Changes in soil organic matter and phosphorus fractions under planted fallows and a crop rotation system on a Colombian volcanic-ash soil

Planted tree or shrub fallows can help increase the fertility of degraded tropical soils. We investigated the effects of planted fallows of Indigofera (IND), Calliandra (CAL), and Tithonia (TTH); a natural, unmanaged fallow (NAT); and a maize/bean rotation (ROT) on the dynamics and partitioning of soil organic matter (SOM) and phosphorus (P). One year after treatment, samples were collected from a fine-textured volcanic-ash soil (Oxic Dystropept) of a mid-altitude hillside in southwestern Colombia. The SOM in the sand-size fraction (150-2000 mum) was subdivided into light (LL), intermediate (LM), and heavy (LH) fractions. Total soil P was also fractionated into inorganic (P-i) and organic (P-o). Of the planted fallows, TTH most increased and NAT least increased plant-available P-i and P-o. The amounts of C, N, and P in the LL and LM fractions of SOM followed the order, TTH > CAL > NAT > ROT > IND and CAL > TTH > IND > NAT > ROT, respectively. Total amounts of N, P, K, Ca, and Mg in the soil were significantly (P < 0.05) highest under TTH and lowest under NAT. The fallow and ROT systems did not affect the C/N, C/P, and N/P ratios in the soil but significantly did so in the LL and LM fractions of SOM. Significant correlations indicated that the P content in the LL and LM fractions of SOM may help determine the amounts of NaHCO3-extractable P-i and P-o, which may therefore serve as sensitive indicators of 'readily available' and 'readily mineralizable' soil P pools, respectively, in the volcanic-ash soils of the Andes.