Soil organic matter assessment in natural regrowth, Pueraria phaseoloides and Mucuna pruriens fallow

Biological and chemical components of soil fertility were quantified under three different fallow types and related to soil quality of an Ultisol in s outhern Cameroon at the end of a 9-month fallow. Soil organic matter (SOM). soil exchangeable Ca2+, Mg2+ and K+ and available P concentrations, effect ive cation exchange capacity (ECEC) and, soil acidity in the 0-10 and 10-20 cm layers were evaluated under: natural regrowth mainly composed of Chromo laena odorata and the legume cover crops velvet bean (Mucuna pruriens var. utilis) and kudzu (Pueraria phaseoloides). SOM quality was assessed by C mi neralisation during a 4-week incubation at 28 degreesC in the laboratory. I n addition, particulate organic matter (POM), the most active part of SOM. was fractionated by wet sieving into coarse (4000-2000 mum). medium (2000-2 50 mum) and fine (250-53 mum) particle size classes and analysed for C and N contents. Under Mucuna, Ca2+,, K+ and P concentrations, ECEC and soil pH were higher and C mineralisation was Lower than under natural regrowth and Pueruria in 0-10 cm depth. Soil under natural regrowth had a higher C miner alisation in 0-10 cm indicating more labile SOM than in Pueraria and Mucuna fallow. There was no difference in weight of total POM. for any of the fra ctions between the three fallow types. However. both leguminous fallow spec ies increased POM quality through a higher N content. Compared to natural r egrowth, Pueraria increased N content in coarse POM by 36% in the 0-10 cm l ayer and by 19% (coarse POM) and 35% (medium POM) in the 10-20 cm layer. Mu cuna increased N content in the 0-10 cm layer by 12% (coarse POM), and by 1 9% (fine: POM), compared to natural regrowth. According to the differences in nutrient concentrations, soil acidity and the biological stability of SO M, the three fallow types tanked: Mucuna greater than or equal to Pueraria > natural regrowth. However. in terms of POM quality the ranking was: Puera ria > Mucuna natural regrowth. (C) 2001 Elsevier Science Ltd. All rights re served.