Management of Tropical Legume Cover Crops in the Bolivian Amazon to Sustain Crop Yields and Soil Productivity

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Abstract
The trend to shorter natural fallow periods in shifting agriculture calls for management alternatives to accelerate nutrient accumulation and aid in weed control. Substituting cover crop‐based managed fallows is one possibility. Our objectives were to (i) quantify nutrient accumulation by three tropical legumes [Canavalia ensiformis (L.) DC.; Desmodium adscendens (Sw.) DC.; and tropical kudzu, Pueraria phaseoloides (Roxb.) Benth.] grown as managed fallows for 12 and 18 mo, (ii) evaluate the effect of residue management (burning vs. not burning) on the efficient use of nutrients by the subsequent rotational crops rice‐cowpea‐rice [Oryza sativa L.‐Vigna unguiculata (L.) Walp.‐O. sativa], and (iii) determine the effect of commercial fertilizer on maintaining crop yields. The study was conducted at two sites (Typic and Aquic Dystropepts) in the Chapare region of Bolivia. Total aboveground nutrient contents of the cover crops, across sites and duration of managed fallow, ranged from 41 to 391 kg N ha−1, −1, 5 to 191 kg K ha−1, 3 to 136 kg Ca ha−1, and 7 to 47 kg Mg ha−1. Averaged across sites and residue management, grain yields for the first rice crop after a 12‐mo managed fallow were highest with pueraria (1.65 Mg ha−1), followed by canavalia (1.38 Mg ha−1) and desmodium (0.65 Mg ha−1). Slashing and burning resulted in a mean rice (1st crop) yield increase of 137% compared with slash only. Cowpea yield was less affected by treatment variables than rice, due perhaps to N2 fixation. For the last rice crop, there were no grain yield differences for any of the variables studied. Nutrient uptake differences were most evident with the first rice crop, and were influenced primarily by residue management. Compared with slash only, and averaged across sites for the first rice crop, slashing and burning fostered a whole‐plant increase of approximately 18 kg N ha−1, 3 kg P ha−1, 24 kg K ha−1, 4 kg Ca ha−1, and 3 kg Mg ha−1. Soil chemical properties were only marginally affected by any variable. Shorter fallow periods with legume cover crops can produce adequate yields of annual crops. Longer cropping periods than we used are likely to require lime and fertilizer to sustain crop production.

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