Impact of tree canopies on selected soil chemical properties and pasture nutritive values in a semi-intensive grazing system in South-western Uganda

D Nabasumba, R Muzira, S Natuha, G Tugume and H Kirunda

Mbarara Zonal Agricultural Research and Development Institute P O Box 389, Mbarara Uganda
buyinzadina@gmail.com

Abstract

This study used 136 soil samples collected from under and outside tree canopies of six species, to assess the composition of pH, total Organic matter, total Nitrogen, extractable Phosphorus, and exchangeable Potassium and Calcium in the semi-intensive grazing system of South-western Uganda. A total of 88 dried and crushed samples of Cynadon dactylon and Brachiaria ruziziensis collected from under and outside tree canopies were analyzed for Crude Protein (CP), Phosphorus, Potassium, and Calcium. The composition of soil and pasture tissue parameters was higher under tree canopies than outside tree canopies. Under tree canopies, soils under Ficus natalensis had the highest composition of pH (6.63) and Calcium (19.1 Ml/100g). Nitrogen (0.31%), Organic matter (6.96%) and Phosphorus (86.7ppm) were richer in soils under Allophylus africanus. Potassium was highest under Grewia mollis (1.07 Ml/100g). For tissue nutrients, Cynadon dactylon recorded highest CP (12.5%) and P (0.55%), K (0.98%) and Calcium (0.30%) under Ficus natalensis, Allophylus africanus and Acacia abyssinica respectively. In Brachiaria ruziziensis, CP (12.9%), P (0.59%), K (0.86%) and Calcium (0.30%) were highest under A. coriaria, Allophylus africanus,Grewia mollis and Acacia abyssinica respectively. This study recommends use of A. africanus and F. natalensis to restore degraded grazing areas in South-western Uganda.

Key words: cattle corridor, soil nutrients, tissue nutrients

Introduction

Semi-intensive grazing system characterized by a combination of grazing and stall feeding or purely paddock grazing (Odero-Waitituh 2017), constitutes the major form of livestock production in the cattle corridor of South-western Uganda. This form of land use provides a livelihood to over 60% of the households in the region (Makuma-Massa et al 2017), yet due to significant dependence on natural pastures alongside large herds of cattle which have led to overgrazing (Tibezinda et al 2016), the rate of land degradation therein is facing a rapid increase. Like elsewhere in the world, the negative effects of unsustainable grazing practices reflected in bare grounds, compacted soils, reduced soil fertility and the resultant shortage of quality pastures (Bolo et al 2019), are visibly evident in the semi-intensive grazing areas of South-western Uganda. Moreover, with the substantial land requirements such a production practice poses, a transition from a semi- intensive grazing system in the area is not predicted in the near future. Thus, there is dire need for innovations that support improved livestock productivity in the region through boosting growth of indigenous pastures of high nutritional value.

As with other agricultural systems, suitability of innovations for growth of nutritious pastures in grazing lands is largely dependent on their effect on soil chemical parameters (Zornoza et al 2015). The interactions between the physical and biological soil environment and the resultant composition of soil chemical properties is reported to influence the nutritional quality of the pastures (Pulido et al 2018). Therefore, in the increasingly degraded semi-intensive grazing areas of South-western Uganda where natural pastures constitute the major source of feed for livestock throughout the year (de Vries 2019), use of land management practices that will improve the status of soil chemical properties and enhance pasture nutritive values are both desirable and essential.

Over the years, there is growing literature highlighting the significant contribution of tree species towards improving soil chemical properties and forage quality in grazing lands (Mugerwa and Emmanuel 2014; Serrano et al 2018). Through increased litter fall and a favorable micro-climate under tree canopies, soil microbial processes and subsequent release of nutrients required for pasture growth and nutrient accumulation have been reported as opposed to tree-less sites (Abdulahi et al 2016). However, due to socio-economic pressures coupled with value mismatch between tree species and ecosystem support services, most tree species in the grazing areas of Uganda have been indiscriminately harvested (Filipová and Johanisova 2017). Thus, there is need to restore tree cover in the semi-intensive grazing areas of South-western Uganda to enhance soil fertility and support growth of higher quality pastures. However, there is still lack of formidable information on suitable tree species which have a high positive bearing on both soil chemical properties and pasture nutritive values in the region. Therefore, the objective of this study was to assess the impact of tree canopies of prevailing indigenous species on selected soil chemical properties and nutritive values of Brachiaria ruziziensis and Cynadon dactylon, as a criterion of identifying potential tree species to use in restoring the productivity of degraded grazing areas in South-western Uganda.

Materials and methods

Study area

This study was carried out in February and March 2019, in two livestock dominated sub-counties in the districts of Isingiro (0.8435° S, 30.8039° E), Kiruhura (0.1928° S, 30.8039° E), Lyantonde (0.2241° S, 31.2168° E) and Mbarara (0.6072° S, 30.6545° E); Figure 1. The study period marked the onset of the first rainy season in the region. In particular, the study districts lie in the southern part of the cattle corridor of Uganda, which stretches from the North-eastern part of the country (Sempiira et al 2018). The climate of the area is predominantly semi-arid with average annual rainfall range of 750 - 800 mm distributed in a bimodal pattern (Nagasha et al 2019). Temperature ranges from 20^o - 30 ^o C with high peaks recorded in January and July. The altitude of the area stands at an average elevation of 1800 m above sea level. Soils are generally sandy loam with low organic matter content. Due to the nature of climate and soils, savannah grassland type of vegetation with scattered Acacia tree species characterize the area. Dominant domestic fauna is cattle mainly the Ankole Longhorn local breed, and the Holstein Friesian and their crosses grazed in a semi-intensive sedentary system (Tibezinda et al 2016).

Figure 1.  Map of the study  area

Selection of the study tree and pasture species

Through focus group discussions with livestock farmers in two livestock dominated sub-counties in each district, six dominant indigenous tree species retained in the grazing areas were identified. The tree species included Acacia abyssinica, Acacia gerrardii, Albizia coriaria, Allophylus africanus, Ficus natalensis, and Grewia mollis (Figure 2). The dominant pastures species were Brachiaria ruziziensis and Cynadon dactylon. In every sub-county, a minimum of 8 farmers who had at least four of the prioritized tree species on the same farm were selected for field assessments.

Acacia abyssinica	Allophylus africanus	Albizia coriaria
Acacia gerrardii	Ficus natalensis	Grewia mollis
Figure 2. Sample photographs of the study tree species

Study design and data collection

Assessment of the impact of tree canopies on soil chemical properties

At every farm, mature (>20 cm Diameter at Breast Height-DBH) and healthy trees were considered. DBH (cm) and canopy radius (m) of each tree were measured using a diameter tape and measuring tape respectively. Using wooden frames, four quadrats of 1m² each were established in different directions under the tree canopy. Using a soil auger, soil samples were collected to a depth of 15 cm in each quadrat, dispensed in a basin and mixed to obtain a composite soil sample. A distance of 5 m away from the outer most edge of the tree canopy was measured and corresponding quadrats of 1 m² were instituted. Soil samples were collected from each quadrat, mixed and a composite soil sample outside tree canopies was obtained.

Each soil sample collected was packed into polythene bags containing a detailed label. A total of 136 samples were collected under and outside canopies of 68 tree species.

The soil samples were delivered to Makerere University soil and plant analytical laboratory, where they were analyzed for pH, total Organic matter (TOM), total Nitrogen (TN), extractable Phosphorous (EP), exchangeable Potassium (EK) and Calcium (ECa). Soil pH was determined using 2.5:1 water to soil suspension ratio while Walkley and Black method was used to determine TOM, Kjeldehal method for total N, Bray 1 for extractable P and flame photometer for exchangeable K and Ca. These methods are fully described by Okalebo et al (2002).

Assessment of the impact of tree canopies on pasture nutritive values

From the already established quadrats, pastures were manually harvested with a panga to ground level. Green samples of B. ruziziensis and C. dactylon were isolated, packed in paper bags and delivered to Mbarara Zonal Agricultural Research and Development Institute. At the institute, samples were sorted further and matched with the corresponding soil samples for each tree species and farm field. A total of 88 pasture samples (44 under and 44 outside tree canopies) were screened. No samples of C. dactylon were obtained under and outside tree canopies of G. mollis.

The green pasture samples (4 samples per pasture species/tree species/canopy site) were oven dried at 60^o C to a constant weight. The dry samples were manually crushed in separate motors to obtain fine powdered samples, which were delivered to Makerere University soil and plant analytical laboratory for analysis. In the laboratory, the samples were analyzed for tissue N (Kjedahl method), P (Calorimetric method), K and Ca (Spectrometric method) as described by Okalebo et al (2002).

Data analysis

Results of the assessed soil and pasture parameters were entered in Microsoft Excel computer package where descriptive statistics in form of graphs and table summaries were generated. For tissue N, a multiplication factor of 6.25 was applied to determine the level of Crude protein (Moran 2005) which was used in the analysis. The data was exported to R statistical package for inferential statistics.

Within R, a normality test was undertaken using Shapiro-Wilk’s test to identify the distribution of the data. To analyze for statistical differences in the means of the study soil and pasture parameters under and outside tree canopies as well as between tree species, two way ANOVA (at 95% Confidence Interval-CI) was used. Where significant differences existed in the means, the data was subjected to Tukey’s Least Square Difference (LSD) at 95% (CI) to identify significantly different means. Pairwise Pearson correlation test was undertaken to establish the relationship between soil nutrients and the corresponding pasture nutrient compositions while Coefficient of multiple determination (r²) was undertaken to determine the strength of the relationship.

Results

Canopy size characteristics of the study tree species

All tree species covered in this study were mature (DBH > 20 cm), with marked variation in canopy size (p < 0.05) A. abyssinica had the widest canopy of 7.45 m while A. africanus had the shortest canopy of 3.91 m (Table 1). Statistically, the canopy radius of A. abyssinica, A. gerrardii, A. coriaria and F. natalensis was far wider than that of A. africanus and G. mollis (Table 1).

Effect of tree canopies on selected soil chemical properties

From Table 2, pH and the composition of TN, TOM, EK and ECa had a remarkable variation under and outside tree canopies (p < 0.05), with higher levels recorded under tree canopies. An exception was recorded in soils under A. coriaria where pH was lower (6.02) compared to the corresponding soils outside tree canopies (6.13). For EP, there was no substantial variation in the composition under and outside tree canopies (p > 0.05), although soils outside tree canopies of A. abyssinica showed an exceedingly higher composition (150 ppm) compared to under tree canopies (28.1 ppm).

Amongst constituent parameters, pH was highest in soil samples under tree canopies of  F. natalensis (6.63) while highest TN (0.31%), TOM (6.96%) and EP (86.7ppm) were in soils under the canopies of  A. africanus. For EK and ECa, highest compositions were recorded under tree canopies of G. mollis (1.07 Ml/100g) and F. natalensis ECa (19.1 Ml/100g) respectively.

Between tree species, there was a more pronounced variation in pH and TOM under tree canopies (p < 0.05). For pH, the variation was contributed by the soils under canopies of A. abyssinica, A. gerradii and A. coriaria which had lower values compared to other tree species. Conversely, soils under tree canopies of A. coriaria exhibited a noticeable lower amount of TOM compared to other tree species (Table 2).

Effect of tree canopies on the nutrient profiles of pastures

The composition of all the tissue nutrients assessed was higher under tree canopies (p < 0.05) for the two pasture species. Between pasture species, the composition of tissue nutrients did not differ greatly (p > 0.05) in both canopy sites. Nonetheless, as shown in Figure 3, C. dactylon under tree canopies was slightly richer in CP (11.8%) and K (0.91%) compared to B. ruziziensis (CP =10.3%, K = 0.83%). Contrastingly, tissue P and Ca existed in equal proportions (0.48% and 0.29% respectively) in samples of the two pastures species collected under tree canopies.

Figure 3. The composition of tissue nutrients in B. ruziziensis and C. dactylon under (UC) and outside (OC) tree canopies

Amongst tree species, the composition of all tissue nutrients did not show a remarkable variation (p > 0.05). Nonetheless as presented in Figure 4, C. dactylon registered highest tissue CP and P under tree canopies of F. natalensis (12.5% and 0.55% respectively), K under A. africanus (0.98%) while tissue Ca was highest under canopies of A. abyssinica (0.30%). Lowest proportions of tissue CP, P, K and Ca were recorded in samples under canopies of A. gerrardii (10.9%), A. coriaria (0.43% for P and 0.28% for K) and F. natalensis (0.28%) respectively. As presented in Figure 5, B. ruziziensis recorded highest proportion of CP in samples under tree canopies of A. coriaria (12.9%), P under A. africanus (0.59%), K under G. mollis (0.86%) and Ca under A. abyssinica (0.30%). Lowest proportions of tissue nutrients were recorded in samples under canopies of A. gerrardii for CP (9 %), F. natalensis for P (0.36%) and K (0.71%) and G. mollis for Ca (0.20%).

Figure 4. Nutrient composicion of C. dactylon under tree canopies	Figure 5. Nutrient composicion of  B. ruziziensis under tree canopies

Relationship between soil nutrients and pasture nutritive values under tree canopies

As presented in Figures 6 and 7 (Graphs A and B), tissue samples of the two pasture species show a higher composition of N and P than in corresponding soil samples under the canopies of all the tree species. For B. ruziziensis tissue K was higher than soil K in samples under tree canopies of A. coriaria, A. gerrardii and F. natalensis but under the canopies of A. abyssinica, A. africanus and G. mollis soil K was higher than tissue K (Figure 6, Graph C). For C. dactylon, with the exception of A. abyssinica where soil and tissue K were equal as well as F. natalensis where soil K was greater than tissue K, the constituent nutrient was higher in the pasture tissues than in the soil under tree canopies of all other tree species (Figure 7, Graph C). On the other hand, soil Ca was higher than tissue Ca for the two pastures species under the canopies of all the study tree species (Figures 6 and 7, Graph D).

Figure 6. Relationship between soil and tissue N(A), P(B), K(C) and Ca(C) of B. ruziziensis under tree

Figure 7. Relationship between soil and tissue N(A), P(B), K(C) and Ca(C) of C. dactylon under tree

From Table 3, Pairwise pearson correlation coefficients show a negative correlation between the soil and all tissue nutrients of B.ruziziensis under tree canopies of A. abyssinica and A. gerrardii. Exceptional for A. abyssinica, the composition of tissue N in B.ruziziensis was not attributed to the amount of soil N recorded (r = -1, p<0.01) while for Ca, up to 86% composition of tissue Ca was not linked to the amount of ECa in the soil (r = -0.93, p<0.05). Nonetheless, soil N and tissue N of B. ruziziensis were positively correlated for all other tree species while soil Ca and tissue Ca were negatively correlated for all the tree species. Worthy noting, soils under tree canopies of A. africanus present a strong positve correlation between soil and tissue N, P and K.

For C. dactylon, the general trend presented in Table 3 show that there was a positive correlation between soil N, P and the corresponding tissue N, P for over 50% of the tree species while for soil and tissue K and Ca, over 50% of the tree species returned a negative correlation. Exceptional to F. natalensis other than soil and tissue Ca, all the other soil and corresponding tissue nutrients of C. dactylon were negatively correlated.

Discussion

Occurrence of remarkably higher levels of the selected soil nutrients under tree canopies of the study species demonstrates that the major tree species retained in the grazing lands of South-western Uganda are vital in improving soil fertility in such areas. Such results collate with the findings of Abdulahi et al (2016), where soils under tree canopies of Acacia senegal and Balanites aegyptiaca in the grazing lands of Ethiopia registered higher levels of soil nutrients compared to the open grasslands. Previous studies have attributed such results to higher levels of plant litter, reducing solar radiation and soil temperature under tree canopies (Tessema and Belay 2017), which factors could have enhanced mineralization processes within the soil leading to higher nutrient releases compared to outside tree canopies. More so, the higher pH values (more alkaline) recorded under tree canopies (6.02-6.49) compared to outside tree canopies (5.95-6.24) compound the observed higher level of organic and mineral nutrients under tree canopies. In a study by Neina (2019), it was revealed that there is higher solubility of organic matter and subsequent mineralization as soil pH increases. Such factor could as well have favored the release of soil nutrients under tree canopies compared to outside tree canopies.

Nonetheless, despite the dominant higher soil pH under tree canopies, soils under tree canopies of A. coriaria presented contrasting results where soil pH was lower under tree canopies (6.02) compared to outside tree canopies (6.13). Such results are comparable to the findings of (Tiruneh 2017), where soils under the canopy of scattered Acacia tortilis trees were more acidic compared to those of the adjoining open area. Desta et al (2018) links the observed pattern of results to accumulation of organic matter under the tree canopies through litter fall and root decay. However, since soils under tree canopies of A. coriaria did not register the highest TOM but instead presented the lowest composition, such observation calls for a thorough scientific investigation. Similarly, despite the dominant composition of EP under tree canopies, soils outside tree canopies of A. abyssinica showed exceedingly higher composition of EP (150 ppm) compared to under tree canopies (28.1ppm). A study by Avendaño-Yáñez et al (2018) revealed that soluble P content may also increase due to herbaceous vegetation in sites without tree cover compared to those under canopies, implying that plant–soil systems could be more important for enrichment of P, and that trees are not necessarily the most effective. Thus, further studies are required to understand the dynamics of P availability in plant–soil systems in association with A. abyssinica.

Amongst tree species, presence of higher amounts of the selected parameters in soils under tree canopies of A. africanus (for TN, TOM and EP), F. natalensis (for pH and ECa) and G. mollis (EK) implies that these tree species possess a higher potential for improving soil fertility in grazing lands. Studies by Ali et al (2018) and Castillo et al (2020), documented that the differences in traits such as litter quality among tree species can influence the production of OM, turnover and decomposition, thus influencing the accumulation of nutrients. Therefore, it’s probable that there could be differences in the litter quality of the respective tree species which may have favored higher nutrient accumulation compared to other tree species. Interestingly, whereas previous literature documents higher accumulation of soil nutrients under the canopies of leguminous trees (Tessema and Belay 2017), none of these species where highest compositions of nutrients was recorded are leguminous. Worth more, much as the respective tree species had the shortest canopy size, findings of Ward et al (2018) still recorded higher soil nutrient availability under tree canopies of large woody species with wider canopies compared to woody plants which had a smaller canopy. Therefore, whereas the C:N ratio was an aspect beyond the scope of this study, exploring this parameter in the leafy litter of the study tree species could provide a bankable explanation for the observed results.

In this study, it was evident that tree species influenced the pH of the soils. Presence of  A. abyssinica, A. gerradii and A. coriaria, facilitated a lower soil pH while A. africanus, F. natalensis and G. mollis fostered more alkaline soil pH conditions. Generally, soil pH is documented to be influenced by the mineralogy, climate, weathering, and agro-management of an agricultural system at appoint in time (Cao et al 2016). Basing on the latter findings, it is highly likely that the tree species in this study had different scales of influence on processes underlying soil pH in the grazing lands of South-western Uganda. This interaction fostered by heterogeneity composition of tree species is of paramount value in enhancing the sustainability of natural pasture production systems for improved livestock productivity. Interestingly, the tree species under whose canopies low pH values were recorded were of a wider canopy size compared to the rest of the tree species, which observation still contradicts the findings of a study by Ward et al (2018). In their study, they found out that whereas soil pH varied between the study tree species, soil pH was noticeably higher under the canopies of large woody species with wider canopies compared to smaller canopy species. Since litter input is documented to influence soil pH (Blume et al 2010), profiling the litter quality of the respective tree species could help in attaining a bankable explanation for the findings of this study.

In terms of TOM, the noticeably lower composition recorded in soils under tree canopies of A. coriaria compared to other tree species could imply limitations in microbial activities in association with the tree species. In a study by Lai et al (2014), it was revealed that the highest soil organic content under tree canopies is largely due to high enzymatic activities supported by higher litter fall and continuous decomposition of dead roots as opposed to tree less sites. Therefore, considering that soils under A. coriaria presented the lowest pH which limits microbial activity, it could be true that the tree species had lower potential for enzymatic decomposition processes compared to other tree species.

The higher composition of pasture tissue nutrients under tree canopies of all species affirms that the major tree species retained in the grazing lands of South-western Uganda improve the quality of natural pastures therein. Comparatively, similar results have been documented in several grazing areas worldwide (Bernardi et al 2016). Such findings are largely attributed to higher levels of soil nutrients, reduced water stress and surface temperatures under tree canopies, which factors favor mineralization of nutrients, and thus support growth of pastures of a higher forage quality as opposed to tree-less sites.

Whereas the composition of tissue nutrients between the two pasture species did not show a substantial variation, dominance of CP and K in C. dactylon reveals a slightly higher potential of this pasture species in extracting the respective nutrients from the soil as opposed to B. ruziziensis. This diversified ability is of a higher positive implication in sustaining the semi-intensive livestock production system South-western Uganda, where natural pastures constitute the major source of feed for livestock throughout the year (de Vries, 2019). Nonetheless under tree canopies, both C. dactylon and B. ruziziensis had equal proportions of tissue P and Ca which finding could be linked to the maturity status of the pasture species during the period of this study. A study by Siulapwa et al (2016) revealed that in many tropical grasses, the concentration of tissue P and Ca tend to decrease with advancing maturity. Therefore, since the period of data collection for this study was at the interface of end of the dry season and onset of the rainy season with limited new growth, it is probable that the pasture species harvested were mature in physiological status with close to maximum nutrient accumulations.

Most importantly, the tree species in this study influenced the CP content of the two pastures species, which parameter is regarded as the most vital for livestock productivity. In this study CP contents recorded under tree canopies were as high as 11.8% for C. dactylon and 10.3% forB. ruziziensis. Conversely outside tree canopies, C. dactylon recorded CP of 7.94% while B. ruziziensis recorded 7.42%. According to Rusdy (2016), dairy cattle requires a minimum of 7% CP for optimum rumen functioning and 10-12% to support the protein needs of dairy cows with moderate milk production (ARC 1984). Therefore, since C. dactylon and B. ruziziensis are the dominant pasture species in the semi-intensive grazing areas of South-western Uganda (Nabasumba et al 2020), retention and restoration of tree cover in such lands will go a long way in contributing towards availability of quality pastures in the largely feed scarce livestock production system.

Worth more, much as all the major tree species retained in the grazing lands of South-western Uganda supported growth of higher quality pastures, the differences in nutrient levels of the pastures in association with certain trees reveal a variation in the complementary strength of tree species towards improving forage quality. Serrano et al (2018), reports that tree species have different capacities to accumulate soil organic carbon and nutrients which in turn affect nutrient levels in pastures. Such results strongly support the findings of this study where soils under canopies of A. africanus and F. natalensis which had the highest composition of up to three (3) of the soil chemical parameters assessed led to highest compositions of CP and P in the two pasture species.

In this study, it was observed that under the canopies of all the tree species, both B. ruziziensis and C. dactylon had a higher composition of N and P than in the soil, which results are comparable to the findings of Yang et al (2018). In their study, it was revealed that the composition of N and P in plant tissues is largely linked to the underlying concentrations of the constituent nutrients in the soil. Therefore, since over 83% of the tree species in this study favored a higher concentration of the constituent elements in soils under their canopies, it is evident that this interaction had a ripple effect on the composition of N and P in the pasture species.

Regarding K, this study has revealed existence of interrelationships between soil K and tissue K with tree species having a micro influence. This is based on the observed pattern of results where for A. coriaria, A. gerrardii and F. natalensis, tissue K of B. ruziziensis was higher than soil K, which was the reverse for A. abyssinica, A. africanus and G. mollis. A study by Prajapatin and Modi (2012) revealed that although the time of K uptake varies with different plants, generally plants absorb the majority of their K at an earlier growth stage and K in older tissues is usually translocated to younger tissues. Banking on such findings it is probable that at the time of the study, B. ruziziensis growing under A. coriaria, A. gerrardii and F. natalensis was physiologically younger and in active stage of higher uptake of K from the soil. Interestingly it is also more likely that under the tree canopies of A. abyssinica, A. africanus and G. mollis, there was faster growth of  B. ruziziensis to a level than no more uptake of the respective nutrient was taking place at the time of sample collection. This is especially true basing on the fact soils especially under tree canopies of A. africanus and G. mollis had higher levels of soil N, which nutrient is reported to foster K uptake (Yarborough et al 2017) and increase growth of the benefiting pasture.

Similarly, the findings of Prajapatin and Modi (2012), could justify that at the time of the study C. dactylon under tree canopies of A. gerrardii, A. coriaria and A. africanus was physiologically younger and actively extracting K from the soil, thus the observed higher tissue K as opposed to soil K. In contrast, due to the relatively higher level of soil N which could have favored high uptake of K from the soil, it is probable that soils under F. natalensis facilitated faster growth and maturity of C. dactylon where no or little K was being extracted from the soil, thus the observed pattern of results where soil K was greater that tissue K.

Comparatively, the findings of this study where a negative correlation existed between soil Ca and tissue Ca for the two pasture species under the tree canopies of all the species are in agreement with the results of a study by Khan et al (2005). In their study, correlation between soil and forages mineral concentrations was positive except in Ca. Like Gizachew and Smit (2012) documents, such observations could be linked to higher concentration of Ca in soil above the critical values which are required for optimum growth of the pastures. This is especially true due to the fact that in this study, the Ca levels in soils under tree canopies of all species were above the critical level of 0.68 cmolc/kg required in Uganda’s soils (Kyomuhendo et al 2020).

Despite the dominant positive soil-tissue relationships displayed graphically, pairwise Pearson correlation coefficients did not show significant relationships between most soil nutrients and the corresponding values in the pasture samples. Previous studies have established that lack of significant relationships in the composition of soil and corresponding tissue nutrients is due to high absorption of these elements by the pastures for their growth, which may lead to a subsequent decline of the elements in the soil (Beyene and Mlambo 2012). Exceptionally, the absolute negative correlation between the soil and all tissue nutrients of B.ruziziensis under tree canopies of A. abyssinica and A. gerrardii could be linked to the complex interactions of processes which impact on uptake by plants. For instance in some cases, Ca uptake may have been hindered by competition with K on the ion carrier sites leading limited accumulation in the tissues (Moore et al 2014). Besides, since A. abyssinica was among the tree species under whose canopies lower pH was recorded, this could as well have influenced soil processes limiting nutrient uptake by the pastures. Nonetheless, the unique soil-pasture interactions recorded under tree canopies of A. abyssinica warrant further investigation.

Conclusions

• Presence of trees in the semi-intensive grazing areas of South-western Uganda has a positive impact on soil chemical properties and nutritive values of the pastures
  
  
• Higher levels of soil chemical properties and pasture tissue nutrients exist under tree canopies as opposed to outside tree canopies
  
  
• Allophylus africanus and Ficus natalensis are of high potential for restoring degraded areas in the semi-intensive grazing system of South-western Uganda
  
  
• Research in litter quantity and quality of the study tree species and mineralization rates under tree canopies will strengthen the body of knowledge on factors underlying higher soil fertility and forage quality under tree canopies

Acknowledgement

The authors extend their appreciation to National Agricultural Research Organization of Uganda and SNV Tides Project for funding this study.

Conflict of interest

The authors declare no conflict of interest regarding the publication of this manuscript

References

Abdulahi M M, Ebro A and Nigatu L 2016 Impact of woody plants species on soil physio-chemical properties along grazing gradients in rangelands of Eastern Ethiopia. Tropical and Subtropical Agroecosystems, 19(3). http://www.revista.ccba.uady.mx/ojs/index.php/TSA/article/view/2254

Ali B, Khan J H, Khan A A, Hussain A, Naveed M, Jamil K and Khurshid H 2018 Comparative effect of Acacia modesta and Olea ferruginea trees on soil characteristics of Dharabi watershed, Chakwal, Pakistan. Range Management and Agroforestry, 39: 87-92 http://www.indianjournals.com/ijor.aspx?target=ijor:rma&volume=39&issue=1&article=013

ARC 1984 The Nutrient Requirement of Ruminant Livestock. Supplement No. 1 (Commonwealth Agricultural Bureaux: Faraham Royal, UK). ARC, 1984.Agricultural Research Council.The Nutrient Requirements of Ruminant Livestock. Commonwealth Agricultural Bureaux, Farnham Royal, UK. In. Rusdy M 2016 Elephant grass as forage for ruminant animals. Livestock Research for Rural Development, 28(4), 1-6. https://lrrd.cipav.org.co/lrrd28/4/rusd28049.html

Avendaño-Yáñez, M D L L, López-Ortiz S, Perroni Y and Pérez-Elizalde S 2018 Leguminous trees from tropical dry forest generate fertility islands in pastures. Arid Land Research and Management, 32(1), 57-70. https://www.tandfonline.com/doi/abs/10.1080/15324982.2017.1377782

Bernard R E, De Jonge I K and Holmgren M 2016 Trees improve forage quality and abundance in South American subtropical grasslands. Agriculture, Ecosystems and Environment, 232: 227-231. https://www.sciencedirect.com/science/article/pii/S0167880916303978?casa_token=arTm791dFE4AAAAA:DQfCzW2qKJzwbLR3ILj1ZgEi98PxfCBW-GN_FfWKcazcInUDmuXKrJeYSkM7ofpiA4BILMuTpeg

Beyene S T and Mlambo V 2012 Yield and nutritive values of grasses in degraded communal savannas of swaziland surrounding dip-tanks and relationship with soil and herbaceous structure. Animal Nutrition and Feed Technology, 12(3), 279-296. http://www.indianjournals.com/ijor.aspx?target=ijor:anft&volume=12&issue=3&article=001

Blume H P, Brümmer G W, Horn R, Kandeler E, Kögel-Knabner I, Kretzschmar R and Welp G 2010 Scheffer/Schachtschabel. Lehrbuch der Bodenkunde, 16. https://link.springer.com/book/10.1007%2F978-3-642-30942-7

Bolo P O, Sommer R, Kihara J, Kinyua M, Nyawira S and Notenbaert A 2019 Rangeland degradation: Causes, consequences, monitoring techniques and remedies. Working Paper. CIAT Publication No. 478. International Center for Tropical Agriculture (CIAT). Nairobi, Kenya. 23 p. https://cgspace.cgiar.org/handle/10568/102393

Cao H, Chen R, Wang L, Jiang L, Yang F, Zheng S and Lin X 2016 Soil pH, total phosphorus, climate and distance are the major factors influencing microbial activity at a regional spatial scale. Scientific reports, 6(1), 1-10. https://www.nature.com/articles/srep25815/

Castillo M S, Tiezzi F and Franzluebbers A J 2020 Tree species effects on understory forage productivity and microclimate in a silvopasture of the Southeastern USA. Agriculture, Ecosystems and Environment 295:106917. https://www.sciencedirect.com/science/article/pii/S016788092030102X?casa_token=_zfAFZlcaDYAAAAA:eeXgX0uvywezwjWy2TdjsNLiVY00nXqw3SLbflNcpJImF5JO0O2R-4HTPTml3NWlXJVvf9f-JPg

Desta K N, Lisanenwork N and Muktar M 2018 Physico-chemical properties of soil under the canopies of Faidherbia albida (Delile) A. Chev and Acacia tortilis (Forssk.) Hayen in park land agroforestry system in Central Rift Valley, Ethiopia. Journal of Horticulture and Forestry, 10(1), 1-8. https://academicjournals.org/journal/JHF/article-abstract/7828F8355705

de Vries M 2019 Vulnerability and adaptation strategies of dairy farming systems to extreme climate events in south west Uganda: results of CSA-PRA workshops (No. 1141). Wageningen Livestock Research. https://library.wur.nl/WebQuery/wurpubs/547084

Filipová Z and Johanisova N 2017 Changes in pastoralist commons management and their implications in Karamoja (Uganda). Journal of Political Ecology, 24(1), 881-900. https://journals.uair.arizona.edu/index.php/JPE/article/view/20972

Gizachew L and Smit G N 2012 The status and importance of crude protein and macro minerals in native pastures growing on Vertisols of the central highlands of Ethiopia. Journal of environmental management, 93(1), 177-184. https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&scioq=ARC+1984+The+Nutrient+Requirement+of+Ruminant+Livestock.+Supplement+No.+1+%28Commonwealth+Agricultural+Bureaux%3A+Farnham+Royal%2C+UK%29.+ARC%2C+1984.Agricultural+Research+Council.The+Nutrient+Requirements+of+Ruminant+Livestock.+Commonwealth+Agricultural+Bureaux%2C+Fa&q=Gizachew+and+Smit+%282012%29&btnG =

Khan Z I, Valeem E E, Hussain A, Ashraf M and McDowell L R 2005 Seasonal variation in calcium and magnesium status of soil, dietary sources and small grazing ruminants in the semi-arid region of pakistan. International Journal of Biology and Biotechnology, 2 (1): 197-214, 2005. http://www.ijbbku.com/assets/custom/journals/2005/1/SEASONAL%20VARIATION%20IN%20CALCIUM%20AND%20MAGNESIUM%20STATUS%20OF%20SOIL,%20DIETARY%20SOURCES%20AND%20SMALL%20GRAZING%20RUMINANTS%20IN%20THE%20SEMI-ARID%20REGION%20OF%20PAKISTAN.pdf

Kyomuhendo P, Tenywa M M, Semalulu O, Lenssen A, Yost R, Mazur R and Kybogola S 2020 Lime requirements for bean production on two contrasting soils of Lake Victoria Crescent agro-ecological zone. African Journal of Agricultural Research, 15(4), 531-537. https://academicjournals.org/journal/AJAR/article-abstract/C8E148D63453

Lai R, Lagomarsino A, Ledda L and Roggero P P 2014 Variation in soil C and microbial functions across tree canopy projection and open grassland microenvironments. Turkish Journal of Agriculture and Forestry, 38(1), 62-69. https://journals.tubitak.gov.tr/agriculture/abstract.htm?id=14443

Makuma-Massa H, Bemigisha J, Kyasimire B, Nyiramahoro E, Begumana J, Mugerwa S and Cho M 2017 Mapping the potential for hay making in rangelands: a methodological proposition. Rangelands 39: 152-162. https://www.sciencedirect.com/science/article/pii/S0190052817300329

Moore A, Hines S, Brown B, Falen C, de Haro Marti M, Chahine M and Satterwhite M 2014 Soil–Plant Nutrient Interactions on Manure‐Enriched Calcareous Soils. Agronomy Journal, 106(1), 73-80. https://acsess.onlinelibrary.wiley.com/doi/full/10.2134/agronj2013.0345

Moran J 2005 Tropical dairy farming: feeding management for small holder dairy farmers in the humid tropics. Csiro publishing. https://books.google.co.ug/books?hl=en&lr=&id=ksAlAXas6xAC&oi=fnd&pg=PR3&dq=Moran,+J.+(2005).+Tropical+dairy+farming:+feeding+management+for+small+holder+dairy+farmers+in+the+humid+tropics.+Csiro+publishing.+&ots=ADPz5C4cVk&sig=cWFGl0dMO2UiSHC-AOU75hAccP0&redir_esc=y#v=onepage&q=Moran%2C%20J.%20(2005).%20Tropical%20dairy%20farming%3A%20feeding%20management%20for%20small%20holder%20dairy%20farmers%20in%20the%20humid%20tropics.%20Csiro%20publishing.&f=false

Mugerwa S and Emmanuel Z 2014 Drivers of grassland ecosystems’ deterioration in Uganda. Applied Science Reports, 2(3), 103-111. https://www.researchgate.net/profile/Swidiq_Mugerwa/publication/279498423_Drivers_of_grassland_ecosystems'_deterioration_in_Uganda/links/55b1c49008aed621ddfd5f7c.pdf

Nabasumba D, Kirunda H, Muzira R, Tugume G, Natuha S and Beyihayo G A 2020 Effect of scattered tree species on the diversity, abundance and biomass of pastures in a sedentary grazing system in South-western Uganda. African Journal of Agricultural Research, 16(8), 1078-1087. https://academicjournals.org/journal/AJAR/article-abstract/F7527D464413

Nagasha J I, Ocaido M and Kaase-Bwanga E 2019 Attitudes, practices and knowledge of communities towards climate change around Lake Mburo National Park Uganda: A Gender Centered Analysis. African Social Science Review, 10(1):3. https://www.researchgate.net/profile/Michael_Ocaido2/publication/333622657_Attitudes_Practices_and_Knowledge_of_Communities_Towards_Climate_Change_Around_Lake_Mburo_National_Park_Uganda_A_Gender_Centered_Analysis/links/5cf7b4f592851c4dd02a4124/Attitudes-Practices-and-Knowledge-of-Communities-Towards-Climate-Change-Around-Lake-Mburo-National-Park-Uganda-A-Gender-Centered-Analysis.pdf

Neina D 2019 The role of soil pH in plant nutrition and soil remediation. Applied and Environmental Soil Science, 2019. https://www.hindawi.com/journals/aess/2019/5794869/

Odero-Waitituh J A 2017 Smallholder dairy production in Kenya; a review. Livestock Research for Rural Development, 29(7), 139. http://lrrd.cipav.org.co/lrrd29/7/atiw29139.html

Okalebo J R, Gathua K W, Woomer P L 2002 Laboratory methods of soil and plant analysis: a working manual second edition. Sacred Africa, Nairobi, 21. file:///C:/Users/User/AppData/Local/Temp/SoilAnalysesmanual.pdf

Prajapati K and Modi H A 2012 The importance of potassium in plant growth-a review. Indian Journal of Plant Sciences, 1(02), 177-186. https://www.researchgate.net/profile/kalavati_prajapati/publication/304246278_the_importance_of_potassium_in_plant_growth_-_a_review/links/576a4b1f08ae7d2478cf0db7/the-importance-of-potassium-in-plant-growth-a-review.pdf

Pulido M, Schnabel S, Contador J F L, Lozano-Parra J and Gómez-Gutiérrez Á 2017 Selecting indicators for assessing soil quality and degradation in rangelands of Extremadura (SW Spain). Ecological indicators, 74, 49-61. https://www.sciencedirect.com/science/article/pii/S1470160X16306537?casa_token=J82ITiT2FvUAAAAA:GWjWOUevtelBUwLZZHLPlRk8Jqe1qtsMd-UqAvhPs0Opo83xTHlR-wc-yTK72YlP9a5LNgkwXHM

Rusdy M 2016 Elephant grass as forage for ruminant animals. Livestock Research for Rural Development, 28(4), 1-6. https://lrrd.cipav.org.co/lrrd28/4/rusd28049.html

Sempiira J E, Katimbo A, Mugisa D J, Kisaalita W S 2017 Ghee-making in the cattle corridor of Uganda. African Journal of Food, Agriculture Nutrition and Development, 17: 11771-11786. https://www.ajol.info/index.php/ajfand/article/view/153802

Serrano J, Shahidian S, Marques da Silva J, Sales-Baptista E, Ferraz de Oliveira I, Lopes de Castro J and Carvalho M D 2018 Tree influence on soil and pasture: Contribution of proximal sensing to pasture productivity and quality estimation in montado ecosystems. International Journal of Remote Sensing, 39: 4801-4829. https://www.mdpi.com/2073-4441/10/10/1422

Siulapwa N, Mwambungu A, Mubbunu L, Siyumbi S, Lungu E and Sichilima W 2016 Seasonal Variation of Nutrients in Hyperrhenia Grass from Liempe Farm in Lusaka, Zambia. Research Journal of Chemical and Environmental Sciences, 4: 59-67. https://www.researchgate.net/profile/Lumamba_Mubbunu4/publication/318432271_O_OR_RI_IG_GI_IN_NA_AL_L_A_AR_RT_TI_IC_CL_LE_E_Seasonal_Variation_of_Nutrients_in_Hyperrhenia_Grass_from_Liempe_Farm_in_Lusaka_Zambia/links/59694071a6fdcc18ea6f1ea2/O-OR-RI-IG-GI-IN-NA-AL-L-A-AR-RT-TI-IC-CL-LE-E-Seasonal-Variation-of-Nutrients-in-Hyperrhenia-Grass-from-Liempe-Farm-in-Lusaka-Zambia.pdf

Tibezinda M, Wredle E, Sabiiti E N and Mpairwe D 2016 feed resource utilization and dairy cattle productivity in the agro-pastoral system of South Western Uganda. African Journal of Agricultural Research, 11: 2957-2967. https://academicjournals.org/journal/AJAR/article-abstract/46F8EF559948

Tiruneh Asaye Z 2017 Effects of scattered Acacia tortilis (Forssk) hayne on soil properties in different land uses in Central Rift Valley of Ethiopia. Journal of Sustainable Forestry, 36(2), 164-176. https://www.tandfonline.com/doi/abs/10.1080/10549811.2016.1270835

Tessema Z K and Belay E F 2017 Effect of tree species on understory vegetation, herbaceous biomass and soil nutrients in a semi-arid savanna of Ethiopia. Journal of Arid Environments 139, 76-84. https://www.sciencedirect.com/science/article/pii/S0140196316302336?casa_token=qOd4E6NdkQsAAAAA:ml-r0-738dA04bPcp5UI7a93FM6PAHb6ng_3Ot66LUdcdX54GSZhsXv4FYBR3sw_V3th9fq2nzk

Ward D, Trinogga J, Wiegand K, du Toit J, Okubamichael D, Reinsch S and Schleicher J 2018 Large shrubs increase soil nutrients in a semi-arid savanna. Geoderma, 310, 153-162. https://www.sciencedirect.com/science/article/pii/S0016706117304214?casa_token=6xdlqtdlS1cAAAAA:xCAWAwQuMPNEJR2HX-jjAjG-HnMAwRJp0N3vZ__d0QhY2ZpgBkoRlotYi6NU9_2nEVqgrMS-M7o

Yang Z, Zhu Q, Zhan W, Xu Y, Zhu E, Gao Y and Peng C 2018 The linkage between vegetation and soil nutrients and their variation under different grazing intensities in an alpine meadow on the eastern Qinghai-Tibetan Plateau. Ecological Engineering, 110, 128-136. https://www.sciencedirect.com/science/article/pii/S0925857417305840?casa_token=cDWbB1O_iCMAAAAA:DUMzN6KcQte7ECeU_ayh1d6uRgsVWdilVI-YVm1IeGBCpC7C2nx2igDzTqk2HdayG_o1iHZNu08

Yarborough J K, Vendramini J M B, Silveira M L, Sollenberger L E, Leon R G, Sanchez J M D and Filho C S 2017 Impact of potassium and nitrogen fertilization on bahiagrass herbage accumulation and nutrient concentration. Agronomy Journal, 109(3), 1099-1105. https://acsess.onlinelibrary.wiley.com/doi/full/10.2134/agronj2016.10.0589

Zornoza R, Acosta J A, Bastida F, Domínguez S G, Toledo D M and Faz A 2015 Identification of sensitive indicators to assess the interrelationship between soil quality, management practices and human health. Soil, 1(1), 173-185. https://core.ac.uk/download/pdf/194990000.pdf