Dynamics of Soil Organic Carbon and Microbial Activities Under Different Land-Use and Agro-Ecological Zones of Ghana

Abstract

Soil organic carbon (SOC) plays major roles in maintaining soil productivity, especially in tropical agriculture, where the application of external inputs is low. The SOC stock is affected by a host of factors such as land-use, soil type, organic resource addition and the dynamics of SOC under varying climate conditions. It is the aim of the study to as sess the impact of land-use type: namely, forest or uncultivated lands (UC), cultivated lands (CT), and woodlot (WL) under different agro-ecological zones on the dynamics of SOC and microbial activities in some Ghanaian soils. The assessment of the SOC com position involved the determination of a range of chemical SOC fractions including (i) hot water extractable C (HWEOC), (ii) potassium permanganate extractable organic C (POX-C), (iii) acid hydrolysable (HCl-C) and (iv) non-acid hydrolysable (non-HCl-C). Biological fractionation included microbial biomass C (MBCmic) and N (MBNmic). Other SOC dynamics were characterized by the basal respiration (BR), metabolic quo tient (qCO2), microbial quotient (qMIC), mineralization quotient (qM) and carbon min eralized (Cmin) in a 365-day incubation study. The cumulative C mineralized during the incubation study was fitted to the first order double exponential models that separate C pools into potential mineralizable C (C0), labile pool (C1) and recalcitrant pool (C2) with their respective turn-over rates (k, k1 and k2) and half-lives (t1/2). The kinetic parameters from the first order equation were correlated with the chemical and biological fractions of C, and the microbial indices correlated with the organic C fractions. The results showed that land-use systems affected the various C pools, with the forest or uncultiv ated system recording the highest total C (TC) stocks, HWEOC, POX-C, MBCmic, MBN mic and C mineralization. Regarding agro-ecological zones, the savannah zones (Interior, Coastal) recorded lower values of total C, and MBCmic, but higher C values in HWEOC, POX-C, and HCl-C. The MBCmic and MBNmic values followed the order uncultivated > woodlot > cultivated across the land-uses systems. With respect to agro-ecological zones, the highest MBCmic and MBNmic were recorded for the Forest-savannah transition zone followed by the Semi-deciduous forest zone. The MBCmic was positively and sig nificantly correlated with the POX-C (r = 0.245*), TC (r = 0.475***), BR (r = 0.541***) and qMIC (r = 0.715***), but negatively significantly correlated with qCO2 (r = 0.715***). Curve fitted to the incubation data showed that the cultivated land-use of the Interior savannah zone recorded the lowest C0 values, whereas the Semi-deciduous zone recorded the highest C0. Most of the C were estimated to be in the Cs (recalcitrant pool) with high half-life. The C0 was positively correlated with POX-C (r = 0.743****), HWEOC (r = 0.540****), HCl-C (r = 0.663***), non-HCl-C (r = 0.614***), Cmin (r = 0.984***). The CMI recorded the highest value in UC and WL soils. It was concluded that the conversion of UC lands to CT resulted in a rapid decline in C stored in soils, shifting the hitherto conserved C to labile and recalcitrant forms. Labile fractions of SOC showed a rapid response to land-use changes and can thus be used as a sensitive indicator of SOC dynamics.