Plant and Soil 147: 1-11, 1992. © 1992 Kluwer Academic Publishers'. Printed in the Netherlands. PLSO 9517 Mulch decomposition under agroforestry conditions in a sub-humid tropical savanna processes and influence of perennial plants GOTZ SCHROTH, WOLFGANG ZECH and GONTER HEIMANN 1 Institute of Soil Science and Soil Geography, University of Bayreuth, P.B. 101251, DW-8580 1 • • • Bayreuth, Germany and Btometrtctan, Crellestrasse 46, DW-IO00 Berlin 62, Germany Received 21 February 1992. Revised July 1992 Key words': agroforestry, Cajanus cajan, decomposition, Ferric Acrisols, mulching, nutrient release, sub-humid savanna, termites Abstract For the effective use of mulch materials in tropical agriculture and agroforestry knowledge of the speed of decomposition and nutrient release is of primary importance. The transfer of these informations from one site to another requires comparability of the processes of decomposition and their intensity at the two sites. In a litterbag experiment the decomposition and release of main nutrients from leaves and branches of Cajanus cajan (L.) Millsp. were investigated with regard to the underlying physical and biological processes during an 81 days period. To test the influence of perennial plants on the decomposition process, the study was conducted on an agricultural field in 1.1 m, 6.9 m and 14.9m distance from a tree and hedge band. During the first 11 days leaching was high, especially for N and P (about 50% lost) and K (75-80% lost). After the llth day consumption of the mulch material by the soil fauna was the dominating process of decomposition. During this phase the perennial plants significantly retarded the decomposition of Cajanus branches, but not leaves, probably by their influence on termite activity. Ca release was also retarded in leaves. After about 6-7 weeks, more than 90% of all main nutrients except Ca had been released from the samples. To minimize nutrient losses from nutrient-rich mulch materials, they should be applied repeatedly in small quantities according to the nutrient demand of the crop. Introduction The benefits of the application of organic materi- als as mulch to the soil surface has widely been appreciated in tropical agriculture and agrofores- try. They include the reduction of erosion hazards, better infiltration of rain water and less evaporation, lower soil temperatures, supply of organic matter and nutrients, higher biological activity, better root growth and suppression of weeds (Webster and Wilson, 1980). Under tropical smaUholder conditions, mulch- ing materials and especially working capacity for their transport and application on the field are often in limited supply (Webster and Wilson, 1980). Considering the effort for effective mulch- ing, a maximum of benefits has to be drawn from mulch application to be an acceptable technique for smallholders. Where the primary goal is the substitution of costly mineral fertilizers by mulch, it is necessary to know the release func- tions of the limiting nutrients to be able to synchronize the periods of maximum supply from the decomposing mulch with those of maxi- mum demand of the crop. Under conditions of high erosion hazard, on the other hand, the organic material should cover the soil when rain-