Dynamics of soil organic Carbon, Nitrogen and Microbial Biomass in a long-term experiment as Affected by inorganic and organic fertilization
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Smallhold farmers are faced with declining soil fertility arising from nutrient depletion through continuous harvests and insufficient return through fertilizers or organic resources and therefore a resulting decrease in crop yield. CheWi8!1 f ilisers directly offset nutrient shortages while organic reso rces recycle nutrients and offset soil organic matter (SOM) losses. Soil nutrient availability in low input farming systems which have low inherent soil fertility depends upon mineralisation of these organic resources which include crop residues, animal manures and native SOM. An 18-year old long-term experiment located at the National Agricultural Research Laboratories (NARL) of the Kenya Agricultural Research Institute (KARl) was used to study SOM dynamics and its effects on soil nutrients. The experiment began in 1976 and is intended to determine the appropriate methods for maintaining and improving soil fertility through the use of fertilisers, farmyard manure and maize stover under a continuous crop rotation of maize and beans. A selection was made of 8 out of 18 experimental treatments for detailed characterization of SOM pools and other soil parameters. These treatments were: maize stover (R), farmyard manure (M = 10T/halyr), fertiliser (F = 120 kg N & P205 /halyr), combinations of each of them (FR, FM, MR, FMR) and the complete control (CT). Soils were collected eleven months after the most recent input applications and organic matter and plant nutrients analyzed. Organic matter fractions were recovered by a procedure: of soil dispersion, wet sieving (53!-lm), decanting to remove sand and density separation using Ludox, a colloidal silica suspension of densities 1.13 and 1.37 g/crn-'. Analysis of variance, regression and correlation were then performed on the data. Manure application had the greatest effect on chemical soil properties, causing significant increases (up to 50%) in the measured parameters. Total SOM varied only slightly among the treatments (1.4-1.7% C and 0.14-0.20% N), but was significantly (P<O.OO1) affected by manure. Particulate organic matter (POM) and its density separates varied between land managements to a greater extent than total SOM. The medium (1.13-1.37 g/cm3) fraction showed the greatest sensitivity to land management (P<O.OO 1). All the organic matter pools correlated strongly with maize and bean crop yields except the heavy fraction (> 1.37 g/cm3) N which correlated poorly with both maize(R2 = 0.13, P< 0.05) and bean (R2 = 0.10, P> 0.05) yield. Manure and maize stover had a highly significant effect on soil microbial biomass (SMB) C and N (P< 0.001). There was a significant correlation between net anaerobic mineralizable nitrogen (AMN) and soil microbial biomass C (r= 0.91) and N (r= 0.93). Maize stover addition reduced AMN significantly (P< 0.001) by 14% compared to the manure treated soils. When stover was applied in combination with fertilizer however, AMN increased. Extractable Nand P and exchangeable cations (calcium, magnesium and potassium) varied significantly among the different land managements and they correlated well with total soil organic C and N (P<O.OOl). Soils recieving only fertilizer showed significant (P< 0.01) reduction in pH while manure and stover treatments showed significant (P<O.OOl) increases in soil pH. Effective cation exchange capacity increased with the addition of both organic and inorganic fertilizers in the order M > F > R. Use of manure offered the greatest return ratio to investments (4.4) and a high C sequestration efficiency (5.5%, 18 g/KSh input). In the Kikuyu Red Clay-Loam (Humic Nitisols) Highland areas of Kenya, smallhold farmers who recycle nutrients through livestock may be managing their soil organic matter resources in a more efficient manner than those who rely on fertilisers or crop residues alone.