Effects of increasing phosphorus rate on microbial dynamics and soil available P under a Lixisol soil in Zimbabwe

Soil phosphorus (P) deficiency is a major challenge to the attainment of food security in most parts of sub-Saharan Africa, including Zimbabwe, where farmers largely depend on local organic nutrient resources in cropping. A greenhouse study was conducted to evaluate the influence of increasing inorganic P fertilizer rates (16, 26 and 36 kg P ha-1) on microbial dynamics, soil P pools and maize P uptake. using soils collected from a long-term (16 seasons) maize-monocropped field experiment where organic nutrient resources of different qualty namely Crotalaria juncea (high quality), Calliandra Calothyrus (medium quality), cattle manure ( variable quality), maize stover and Pinus patula sawdust ( both low quality) were applied at 4 t C ha-1 with 16 kg P ha-1 at the start of every season, a 3 x 6 factorial experiment was established by adding single super phosphate (8.5% P) as a basal fertilizer. Maize was used as the test crop and the experiment was conducted at Soil Productivity Research Laboratory in Zimbabwe. Using Pokovskays agar medium and serial dilution methods, phsophate-solubilizing microbes were monitored forthnightly from day 1 to 57 after maize planting. Ninetten (19) fungal and forty-two (42) bacterial colonies were identified over the study period, Fungi dominated bacteria on dayone, wth Aspergillus niger showing 20-98% abundance that depended on organic resources quality. Overall, a microbial explosion bulge characterized sucession on day 29 which coincided with a significant (P< 0.05) increase in diversity (H') and soil available P. Increasing P rate to 26 kg ha-1 amplified the explosion bulge under medium-high quality resources while under the control the bulge emerged earlier on day 15. Mucor and Bacillus had peak abundance on day 43 and 57, respectively, across treatments regardless of P rates. Treatment and P rate had a significant (P< 0.01) effect on microbial P. Bacteria were more rsponsive to added P than fungi. Increasing P to 36 kg P ha-1 also stimulated an earlier bulge under maize stover on day 15. Addition of P alone, without supplying complementary nutrients such as nitrogen,did not have a positive effect on maize P uptake. Farmers need to co-apply medium-high quality organic resources with high fertilzier P rates to increase microbial diversity, soil available P and maize growth on sandy soils (Lixisols). Our results suggest the need to recondiser existing P fertilizer recommendations, currently pegged at between 26 and 30 kg P ha-1, for maize production on sandy soils as well develop new fertilizer formulations to intensify crop production in Zimbabwe