.
Values in parentheses are standard errors.CS: compst applied at the soil surface, CB: buried compost.Table 3. Cucumber root growth parameters [Mean(SE)]

Figure 2 .
Figure 2. Correlation between soil acidity and lateral root density

Figure 3 .
Figure 3. Fresh fruit yield (a) and total dry biomass yield (b) in the different treatments Note.CS: compst applied at the soil surface, CB: buried compost.

Figure 4 .
Figure 4. Correlation between root attributes and cucumber dry mass yields

Table 1 .
Nutrient concentration and quantity of nutrient supplied though composted poultry litter

Table 2 .
Change in soil acidity between the start and the end of the cucumber cropping cycle

Table 4 .
Indexes of biomass allocation by cucumber [Mean(SE)]

Table 5 .
Indexes of composted poultry litter-derived nutrient use efficiency by cucumber