Variations in soil microbial biomass and N availability due to residue and tillage management in a dryland rice agroecosystem

Seasonal changes in the levels of soil microbial biomass C (MBC) and N (MBN), N-mineralization rate and available-N concentration were studied in rice±barley supporting tropical dryland (rainfed) agroecosystem under six combinations of tillage (conventional, minimum and zero tillage) and crop residue manipulation (retained or removed) conditions. Highest levels of soil MBC and MBN (368±503 and 38.2±59.7 mg gÿ1, respectively) were obtained in minimum tillage residue retained (MT‡R) treatment and lowest levels (214±264 and 20.3±27.1 mg gÿ1, respectively) in conventional tillage residue removed (CTÿR, control) treatment. Along with residue retention tillage reduction from conventional to zero increased the levels of MBC and MBN (36±82 and 29±104% over control, respectively). The proportion of MBC and MBN in soil organic C and total N contents increased signi®cantly in all treatments compared to control. This increase (28% in case of C and 33% N) was maximum in MT‡R and minimum (10% for C and N both) in minimum tillage residue removed (MTÿR) treatment. In all treatments concentrations of N in microbial biomass were greater at seedling stage, thereafter these concentrations decreased drastically (21±38%) at grain-forming stage of both crops. In residue removed treatments, N-mineralization rates were maximum during the seedling stage of crops and then decreased through the crop maturity. In residue retained treatments, however, N-mineralization rates were lower than in residue removed treatments at seedling stage of both crops. At grain-forming stage in all instances the N-mineralization rates in residue retained treatments considerably exceeded the rates in corresponding residue removed treatments. Tillage reduction and residue retention both increased the proportion of organic C and total N present in soil organic matter as microbial biomass. Microbial immobilization of available-N during the early phase of crops and its pulsed release later during the period of greater N demand of crops enhanced the degree of synchronization between crop demand and N supply. The maximum enhancement effects were recorded in the minimum tillage along with residue retained treatment. In the dryland agroecosystem studied, two management practices in combination proved more advantageous than either practice alone in maintaining soil fertility levels. For soil fertility amelioration in dryland agroecosystems with least dependence upon chemical fertilizer input, post-harvest retention of about 20 cm shoot biomass (accounting for 25±40% aboveground biomass) of previous crop and its incorporation in soil through minimum tillage in the succeeding crop is suggested, especially in the case of cereal. # 2000 Elsevier Science B.V. All rights reserved.