Decline of vesicular-arbuscular mycorrhizae in long fallow disorder of field crops and its expression in phosphorus deficiency of sunflower

Abstract

Poor growth of crops after long fallows (> 12 months) in cracking clay soils of the northern areas of the Australian grain belt is known as 'long fallow disorder'. Various crop species, including wheat (Triticum aestivum L.), chickpea (Cicer arietinum L.), grain sorghum [Sorghum bicolor (L.) Moench], sudan grass [Sorghum sudanense (Piper) Stapf], sunflower (Helianthus annuus L.), soybean [Glycine max (L.) Merr.] and maize (Zea mays L.), had less root colonization with vesicular-arbuscular mycorrhizal (VAM) fungi and plant weight after long fallows than after short fallows. An experiment was conducted with a phosphorus-deficient soil that had been either fallowed for 3 years or sequentially cropped to cotton, sorghum and sunflower. Cropped soil had more mycorrhizal propagules consisting of intact spores and colonized roots than long fallow soil. In the glasshouse, mycorrhizal colonization of sunflower (cv. Hysun 33) developed quickly in previously cropped soil to peak at 80% of root length at 72 days (flowering), but in long fallow soil it proceeded slowly, attaining 35% of root length at 72 days. Inoculation of long fallow soil with 20% w/w cropped soil resulted in extensive root colonization (89% at 72 days), eliminated P deficiency symptoms and more than doubled plant growth and final P uptake. Inoculation with similar soil treated with gamma radiation to kill propagules of mycorrhizal fungi had no effect on plant growth. Sunflower grew extremely poorly in irradiated soil with considerable leaf necrosis due to P deficiency. Reinoculation with cropped soil resulted In high levels of mycorrhizal colonization and good plant growth. It was concluded that long fallow disorder is caused by a decline in viable propagules of mycorrhizal fungi during fallowing, resulting in poor root colonization and symbiotic effectiveness of a subsequent crop. Fertilizing with phosphorus (50 mg P/kg soil) delayed the development of mycorrhizal colonization, but increased final lengths of colonized roots at 72 days. Zinc fertilizer (15 mg Zn/kg soil) slightly improved mycorrhizal colonization, and basal fertilizer (N, K, S, Ca) substantially improved colonization in long fallow soil inoculated with cropped soil.

Survival of propagules of arbuscular mycorrhizal fungi in soils in eastern Australia used to grow cotton

“…In the absence of a host, the propagules die if they cannot re-establish quiescence (Braunberger, Abbott & Robson, 1994). Thus, densities of arbuscular mycorrhizal fungi are likely quickly to reach critically low densities for adequate plant growth, as observed by Thompson (1987).…”

“…Email: peterm(^hio.usyd.edu.au Cotton can be grown in rotation with other summer or winter growing crops, or 'back-to-back' as a monoculture over consecutive seasons. Insufficient rainfall can extend the fallow to more than 12 months, a period associated with a severe reduction in initiation of AM in many crops, in a syndrome called 'Long Fallow Disorder' (Thompson, 1987). Cotton appears to be dependent on AM for maximum rates of plant growth and production of lint (Rich & Bird, 1977;G.…”

High densities of arbuscular mycorrhizal fungi maintained during long fallows in soils used to grow cotton except when soil is wetted periodically

“…The patterns did not change over 18 months, indicating that sufficient propagules of mycorrhizal fungi survived for 18 months in dry soil to establish maximum rates of infection in the cotton plants. Implicit in the description of long fallow disorder (Thompson, 1987(Thompson, , 1994) is a decline in viability as the propagules age leading to reduced infection in crops following fallows longer than 12 months. In these experimental soils, time was not the only factor in the depletion of propagules of mycorrhizal fungi.…”

“…The disorder is associated * To whom correspondence should be addressed. E-mail; fbioUaJbio.usvd.edu.au with low densities of AM fungal propagules in soil, resulting in reduced mycorrhizal infection of the plant (Sanders & Reed, 1978;Thompson, 1987Thompson, , 1994. By implication, the quantity of propagules of AM fungi had decreased with time.…”

“…Thus the survival of spores and hyphae in soil during the fallow will determine the propagule density of AM fungi following cotton. \'^iability of AM fungi can decline over time (Thompson, 1987;McGee et al, 1997), be reduced by cultivation or disturbance (Evans & Miller, 1988, 1990Fairchild & Miller, 1988, 1990Jasper, Abbott & Robson, 1989;McGonigle, Evans & Miller, 1990;McGee et al, 1997), environmental factors such as soil temperature and relative humidity (Sylvia & Schenck, 1983;Daft, Spencer & Thomas, 1987;Ruiz-Lozano & Azcon, 1996), and application of pesticides (Trappe, Molina & Castellano, 1984) and perhaps fertilizers (Johnson & Pfleger, 1992). The rate or extent to which each factor affects \'iability of AA'I fungi is unclear.…”