ARTHROPOD, NEMATODE AND WEED DYNAMICS IN A MAIZE-BASED CROPPING SYSTEM UNDER CONSERVATION AGRICULTURE
Two field experiments were conducted to investigate the effects of two conservation agriculture principles (minimum tillage and mulching) implemented simultaneously with weeding and fertiliser treatments on spiders, beetles, nematodes and weeds in a sub-Saharan African tropical environment. The experiments were conducted in Zimbabwe at Chinhoyi University of Technology farm between January 2014 and April 2016. Both experiments were laid out in a split-plot arrangement using a randomized complete block design and replicated three times. Treatments were:- i) tillage system: basin planting, rip line seeding and conventional tillage (CT); ii) mulching rate: no mulching, 4 t ha¬-1 maize stover mulch, 8 t ha¬-1 maize stover mulch, and 12 t ha¬-1 maize stover mulch; iii) fertiliser regime: zero fertiliser; 35.2 kg ha¬-1 N + 12.2 kg ha¬-1 P2O5 + 6.6 kg ha¬-1 K2O; 41.5 kg ha¬-1 N +14 kg ha¬-1P2O5 + 7 kg ha¬-1K2O; and 83 kg ha¬-1 N + 28 kg ha¬-1P2O5 + 14 kg ha¬-1 K2O and; iv) weeding intensity: clean weeding, weeding twice, weeding three times and weeding four times. Densities of wandering spiders (ground and plant wanderers) and total spiders were 69% higher in minimum tillage than CT treatments. Application of 4 tons ha-1 maize stover mulch resulted in 60% more ground wanderers and total spider abundances compared to other mulching treatments. Tillage and fertilizer treatments had no effect on spider diversity (Shannon-Weaver index) and functional group abundance, respectively. Spider evenness index was lower whilst richness index was 23% higher in minimum tillage treatments than CT treatments. Plots treated with basin planting and rip line seeding contained higher densities of beetle species such as Gonocephalum simplex, Renatiella reticulata and Zophosis boei than CT plots. Detritivorous, herbivorous, omnivorous and predatory beetles were higher in basin planting and rip line seeding than CT. However, fertilizer and mulching treatments had no effect on beetle feeding functional groups. Herbivorous beetle densities were highest in moderate weeding intensity (weeding three times). Beetle species diversity, evenness and richness were higher in basin planting and rip line seeding whilst application of 4 tons ha-1of mulch had the highest beetle species diversity compared to other mulching treatments. Mean densities of semi-endoparasitic nematodes 100 g-1 of soil were 140.32 in basin planting, 99.85 in rip lines and 50.17 in CT. Conventional tillage had the least densities of total nematode densities (117.5 nematodes 100 g-1) compared to rip line seeding (153.3 nematodes 100 g-1) and basin planting (199.2 nematodes 100 g-1). On the other hand higher nematode richness was recorded when basin planting was combined with low to medium fertilizer application. Weed species richness was highest (richness index = 4.34) in plots that received 4 t ha-1 maize stover mulch and lowest (richness index = 3.58) in plots that had no mulch. Germinable weed seed bank species evenness was also highest in rip line seeding. Weed species such as Rottboellia cochinchinensis and Eleusine indica were higher in basin planting and rip line seeding whilst species such as Ageratum conyzoides and Galinsoga parviflora were higher in CT system. Plots treated with fertilizer had approximately 20% lower densities of germinable seeds of I. plebia and E. indica. Moderate weeding intensity (weeding three times per cropping season) resulted in increased germinable weed seed banks of some species such as Amaranthus hybridus and G. parviflora. Significant interactions of soil depth and treatment factors such as tillage and mulching were observed. A. conyzoides and G. parviflora were higher in basin planting in the 0-10 cm soil depth. Multivariate analysis revealed association between management practices and biota i.e. spider, beetle, nematode and weed assemblages. This study has also shown that maize (Zea mays) grain yield decreases with increasing intensity of mechanical soil disturbance. The results of this study suggest that minimum tillage and mulching are important drivers of arthropod, nematode and weed community structure. There is evidence to suggest that minimum tillage and application of up to 4 ha-1 of mulch increase predatory arthropods and therefore enhance biological pest control. However, there is need to investigate the long-term effects of these treatments in different agro-climatic conditions.