Soil Viol. Eiochrm. Vol. 15, No. 2, pp. 221-222, 1983 0038-0717:83/020221-02$03,00/O Printed m Great Britain Pergamon Press Ltd zyxwvut SHORT COMMUNICATION MINIMIZING THE POTENTIAL PHYTOTOXICITY OF WHEAT STRAW BY MICROBIAL DEGRADATION J. M. LYNCH’ and L. F. ELLIOTT* ‘Agricultural Research Council Letcombe Laboratory, Wantage, Oxon OX12 9JT, U.K. ‘USDA, Agricultural Research Service, Pullman, WA 99164-6421, U.S.A. zyxwvutsrqponmlkjihgfed (Accepted 1 Sepfember 1982) The microbial degradation of wheat straw in soil under wet conditions can retard plant establishment and decrease crop yields (Cochran et al., 1977, 1981). The problem is caused at least in part by phytotoxins, principally acetic acid, formed by bacterial fermentation (Elliott et al., 1980; Lvnch, 1977, 1978; Lynch et al., 1980; Tang and Waiss, -1978; Wallace and Elliott. 1979). After a few months. when the straw has become degraded the potential for phytotoxin production lessens because the necessary substrates are no longer present (Cochran et nl., 1977; Harper and Lynch, 1981). This natural loss of substrates might be accelerated and the potential for phytotoxin production reduced if the straw could be degraded on-farm prior to seeding. We therefore investigated this possibility, using ground wheat straw under defined conditions in the laboratory, to achieve maximum control. In our first experiment wheat straw (10 g I- ‘) ground ~0.5 mm was degraded up to I month at 20’C in liquid medium (100 ml) containing mineral salts (Bravery, 1968) in 325 ml shaken flasks. Soil micro-organisms, which had been isolated as dominant colonists of straw for earlier liquid cultures were the inocula. The inocula suspended in 20ml distilled water were prepared from washed S-day cultures harvested from slanted tubes (20 x 75 mm) containing 20 ml malt (fungi) or nutrieht (bacteria) agar; I ml was added to each flask. Samples (1 ml) of the degraded straw suspensions were inoculated into 10 ml melted 0.9% agar (50°C) in test tubes (20 x 75 mm) and then slanted. Undegraded straw (non-treated) suspensions (1 ml of 0.1% v/w) containing the inocula and a non-inoculated control were added to the agar to check the potential activity of the fresh straw. Distilled water (I ml) was added to the agar as a control. A germi- nated wheat zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA (Triticum aestivum var. “Daws”) seedling (36 h after germination) was placed midway on the agar face (tube in vertical position) to avoid submerging it in the condens- ate. Ten replicates were used for each treatment. Seedlings were grown for 3 days at 23’C, after which the length of the longest root, a satisfactory index of root growth (Harper and Lynch, 1980), was measured. The non-treated straw produced a phytotoxic effect on the wheat seedlings (Table I). The effect was sienificant but smaller than that sometimes observed from field samples. This was probably because the relevant fermentative or- ganisms had only 3 days to function (during the assay itself), and also the assay conditions did not allow the necessary reducing conditions to develop fully. However, after 5 days of straw decomposition, the toxicity was decreased, and for the succeeding month no toxicity was evident. The differences in effect between micro-organisms were small and the micro-organisms themselves seemed to have no secondary effects on plant growth, perhaps because the seedlings were not stressed, and large microbial biomasses did not develop. During the early stages of decomposition, adding cellulolytic fungi to straw seems to offer no particu- lar advantage; presumably enough are present on the non- sterilized straw. In our second experiment cones of 0.4g ground wheat straw (2.5 cm dia, 0.5 cm deep) were added to the surface of 60g Palouse silt loam (mixed mesic, pachic, ultic Hap- loxerolls) or Hamble series silt loam (typic Hapladalf in Alfisols) in petri dishes (85 mm dia) and the soil wetted to its maximum water-holding capacity (0.25 g 8-l) with no suction. Decomposition was allowed to proceed for 10 days at 20°C. Then further cones of straw were added to the soil and wheat seeds added to the surface of the soil or straw. The wheat varieties used for seedling bioassay were Daws in Table 1. Effect of microbial decomposition of wheat straw on its phy- totoxicity to wheat seedlings Decomposer micro-organisms Root extension (% control without straw)* Azotobacter sp. Enlerobacter cloacae Mucor plumbeus Penicillium purpurescens ‘r Trichoderma harzianum t Azotobacter sp. + T. harzianum 7 M. plumbeus + P. purpurescenst E. cloacae + T. harzianumt Non-inoculated Non-treated straw Control (no straw) 920h’ 86” 98”* 86*’ 88”b 9O”h’ 96”h 96”h 95”h 81’ 100 *Results not followed by the same letter are significantly different (P = 0.05) using Duncan’s multiple range test. _FCellulase positive. 221