Nematodes as an indicator of plant-soil interactions associated with desertification

Authors: KLASS, JEREMY - New Mexico State University; Peters, Debra; TROJAN, JACQUELINE - New Mexico State University; THOMAS, STEPHEN - New Mexico State University

Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/8/2012
Publication Date: 4/13/2012
Citation: Klass, J., Peters, D.C., Trojan, J.M., Thomas, S.H. 2012. Nematodes as an indicator of plant-soil interactions associated with desertification. Applied Soil Ecology. 58:66-77.

Interpretive Summary: While many unknowns exist in the evaluation of nematode populations within natural systems, previous research along with results from this study provide information in the role of soil biota in desertification of arid grasslands. The shift in soil community structure and the subsequent change in soil biotic energy pathways, associated with desertification can lead to ecosystem level alterations of nutrient cycling and changes in aboveground production. As desertification increases, soil biotic populations become less diverse and more heterogeneously distributed in vegetation patches that further alter nutrient dynamics on an ecosystem scale. Furthermore, loss of the associated soil biotic consortia that is intimately tied to ecosystem function of semi-arid grasslands may have real and long-term consequences for the sustainability and recovery of B. eriopoda grasslands. A more diverse trophic structure, as observed in the Otero Mesa soils, may be indicative of stability in ecosystem interactions between roots and the fungal and bacterial energy channels. Dominance of any one energy channel might be indicative of dynamic instability as observed within the JER grassland (fungal) and the mesquite duneland (bacterial), where unimpeded vertical energy flow may provide increased nutrient supplies, nutrient imbalance or losses, and a change in facilitative soil biotic communities vital to plant production and resilience in arid systems. However, the dominance of the fungal energy channel in black grama grasslands on the Jornada may be indicative of a fully functioning, connected system of hyphal networks, plant patches and soil biotic crusts. Linking nematode populations and trophic structure to function at individual plant scales is more complex than just enumeration and identification because of their central role in food webs and connections to ecological processes in soil. In order for nematode community and trophic analyses to be more useful, a more complete understanding of feeding habits and tolerance levels of each dominant nematode genus to abiotic stresses are needed before soil nematode community indices can be interpreted and calibrated accurately as indicators of arid land soil condition, function, and soil biotic interactions. This improved understanding will require improved species level identification using morphological and molecular criteria, where life history information relating to specific environments can be applied to community level surveys.

Technical Abstract: Conversion of perennial grasslands to shrublands is a desertification process that is important globally. Changes in aboveground ecosystem properties with this conversion have been well-documented, but little is known about how belowground communities are affected, yet these communities may be important drivers of desertification as well as constraints on the reversal of this state change. We examined nematode community structure and feeding as a proxy for soil biotic change across a desertification gradient in southern NM, USA. We had two objectives: (1) to compare nematode trophic structure and species diversity within vegetation states representing different stages of desertification, and (2) to compare nematode community structure between bare and vegetated patches that may be connected via a matrix of endophytic fungi and soil biotic crusts. The gradient included a perennial grassland dominated by Bouteloua eriopoda, the historic dominant in the Chihuahuan Desert, a duneland dominated by the shrub, Prosopis glandulosa, and the ecotone between them. We also sampled a relatively undisturbed, ungrazed B. eriopoda grassland at a nearby site to serve as an end member of our gradient. Nematode communities were sampled using soil cores to depth of 50 cm at each location in 2009 and 2010. Results showed that grasslands and mesquite dunelands had different trophic groupings and herbivorous nematode communities with lower species diversity and evenness compared with the ecotone. Nematode trophic structure and herbivore communities were significantly different in all vegetation states with the highest observed diversity in the undisturbed, ungrazed B. eriopoda grassland in 2010. Vegetated and bare ground patches within the two grassland sites had similar herbivore communities, especially species from the family Tylenchinae. However, the mesquite duneland showed the lowest sampled diversity of all sites, but had significantly larger nematode abundances in vegetated dunes than interdune areas that are void of vegetation and soil biotic crusts where bacteriovores dominated. Decreased nematode trophic structure and species diversity in the Jornada black grama grassland samples compared with the undisturbed grassland illustrate the effect of desertification on the soil biotic community. Our results show that nematodes can be used to identify changes in belowground community structure based on trophic interactions. Large-scale disturbances like desertification can have consequences on the diversity and soil biotic functioning at finer spatial scales.