Short-term effects of defoliation on the soil microbial community associated with two contrasting Lolium perenne cultivars
Introduction
The physiological adaptations of grasses to tolerate periodic defoliation by grazing animals have been studied previously (Ferraro and Oesterheld, 2002). However, much less is known about how plant physiological responses to constraints on growth affect feedbacks between the soil microbiota and plant productivity. In systems receiving little or no fertiliser input, the availability of nutrients for plant growth is heavily dependent on cycling through the microbial biomass. The quantity and quality of plant inputs to soil (litter, root turnover, and root exudation) are key drivers of microbial activity and community structure (Campbell et al., 1997, Grayston et al., 1998). Plant productivity is therefore coupled to the functioning of the soil microbial community (SMC) through mineralisation and immobilisation processes that are driven by root derived C (rhizodeposition). Determining the factors which illicit change in the SMC is of fundamental importance to understanding these interactions.
Plant growth in grassland ecosystems, where fertiliser inputs are uncommon, is often limited by available nutrients (Tisdale and Nelson, 1975). There is evidence that root exudation accounts for a greater proportion of assimilated C under N limited conditions compared with N sufficient conditions (Paterson and Sim, 1999, Paterson and Sim, 2000), possibly contributing to a larger, more active SMC in unfertilised grassland soils (Yeates et al., 1997). Through stimulating the SMC, increased exudation may increase nutrient turnover. Bardgett et al., 1996, Bardgett et al., 1997 have demonstrated a positive relationship between grazing intensity, the size and activity of the SMC and dominance by bacteria. Quantitative and qualitative changes in detrital inputs to the soil are likely determinants of herbivore-induced changes in the SMC (Bardgett et al., 1997). However, the control of plant physiological responses affecting these inputs on microbial activity and community structure are less well understood. Defoliation is a strong perturbation to plant C flow, dramatically reducing photosynthetic capacity, and resulting in preferential carbon allocation above-ground at the expense of below-ground allocation (Morvan-Bertrand et al., 1999). Re-growth potential has been correlated to the concentration of stored carbohydrates in the basal stubble region of forage species (Donaghy and Fulkerson, 1998). Water-soluble carbon (WSC) and amino components, collectively termed non-structural carbon (NSC) (Davis et al., 1995), are rapidly re-mobilised following defoliation to ensure continued assimilate supply to the growing meristems (Morvan-Bertrand et al., 1999). As a result of differential effects on C partitioning and ability to re-grow, the extent to which defoliation alters the quality and quantity of carbon flow from roots is likely to differ between species and cultivars. The SMC often responds to defoliation independently of changes in the root biomass, suggesting that they result from changes in root exudation (Guitian and Bardgett, 2000). While species comparisons are common, the effects of cultivar-specific responses to defoliation have been largely neglected in terms of the SMC, despite significant cultivar-specific responses being reported in terms of altered C partitioning and root functioning (Fulkerson et al., 1994). New forage varieties are often developed for high productivity and nutritional value, without particular attention being paid to the effects on below ground carbon and nutrient cycling. Understanding the driving forces on the SMC is particularly important in developing sustainable management practices.
This study aimed to determine the influence of defoliation and soil fertiliser amendment on the structure of the rhizosphere SMC associated with two Lolium perenne cultivars with contrasting carbon storage characteristics. The L. perenne cultivars selected were AberDove and S23, previously reported as high and low WSC accumulating cultivars, respectively (Humphreys, 1989a, Humphreys, 1989b, Smith et al., 2001). It was hypothesised that: (i) due to differences in below-ground C allocation, the SMC associated with the two L. perenne cultivars would differ in size and structure; (ii) following defoliation the SMC is likely to respond to altered resource availability resulting from changes to C assimilation, C allocation, and nutrient uptake. The perturbation is likely to be more evident in L. perenne S23 due to lower C reserves available to buffer the defoliation effect. In AberDove, greater C reserves are likely to be used for rapid re-growth and a faster return to undisturbed C assimilation, C allocation and nutrient acquisition; (iii) soil fertility will affect the SMC directly through influencing microbial available N, and indirectly as a consequence of increased primary production, and through cultivar specific effects on C partitioning, root production, and rhizodeposition.
Section snippets
Soil and sward establishment
Soil was collected from Fasset Hill, Sourhope, in the Scottish Borders (National Grid Reference NT 852207), an unimproved permanently grazed Agrostis–Festuca grassland, classed U4A (Rodwell, 1992). The soil, a brown ranker (Hapumbrept; FAO/UNESCO, 1994) derived from Old Red Sandstone (Dry, 1993) (7.46% C, 0.61% N, pH 3.55 (CaCl2), 23% clay, 13.7% organic matter), was sampled to a depth of 30 cm and sieved moist (6 mm). Unimproved soil (UNI) was packed into 12 boxes (556 mm×356 mm×222 mm deep), to
Results
Total plant biomass production did not differ significantly between cultivar, but was significantly (P<0.05) influenced by a soil–defoliation interaction (Table 2a, Table 2b). Contrary to improved (IMP) soil, total plant biomass produced in unimproved (UNI) soil was unaffected by defoliation (D). As a result plant biomass produced in UNI soils was significantly (P<0.05) less than that of improved non-defoliated (IMP ND) swards, and significantly more than that of improved defoliated (IMP D)
Discussion
Due to differences in C partitioning, ability to re-grow following defoliation, and growth response to N amendment, it was expected that cultivar- and treatment-specific differences in biomass production and allocation would be reflected below ground in the SMC. While total plant biomass production did not significantly differ between cultivar, there were a number of indicators among the measured plant parameters that would indicate cultivar-specific response to defoliation and soil amendment
Acknowledgments
This work was funded by the Macaulay Development Trust and the Scottish Executive Environment and Rural Affairs Department. The authors gratefully acknowledge the Institute of Grassland and Environmental Research, Aberyswyth for the provision of the two grass cultivars used. Allan Sim and Jasmine Ross are thanked for technical assistance, and Sue Grayston and the anonymous referees for useful comments on this manuscript.
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Present address: CSIRO Land and water, PMB2, Glen Osmand, SA 5064, Australia.