MANAGEMENT OF TROPICAL LEGUME COVER CROPS IN THE BOLIVIAN AMAZON TO SUSTAIN CROP YIELDS AND SOIL PRODUCTIVITY

The trend to shorter natural fallow periods in shifting agriculture ca lls for management alternatives to accelerate nutrient accumulation an d aid in weed control. Substituting cover crop-based managed fallows i s one possibility. Our objectives were to (i) quantify nutrient accumu lation by three tropical legumes [Canavalia ensiformis (L.) DC.; Desmo dium adscendens (Sw.) DC.; and tropical kudzu, Pueraria phaseoloides ( Roxb.) Benth.] grown as managed fallows for 12 and 18 mo, (ii) evaluat e the effect of residue management (burning vs. not burning) on the ef ficient use of nutrients by the subsequent rotational crops rice-cowpe a-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 Dystrop epts) in the Chapare region of Bolivia. Total aboveground nutrient con tents of the cover crops, across sites and duration of managed fallow, ranged from 41 to 391 kg N ha(-1), <1 to 27 kg P ha(-1), 5 to 191 kg K ha(-1), 3 to 136 kg Ca ha(-1), and 7 to 47 kg Mg ha(-1). Averaged ac ross sites and residue management, grain yields for the first rice cro p 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 i ncrease of 137% compared with slash only. Cowpea yield was less affect ed by treatment variables than rice, due perhaps to N-2 fixation. For the last rice crop, there were no grain yield differences for any of t he variables studied. Nutrient uptake differences were most evident wi th the first rice crop, and were influenced primarily by residue manag ement. Compared with slash only, and averaged across sites for the fir st 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 h a(-1), and 3 kg Mg ha(-1). Soil chemical properties were only marginal ly affected by any variable. Shorter fallow periods with legume cover crops can produce adequate yields of annual crops. Longer cropping per iods than we used are likely to require lime and fertilizer to sustain crop production.