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Dynamics of soil organic carbon, nitrogen and microbial biomass in an organically manured ultisol in a guinea savanna agroecological zone of Nigeria

S.O.Agele, S.O.Ojeniyi, S.K.Ogundare

A field study was conducted to evaluate the effects of integrated use of agricultural wastes and a compound mineral fertilizer NPK (commercially available compound fertilizer containingN. P. K) on the fluxes (sources and sinks) of soil nutrients.Agricultural wastes applied were: livestock manure (cow dung and poultry litter), shoots of Chromolaena odorata and Parkia biglosa (locust bean), neem (Azadiracta inidca)seed powder/cake and melon shell. These materials were applied at zero (control), 100% (i.e. organic wastes applied at the recommended rates of 10t/ha) and 70% of their recommended rates plus 30%of the recommended rate of the mineral fertilizer (NPK:400 Kg/ha). The dynamics of biota population and soilmineralization (soil nutrients)measured in terms of organic carbon status, netNmineralization and microbial biomass (microbial biomass C and N) pool differed among the treatments. Differences were obtained for inorganic N released from the soil at the various dates of sampling. The trends of the time dynamics of SOC and plant available forms of N (NH4-N and NO3-N) show that peak values were obtained at 30 and 60 days after planting (DAP) and these values declined subsequently after.Average values of SOC were 1.94, 1.68, 1.36 and 1.38 for organicwastes alone, organic waste plusmineral fertilizer (NPK),mineral fertilizer and unamended control. Across sampling dates, SOC values were highest in poultrymanure and neemseed cake. The average values of SOC, NH4 + andNO3 - turnover rates indicated that these parameters were comparatively greater in the organic amended (1.94; 19; 119) than in the unamended (1.36; 15.5; 54) soils. The values of NO3 - N plus exchangeable NH4 + N which constitutes plant available nitrogen (PAN) that were recovered were significantly higher for organically amended soils (550) andwastes applied at reduced rates combined with 120 kg/hamineral NPK (470) than the unamended control (277). NO3 - N plus exchangeable NH4 + N were relatively high at 30 and 60 days after planting, and this trend was consistent among the agricultural wastematerials applied. The higher values were followed by consistent decline afterwards especially at the end of the experiment (120 DAP). This indicates that the manures whether applied solely or at the reduced rates combined with 120 kg/ha mineral NPK had high mineralisation rates. Mineral N (NO3 - N plus NH4 + N) pools and % C microbial to C organic ratio were higher in the nutrient-rich organically amended soils which indicated that increased Nmineralisation were facilitated by higher amounts of SOC. The time changes in SOC, NH4-N and NO3-N contents was monitored, declines in the values of SOC with time was obtained. This time dependent declining in SOC is linearly relatedwith (Y= 0.18x + 1.07; R2 = 0.34)Asharp decline inNO3-Nwith time under organic amendment alone and the control The nature of the decline in NO3-N is related with time by a power function (Y = 48.084x-1.79; R2 = 0.91). The nature of the decline in NH4-N is related with time by a polynomial function (Y= -28.75x + 130.65x -57.25; R2 = 0.61).Although the trend of the effects ofwastes application on cfu were inconsistent however, the time dynamics of microbial population (cfu) follows trends obtained for SOC. The differences in the quality of the agricultural wastes measured in terms of C/N ratios differed and could have driven the observed temporal variations in soil chemical properties (soil organic carbon,mineralN andmicrobial biomass-C andN). The values of%Cmic: Corg (indicator of microbial activity in terms of the utilisation of organic carbon by the microbes and hence organic matter turnover rate) obtained could be indicative of greater access of nutrients for microbes.Although the%microbial carbon to organic carbon ratios were stable for all treatments, itsmag