Legume defoliation affects rhizosphere decomposers, but not the uptake of organic matter N by a neighbouring grass

Abstract

Legume-grass interactions have a great influence on grassland primary production and it was recently shown how defoliation of a legume can increase the transfer of fixed N to a neighbouring grass. It has also been shown that defoliation of a plant can increase soil microbial activity and lead to better soil N availability in the rhizosphere of the defoliated plant. We combined these two perspectives and tested whether defoliation of a legume (Lotus corniculatus) can enhance N nutrition of the neighbouring grass (Holcus lanatus) by increasing growth of soil decomposer biota and the availability of soil organic matter N for grass uptake. We grew mixtures of L. corniculatus and H. lanatus in grassland soil that included 15 N-labelled L. corniculatus litter. In half of the systems, we subjected L. corniculatus to a defoliation treatment mimicking insect larvae feeding. At destructive harvests 1, 3, 9 and 30 days after the last defoliation event, we determined how L. corniculatus defoliation affected decomposer microbes, protozoa and nematodes and whether these changes among decomposers created a feedback on the growth and 15 N uptake of the neighbouring H. lanatus. Defoliation reduced the growth and litter-N uptake, but increased shoot N concentration of L. corniculatus. Of the soil variables measured, defoliation doubled the number of bacterial-feeding protozoa, but did not affect the abundance of decomposer microbes and bacterial-and fungal-feeding nematodes. Defoliation did not have statistically significant effects on H. lanatus shoot growth, shoot N concentration or litter-N uptake. Our results demonstrate how defoliation-induced changes in legume ecophysiology can affect the growth of decomposers in soil. However, these effects did not appear to lead to a significant change in the availability of soil organic N to the neighbouring grass. It seems that when positive effects of legume defoliation on grass N nutrition are found in grassland ecosystems, these are more likely to be explained by direct transfer of fixed N rather than changes in the availability of soil organic matter N.

Soil fertility and disturbance interact to drive contrasting responses of co‐occurring native and nonnative species

“…Similarly, soil microbial activity should be strongly coupled to both leaf trait and plant growth responses, and promoted by nonnative and N-fixing species (Wardle et al 2004, Reinhart and Callaway 2006, De Deyn et al 2008, Peltzer et al 2009, Orwin et al 2010. Consistent with these expectations, we found that soil microbial activity was stimulated particularly by nonnative N-fixing plant species on fertile soils or with defoliation, but did not determine the belowground mechanisms involved (Bardgett and Wardle 2003, Ayres et al 2007, van der Putten et al 2007, Saj 2008.…”

The effect of simulated herbivory on growth and nutrient status of focal and neighbouring early successional woody plant species

“…This indicates likely differences in the consequences of defoliating woody versus herbaceous plants. In that light, most previous studies showing a positive effect of defoliation on N availability have involved grassland plants (Mikola et al 2005, Ayres et al 2007, Hamilton et al 2008, Saj et al 2008), but not woody plants (Pastor et al 1993, Stark et al 2000, Harrison and Bardgett 2004). While it could be predicted that Carmichaelia may have positive effects on neighbouring plants through enhancing soil N status (Bellingham et al 2001), our results reveal that this is more than offset by a net negative competitive effect which is reversible only by substantial defoliation (Del‐Val and Crawley 2005).…”

Species-specific effects of live roots and shoot litter on soil decomposer abundances do not forecast plant litter-nitrogen uptake

“…The reason for this is that these organisms are abundant in the rhizosphere and thus are directly affected by plant roots (Clarholm 1985;Griffiths 1994). They also have high turnover rates and can respond to changes in resource availability within a few days (Bazot et al 2005;Mikola et al 2005;Saj et al 2008), which allows examination of plant-soil feedbacks in a short temporal scale (Hamilton and Frank 2001). It is, however, possible that the effects of changes in micro-food web structure on litter-N availability were outweighed by the effects of changes in the abundance of bigger organisms, such as enchytraeids and microarthropods, although populations of these animals should be less able to respond to changes in resource availability in the time scale of our study.…”