Conservation agriculture and its impact on soil quality and maize yield: A South African perspective.

Abstract

The countries in sub-Saharan Africa (SSA) are faced with the problem of soil degradation resulting from unsustainable soil management practices such as conventional tillage (CT) and the removal of soil biomass from crop land. Conventional tillage leads to deterioration of important soil physical properties, the decline in soil organic carbon (SOC) and increase the risk of soil erosion. The reduction of SOC further affects soil macrofauna which has important key roles in soil processes such as soil structural formation, decomposition of soil organic matter (SOM) and recycling of soil important nutrients. This combined with water scarcity, low inherent soil fertility, increasing population and the predicted negative impacts of climate change poses threat to the regions ability to self-supply enough food for current and future generations. In response to this conservation agriculture (CA) has been endorsed because of its powerful mechanism to adapt by increasing resilience to land degradation, drought and increasing water use efficiency. Soils under no-till CA have been recognised widely that they generally contain higher SOC, the key principal indicator of soil quality, than CT system. These responses, however, are site-specific and depends on soil type, cropping systems, climate, fertilizer application and other management practices. Moreover, most of the published literature on the effect of CA on soil quality parameters comes from cooler temperate regions. As a result, the effect of CA on soil quality parameters in sub-tropical semi-arid environments remains unknown or controversial. Therefore, the primary objective of the study was to assess the effect of no-till (NT), rotational tillage (RT), CT and nitrogen fertilizer application rates on selected physical, chemical and biological properties of the soil and, their influence on maize yield. The secondary objective of the study was to explore the use of visible to near infrared spectroscopy (VIS-NIRS) as a possible cheap alternative for SOC quantification. The study was conducted at Bergville in KwaZulu-Natal Province of South Africa. The trial was established in 2003/04 growing season. This area forms the most important part of rainfed maize production in KwaZulu-Natal Province. The trial was arranged as a split plot with randomized tillage strips forming the whole plot and rate of application forming the sub-plots which are randomized within the whole plots. The experiment included three tillage treatments: 1) no-till (NT) with permanent residue cover, 2) annual conventional tillage (CT) and 3) rotational tillage (RT) every after four years. Nitrogen was applied at three rates, namely; 0 kg/ha, 100 kg/ha and 200 kg/ha. Lime ammonium nitrate (LAN) was used as a source of nitrogen. Unsuitable soil management in agriculture is known to results in deterioration of soil health and the decline in biodiversity. The objective of the study in soil biological properties was to assess the effect of no-till CA on the abundance and order diversity of soil macrofauna in continuous maize monocropping system. Soil macrofauna was sampled at the end of the 2015/2016 growing season using 25 × 25 × 25 cm steel monoliths. The mean density of individual orders was significantly higher (p < 0.001) under NT (46%) and RT (38%) compared with CT (16%). However, the Shannon-Weaver index (H, E index) revealed that the diversity and evenness of orders were similar, H= 2.6 and E~ 1, for all treatments. Macrofauna patterns revealed that NT and RT contained a significantly (p < 0.001) higher population of orders Isoptera and Diplopoda. Order Isoptera was 51% and 17% higher in NT than CT and RT, respectively while in Diplopoda, NT was 39% and 2% higher than CT and RT, respectively. It was concluded that NT and RT mulch-based system favoured the development of macrofauna communities in the studied maize continuous monoculture cropping system but did not favour order diversity of macrofauna. This suggests the importance of crop rotation for the development of the more diverse macrofaunal population. Soil degradation associated with the loss of soil organic carbon (SOC) has been a major concern in sub-Saharan Africa because of the subsequent yield reduction. The objective of the study in soil physical properties was to investigate the effect of NT, RT, CT and N fertilizer applications rate on soil aggregate stability, infiltration, SOC and its size fractions at 0-10, 10-20 and 20-30 cm depth. Soil samples were taken at the end of 2015/16 growing season using soil auger. On average, total SOC did not vary (p > 0.05) across the tillage treatments, 27.1 t/ha (NT) vs 26.0 t/ha (RT) and 26.6 t/ha (CT), but varied with depth where it was stratified in the 0-10 cm depth in NT and RT. Particulate organic C, however, varied significantly (p < 0.05) across the treatments where it decreased with increase in tillage intensity but only in the 0-10 cm depth. Mean weight diameter (MWD) was high under NT and RT and this was correlated to higher infiltration observed in these treatments. The results of this study showed that reduced soil disturbance improves physical protection of SOC, soil structure and infiltration. Soil management practice may change soil chemical properties and thus fertility. The magnitude of change varies depending on soil type, cropping systems, climate, fertilizer application and management practices. The objective of this study on soil chemical characteristics was to assess the effects of tillage systems, residue retention and fertilizer application rates on the amount and distribution of soil major nutrients in the 0-10, 10-20 and 20-30 cm depth. The soil samples were taken at the end of 2015/2016 growing season using soil auger and transported to the University laboratory for chemical analysis. The concentration of total Nitrogen (N) followed the same trend as that observed in soil physical properties. SOC and N were found to be concentrated on the soil surface (0-10 cm depth). Phosphorus was significantly higher (p < 0.001) under NT (0.0213 t/ha) than in RT (0.0127 t/ha) and CT (0.00704 t/ha). A large amount of P was in the 0-10 cm depth in NT and it was distributed more uniformly under RT and CT. Potassium was also higher (p < 0.05) under NT (9.73 t/ha) than in CT (8.00 t/ha) and RT (9.52 t/ha). It was found to be uniformly distributed across the soil depths in all tillage treatments. The soils from NT and RT had lower pH values than CT at 0-10 cm depth while increased significantly in the lower depths. Cation exchange (CEC) capacity followed the same trend. The results indicated that NT treatment increased nutrient availability in the studied soil which was more linked to the distribution of SOC and variability of pH along the soil profile, thus this indicating the potential of implementing NT in the semi-arid environment. Resilient and sustainable soil management systems are needed to overcome soil degradation, arrest soil fertility decline and to offset the predicted negative impact of climate change. This study investigated the long-term (13 years) impact of soil quality parameters (soil physical, chemical and biological properties), N fertilizer application rate and rainfall on maize grain yield. On average (across the years) maize yields were higher in NT (12.3 t/ha) and RT (12.4 t/ha) under higher rate on N fertilizer application (200 kg/ha) than CT (11.8 t/ha). However, yields decreased in NT with the reduction of N fertilizer application rate in medium N rate (100 kg/ha) and low rate (0 t/ha). The yields decreased by 1.7 t/ha, 1.4 t/ha and 0.4 t/ha from high N application rate (200 t/ha) to medium N application rate (100 t/ha) in NT, RT and CT, respectively. Under low rainfall of < 400 mm/year and high N application rate (200 kg/ha), the yield was 9.13 t/ha, 7.96 t/ha and 7.00 t/ha in NT, RT and CT, respectively across the years. However, when the average rainfall was above 600 mm/year, yields averaged at 13.3 t/ha, 13.7 t/ha and 13.5 t/ha in NT, RT and CT under high N fertilizer application rate across the years. Principal component analysis (PCA) was performed to VI assess some biological, physical and chemical properties of the soil that contributed to maize yield. The results showed no parameter that seemed to be related to maize yield. This was attributed to the complex interaction of bio-physio-chemical parameters with the environment. The results of this study found that yields improve over time under CA and this was more pronounced during the drought period. Yields improvements under CA require the application of the higher rate of N fertilizer in correct amount. Therefore, it is recommended that CA is implemented in semi-arid subtropical areas to improve soil conditions, water conservation and to achieve optimum yields. Application of spectroscopy for assessment of soil nutrition in the field may be affected by the depth at which the radiation spreads to, the analysed nutrient, and management practices such as tillage systems. The visible to near infrared spectroscopy (VIS-NIRS) was explored as a technique to predict soil organic carbon (SOC) and soil organic nitrogen (SON) in soils differing in soil tillage management practices. Partial least square regression (PLSR) models were developed using the leave-one-out cross validation method. The models were then tested on independent samples (54) randomly selected from the total 324 samples. The best prediction model was observed for SOC with the coefficient of determination (R2) = 0.993, root mean square error of prediction (RMSEP) = 0.157% and residual predictive deviation (RPD) = 2.55 compared with R2= 0.661, RMSEP= 0.019%, RPD= 2.11 for SON. Considering the predictive statistics and accuracy created by the model in the prediction of SOC, VIS-NIRS can be recommended as a fast, accurate technique for SOC determination in the studied soil. This will significantly reduce the cost associated with SOC and SON analysis for researchers and farmers.

UKUFINYEZWA KWENDABA YONKE JIKELELE Amazwe asezansi ne Afrikha abhekene nengwadla enkulu yokudicileka phansi nokuphelelwa umsoco komhlabathi. Lokuphelelwa umsoco komhlabathi kubangelwa izindlela zokutshala ezingalungile ezingahlali isikhathi eside njengokutshala lapho oqale ulime umhlabathi khona bese usebenzisa igeja ususe nokhula. Lokhukulima ngegeja bese ususa ukhula kubangela ukuthi umhlabathi unganothi ngoba usuke ususe amacembe namagatsha agayekile (noma ayimvuthuluka) abaluleke kakhulu ekwakheni inqalasizinda sokuthi umhlabathi ubumbane ubeyimbumba futhi ukwazi ukuthi unikeze izitshalo umsoco wokuthi zikhule kahle. Lezizimvuthuluka zezitshalo um