Tesi etd-04222022-121445
Link copiato negli appunti
Tipo di tesi
Dottorato
Autore
MHLANGA, BLESSING
Indirizzo email
blessing.mhlangah@gmail.com
URN
etd-04222022-121445
Titolo
Crop nutrient acquisition, yield, and weed communities as related to arbuscular mycorrhizal fungal communities under conservation agriculture practices
Settore scientifico disciplinare
AGR/16
Corso di studi
Istituto di Scienze della Vita - AGROBIOSCIENCES
Commissione
relatore Prof.ssa ERCOLI, LAURA
Parole chiave
- Sustainable intensification
- No-tillage
- southern Africa
- Conservation agriculture
- Yield stability
- Arbuscular mycorrhizal fungi
- Weeds
- Host specificity
- Rotation
- Mulching
Data inizio appello
28/06/2022;
Disponibilità
completa
Riassunto analitico
Background and aims
Crop productivity in sub-Saharan Africa (SSA) is low and is affected by numerous biotic and abiotic stresses. Agricultural production of smallholder farmers in SSA accounts for most of the food consumed in the region and yet their yields are as low as 1.8 t ha-1 which is very low compared to global standards. These low yields are due to variable climate and inadequate management due to low input use. Maize productivity could be enhanced through increased input use but smallholder farmers in SSA cannot afford, and in some cases, do not have access to inputs such as mineral fertilizers and herbicides etc. An alternative cropping system called conservation agriculture (CA) has been advocated for by researchers to increase yields, improve system resilience to climate change, and protect the environment. CA is based on three components: minimum soil disturbance, permanent soil cover using organic material, and diversification of plant associations. Despite the well documented positive impact of CA, farmers in SSA usually do not implement all the components due to their socioeconomic situations and this has resulted in variable yield responses across the region. Thus, there is need to understand yield responses to CA across the region and how different abiotic and biotic factors shape these responses. The objectives of this study were: (i) to evaluate the effects of the components of CA and their interactions on crop yield responses and on yield stability across various environments in southern Africa; (ii) to evaluate the effects of the components of CA and their interactions on crop nutrient uptake, soil chemical properties, arbuscular mycorrhizal fungi (AMF) communities, and how they are interrelated; (iii) to evaluate the effects of the components of CA and their interactions on weed community diversity and composition, and the interrelatedness of weed community parameters with maize yield; and (iv) to assess whether mycorrhizal weeds surrounding the experimental fields, and among these weeds, those commonly found in experimental plots would act as hosts of AMF during the off-season and during the season, respectively, and if AM fungal assemblages would be affected by host identity and by the different combinations of CA components.
Materials and methods
For the first objective, the study was carried out at eight on-station experimental locations in four countries around southern Africa while for the other objectives two of these eight locations were used. For the first objective, the effects of eight treatments based on the combinations of the CA components (called ‘cropping systems’ hereafter) on crop yield response was evaluated using linear mixed models, while yield stability was assessed using Shukla stability variance and additive main effects and multiplicative interaction (AMMI) analysis. For the second objective, mixed models were used to assess cropping system effect on investigated variables and redundancy analysis (RDA) piecewise structural equation modeling (PSEM) was used to assess the relationship and effect pathway of the variables on maize yield, respectively. For the third objective, cropping system and seasonal effect on weed community diversity indices were assessed using linear mixed models while their effect on weed community structure was assessed using permutational multivariate analysis of variance (PERMANOVA). For objective four, the effect of host identity and cropping system on AMF community diversity indices and on AMF community structure within weed roots were assessed as was for weed communities in the third objective. For both objectives 3 and 4, the interrelatedness of the community parameters and maize yield were assessed using PSEM.
Results and discussion
The results of the study show that the adoption of mulch and rotation either under tillage or no-tillage, (i.e., CT+M+R and NT+M+R, respectively) results in equivalent yield responses across southern Africa. These equal responses may be due to enhanced biological, chemical, and physical processes associated with them. The study showed that these systems are associated with higher total N uptake, which exerted a direct and positive effect on maize grain yield. Mulching was shown to be the most crucial component of CA across southern Africa in improving yield stability and the systems that involved mulching, such as NT+M, resulted in highest soil N and soil organic C, and yet soil N had a direct and positive effect on maize yield. The same system also resulted in highest weed community diversity, and hence a positive and direct effect on grain yield since diverse weed communities are less competitive. However, weed community structures depended on the CA practices implemented, with the adoption of rotation in CT systems showing more distinct structure as compared to other systems. Core weed species, such as Richardia scabra and Bidens pilosa contributed to most of these differences, and yet the same species can harbour AMF during the off-season and in-season, thus resulting in improved AMF-based ecosystem services. Due to host preferences, AMF communities differed within weed roots depending on the weed species.
Conclusions
In conclusion, the study recommends the adoption of CA practices, but there is need for optimization depending on the environment in which a farmer is located, and on the aspect to be enhanced. For example, farmers can adopt mulching, rotations, and no-tillage to enhance yield and improve soil structure in the longer-term. If farmers intend to increase soil organic carbon and nitrogen in the soil while stabilizing their yields, they can adopt no-tillage and mulching. Overall, the results show that there was no “one-size-fits-all” combination of CA components suggesting that farmers need to optimize combinations of CA components depending on environment and targeted aspect to be improved to realize benefits across southern Africa.
Crop productivity in sub-Saharan Africa (SSA) is low and is affected by numerous biotic and abiotic stresses. Agricultural production of smallholder farmers in SSA accounts for most of the food consumed in the region and yet their yields are as low as 1.8 t ha-1 which is very low compared to global standards. These low yields are due to variable climate and inadequate management due to low input use. Maize productivity could be enhanced through increased input use but smallholder farmers in SSA cannot afford, and in some cases, do not have access to inputs such as mineral fertilizers and herbicides etc. An alternative cropping system called conservation agriculture (CA) has been advocated for by researchers to increase yields, improve system resilience to climate change, and protect the environment. CA is based on three components: minimum soil disturbance, permanent soil cover using organic material, and diversification of plant associations. Despite the well documented positive impact of CA, farmers in SSA usually do not implement all the components due to their socioeconomic situations and this has resulted in variable yield responses across the region. Thus, there is need to understand yield responses to CA across the region and how different abiotic and biotic factors shape these responses. The objectives of this study were: (i) to evaluate the effects of the components of CA and their interactions on crop yield responses and on yield stability across various environments in southern Africa; (ii) to evaluate the effects of the components of CA and their interactions on crop nutrient uptake, soil chemical properties, arbuscular mycorrhizal fungi (AMF) communities, and how they are interrelated; (iii) to evaluate the effects of the components of CA and their interactions on weed community diversity and composition, and the interrelatedness of weed community parameters with maize yield; and (iv) to assess whether mycorrhizal weeds surrounding the experimental fields, and among these weeds, those commonly found in experimental plots would act as hosts of AMF during the off-season and during the season, respectively, and if AM fungal assemblages would be affected by host identity and by the different combinations of CA components.
Materials and methods
For the first objective, the study was carried out at eight on-station experimental locations in four countries around southern Africa while for the other objectives two of these eight locations were used. For the first objective, the effects of eight treatments based on the combinations of the CA components (called ‘cropping systems’ hereafter) on crop yield response was evaluated using linear mixed models, while yield stability was assessed using Shukla stability variance and additive main effects and multiplicative interaction (AMMI) analysis. For the second objective, mixed models were used to assess cropping system effect on investigated variables and redundancy analysis (RDA) piecewise structural equation modeling (PSEM) was used to assess the relationship and effect pathway of the variables on maize yield, respectively. For the third objective, cropping system and seasonal effect on weed community diversity indices were assessed using linear mixed models while their effect on weed community structure was assessed using permutational multivariate analysis of variance (PERMANOVA). For objective four, the effect of host identity and cropping system on AMF community diversity indices and on AMF community structure within weed roots were assessed as was for weed communities in the third objective. For both objectives 3 and 4, the interrelatedness of the community parameters and maize yield were assessed using PSEM.
Results and discussion
The results of the study show that the adoption of mulch and rotation either under tillage or no-tillage, (i.e., CT+M+R and NT+M+R, respectively) results in equivalent yield responses across southern Africa. These equal responses may be due to enhanced biological, chemical, and physical processes associated with them. The study showed that these systems are associated with higher total N uptake, which exerted a direct and positive effect on maize grain yield. Mulching was shown to be the most crucial component of CA across southern Africa in improving yield stability and the systems that involved mulching, such as NT+M, resulted in highest soil N and soil organic C, and yet soil N had a direct and positive effect on maize yield. The same system also resulted in highest weed community diversity, and hence a positive and direct effect on grain yield since diverse weed communities are less competitive. However, weed community structures depended on the CA practices implemented, with the adoption of rotation in CT systems showing more distinct structure as compared to other systems. Core weed species, such as Richardia scabra and Bidens pilosa contributed to most of these differences, and yet the same species can harbour AMF during the off-season and in-season, thus resulting in improved AMF-based ecosystem services. Due to host preferences, AMF communities differed within weed roots depending on the weed species.
Conclusions
In conclusion, the study recommends the adoption of CA practices, but there is need for optimization depending on the environment in which a farmer is located, and on the aspect to be enhanced. For example, farmers can adopt mulching, rotations, and no-tillage to enhance yield and improve soil structure in the longer-term. If farmers intend to increase soil organic carbon and nitrogen in the soil while stabilizing their yields, they can adopt no-tillage and mulching. Overall, the results show that there was no “one-size-fits-all” combination of CA components suggesting that farmers need to optimize combinations of CA components depending on environment and targeted aspect to be improved to realize benefits across southern Africa.
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